Question 1

In classical mechanics, the law of falling bodies stated by Galileo is commonly known by which other descriptive name?

Accepted Answer

Introduction / Context: Before Newton formalised the laws of motion and universal gravitation, Galileo carried out systematic experiments on falling objects. His work on falling bodies overturned the old Aristotelian idea that heavier objects fall faster than lighter ones. Galileo showed that, in the absence of air resistance, all bodies near the surface of the Earth fall with the same constant acceleration. This result is often referred to as Galileos law of falling bodies. The question asks you to identify another descriptive name used for this same idea in basic mechanics textbooks. Given Data / Assumptions: • We are dealing with motion of bodies falling freely under gravity near the Earth surface. • Air resistance and other resistive forces are neglected. • The law attributed to Galileo states that all freely falling bodies have the same constant acceleration. • The options list several well known laws and principles from mechanics. Concept / Approach: Galileos experiments showed that in uniform gravitational fields, the acceleration of a freely falling body is constant and independent of its mass. This result implies that the displacement of a falling body is proportional to the square of the time of fall when starting from rest. Because the acceleration is constant, the motion is described as uniformly accelerated motion. Therefore, Galileos law of falling bodies is commonly described as the law of uniform acceleration of freely falling bodies. This descriptive name captures the key content of his law without using more modern Newtonian terminology. Step-by-Step Solution: Step 1: Recall that Galileo emphasised experimental observation of bodies falling from towers and sliding on inclined planes. Step 2: From these experiments, he concluded that all objects in free fall (ignoring air resistance) accelerate at the same constant rate. Step 3: Constant acceleration is referred to in mechanics as uniform acceleration. Step 4: Combine these ideas to obtain a descriptive title: law of uniform acceleration of freely falling bodies. Step 5: Compare this descriptive name with the options given in the question. Step 6: Select the option that exactly mentions uniform acceleration of freely falling bodies as the alternative name. Verification / Alternative check: Looking at introductory physics textbooks, the equations of motion for bodies under constant acceleration due to gravity are often derived in the context of Galileos findings. Relationships such as s = (1/2) * g * t^2 and v = g * t for a body starting from rest are direct consequences of uniform acceleration. Historical notes frequently credit Galileo with discovering that the acceleration of falling bodies is constant and independent of mass. This confirms that the phrase law of uniform acceleration of freely falling bodies accurately reflects the content of his law of falling bodies. Why Other Options Are Wrong: Option B, law of inertia of motion, is more closely associated with Newtons first law, which states that a body remains at rest or moves with constant velocity unless acted on by a net external force. It does not emphasise uniform acceleration under gravity. Option C, law of universal gravitation, is Newtons formulation describing the gravitational attraction between masses and is different in form and scope from Galileos simple falling body law. Option D, law of conservation of momentum, relates to interactions and collisions between bodies, not to the uniform acceleration of a single falling object. None of these alternatives captures Galileos specific result about uniform acceleration in free fall. Common Pitfalls: A common mistake is to assume that all classical mechanics laws must be one of Newtons three laws or his law of gravitation, and to ignore earlier contributions. This can lead students to incorrectly associate Galileos work directly with Newtons named laws. Another pitfall is not recognising that the phrase uniform acceleration directly connects to the constant acceleration due to gravity, g, near the Earth surface. Keeping track of which law deals with which physical situation helps you match historical names and descriptive titles correctly in exam questions. Final Answer: The correct choice is Law of uniform acceleration of freely falling bodies, because Galileos law of falling bodies states that all freely falling objects near the Earth surface experience the same constant acceleration, which is uniform acceleration under gravity.

Question 2

Which one of the following is not a unit used for measuring energy in physics?

Accepted Answer

Introduction / Context: In physics and everyday science, energy is measured in specific units that quantify the capacity to do work or produce heat. Recognising which quantities are units of energy and which are units of other related concepts such as power, force, or charge is essential for interpreting formulas and practical data. This question presents several commonly used physical units and asks you to identify the one that is not a unit of energy. It tests both your familiarity with standard units and your ability to distinguish energy from power and other quantities. Given Data / Assumptions: • The options include calorie, joule, electron volt, and watt. • We assume the SI system of units and some commonly used non SI units. • Energy is the ability to do work or produce heat. • Power is defined as the rate at which work is done or energy is transferred. Concept / Approach: The joule is the SI unit of energy and work, defined as one newton metre. The calorie is a non SI unit of energy, commonly used in chemistry and nutrition to measure heat and food energy. The electron volt is another energy unit, especially in atomic and nuclear physics, defined as the energy gained by an electron when it is accelerated through a potential difference of one volt. Watt, however, is the SI unit of power, not energy. One watt equals one joule per second and measures how quickly energy is used or produced. Therefore watt is not a unit of energy, but rather a unit describing the rate of energy transfer. Step-by-Step Solution: Step 1: Identify joule as the standard SI unit of energy; it clearly measures energy directly. Step 2: Recall that a calorie is defined as the amount of heat needed to raise the temperature of one gram of water by one degree Celsius, also a measure of energy. Step 3: Recognise that an electron volt is defined as the energy acquired by an electron when accelerated through one volt of potential difference and is widely used in particle and nuclear physics. Step 4: Note that watt is defined as one joule per second, which measures power rather than energy itself. Step 5: Conclude that watt is not a unit of energy, making it the odd one out in the list. Verification / Alternative check: You can verify by considering dimensional analysis. The dimension of energy is mass times length squared divided by time squared. Joules, calories, and electron volts all have these dimensions expressed in different unit choices. Power, on the other hand, has the dimensions of energy per unit time. Watt, which equals joule per second, therefore has dimensions that include an extra time in the denominator compared to energy. Checking physical contexts also helps: electric bills measure energy in kilowatt hours, where kilowatt is power and hours is time, combining to form an energy unit, not kilowatt alone. This confirms that watt by itself is not an energy unit. Why Other Options Are Wrong: Option A, calorie, is widely used as a unit of energy in thermodynamics and food nutrition, even though it is not an SI unit. Option B, joule, is the fundamental SI unit for work and energy. Option C, electron volt, is also a well established energy unit used for atomic scale processes. Because all three directly measure energy, they cannot be the correct answer to a question asking for a unit that is not energy. Only watt does not satisfy this requirement. Common Pitfalls: Learners sometimes confuse power and energy in everyday language, for example saying a device uses many watts of energy, when watts actually measure power. This confusion can cause mistakes when identifying units. It is useful to remember that energy tells you how much work is done, while power tells you how fast it is done. Another pitfall is thinking that unfamiliar units like electron volt are not energy, simply because they are less common than joules or calories. Always check definitions rather than relying only on familiarity. Final Answer: The correct choice is Watt, because watt is the SI unit of power (energy per unit time), whereas calorie, joule, and electron volt are all units of energy.

Question 3

The boiling point of a liquid does not have one fixed universal value but changes mainly as which external condition is varied?

Accepted Answer

Introduction / Context: Boiling point is a key concept in thermodynamics and everyday science. We often hear that water boils at 100 degree Celsius, but this value is not universal. It applies only at a specific pressure, namely one standard atmosphere. In real situations such as high altitude locations or pressure cookers, the temperature at which a liquid boils changes. This question asks you to identify the main external factor that determines how the boiling point of a liquid varies, which is crucial for understanding cooking, industrial processes, and phase diagrams. Given Data / Assumptions: • We are considering the boiling point of a generic liquid. • The options include pressure, temperature of the surroundings, volume of the liquid, and density. • Standard definitions of boiling point and vapour pressure are assumed. • The liquid is heated until its vapour forms bubbles throughout the bulk of the liquid. Concept / Approach: A liquid boils when its vapour pressure becomes equal to the external pressure acting on its surface. Vapour pressure is the pressure exerted by the vapour in equilibrium with the liquid at a given temperature. If the external pressure is higher, the liquid must reach a higher temperature for its vapour pressure to match this value, so the boiling point rises. If the external pressure is lower, as at high altitudes, the vapour pressure equals the external pressure at a lower temperature and the boiling point falls. Volume of liquid and density do not directly determine the boiling point, and the ambient temperature alone is not the fundamental controlling variable; it is the pressure condition that matters most. Step-by-Step Solution: Step 1: Recall the definition of boiling point as the temperature at which vapour pressure of the liquid equals external pressure. Step 2: Recognise that external pressure is usually atmospheric pressure for open containers. Step 3: Imagine moving to a high mountain, where atmospheric pressure is lower; the liquid boils at a lower temperature. Step 4: Imagine using a pressure cooker, where pressure is deliberately increased; the liquid then boils at a higher temperature. Step 5: Note that changing the amount of liquid or its density does not fundamentally change the vapour pressure dependence on temperature. Step 6: Conclude that external pressure is the main factor that makes boiling point vary. Verification / Alternative check: This explanation is supported by phase diagrams, which plot temperature versus pressure for a substance and show a boiling curve where liquid and vapour phases coexist. At each pressure on this curve, there is a corresponding boiling temperature. Practical experience also confirms this: cooking times can be longer at high altitudes because water boils at a lower temperature, and pressure cookers are used to raise boiling temperatures for faster cooking. Thermodynamic relations such as the Clausius Clapeyron equation further quantify how boiling point changes with pressure. All these sources agree that pressure is the primary variable controlling boiling point. Why Other Options Are Wrong: Option B, temperature of the surrounding air, may influence heat transfer rate but does not define the boiling point; the boiling temperature is determined by equality of vapour pressure and external pressure, not by ambient air temperature alone. Option C, volume of the liquid, affects how long it takes to reach boiling but not the temperature at which boiling occurs for a given pressure. Option D, density, changes slightly with temperature and pressure but does not directly set the boiling point. Thus, none of these options correctly captures the fundamental control of boiling point. Common Pitfalls: Students often memorise that water boils at 100 degree Celsius without attaching the crucial phrase at one atmosphere pressure. This can lead to confusion when studying high altitude cooking or industrial boilers. Another pitfall is to think that if you apply more heat, the boiling temperature will keep rising, but at fixed pressure, additional heat simply converts more liquid to vapour once boiling has started. Remembering that boiling point is defined via vapour pressure equalling external pressure helps avoid these conceptual errors. Final Answer: The correct choice is External pressure acting on the liquid surface, because boiling point varies mainly with pressure, and liquids boil when their vapour pressure equals the external pressure imposed on them.

Question 4

When small bubbles of air rise upward through a liquid at rest, which main physical effect is responsible for their upward motion?

Accepted Answer

Introduction / Context: The motion of bubbles in liquids is a familiar sight in boiling water, carbonated drinks, and underwater scenes. From a physics point of view, bubbles provide a clear example of how forces act on bodies immersed in fluids. When a small pocket of air or gas is trapped in a liquid, it tends to rise upward. This question asks you to identify the main physical effect responsible for this upward motion. Understanding the role of buoyant force and density differences is fundamental for topics like flotation, fluid statics, and engineering applications involving gases in liquids. Given Data / Assumptions: • The system consists of small bubbles of air inside a liquid. • The liquid is initially at rest and is not flowing violently. • The bubbles are fully surrounded by the liquid and move upward through it. • We apply Archimedes principle and basic fluid statics concepts. Concept / Approach: According to Archimedes principle, any body or volume of fluid immersed in a fluid experiences an upward buoyant force equal to the weight of the fluid displaced. Air bubbles inside a liquid displace liquid whose density is much greater than that of the air inside the bubble. Consequently, the buoyant force on the bubble is significantly larger than the weight of the air it contains. The net result is an upward force (upthrust) that accelerates the bubble upward. Surface tension, viscous forces, and external air currents can influence the shape and speed of the bubble, but they do not provide the main driving force for its rise; buoyancy due to density difference is the primary cause. Step-by-Step Solution: Step 1: Recognise that the bubble of air occupies a certain volume within the liquid and displaces an equal volume of liquid. Step 2: Apply Archimedes principle, which states that the buoyant force equals the weight of the displaced liquid. Step 3: Compare the weight of the displaced liquid with the weight of the air inside the bubble; the liquid is much denser, so its weight is greater. Step 4: Calculate that the upward buoyant force therefore exceeds the downward weight of the bubble, resulting in a net upward force. Step 5: Conclude that this net upward force causes the bubble to accelerate upwards until other factors such as viscous drag balance the motion at some terminal speed. Step 6: Identify this net upward effect as buoyant force or upthrust due to density differences, which is the main cause of the bubbles rising. Verification / Alternative check: You can verify this explanation by analogy with floating objects. Objects less dense than water, such as wood, float on the surface due to buoyant force exceeding their weight. Similarly, hot air balloons rise in the atmosphere because the hot air inside the balloon is less dense than the surrounding cooler air. In all these cases, density differences lead to buoyant forces that cause upward motion. Surface tension mainly affects the shape of the bubble interface, and viscosity provides resistance to motion rather than an upward push. Air currents above the liquid surface can disturb the surface but cannot directly push bubbles deep inside the liquid upwards. This comparison confirms that buoyancy is the dominant effect. Why Other Options Are Wrong: Option B, surface tension, acts along the interface between air and liquid and tends to minimise surface area, but it does not provide a net upward force that can consistently overcome gravity for the bubble volume. Option C, viscous drag, is actually a resistive force that opposes the motion of the bubble through the liquid rather than causing it to rise. Option D, air currents over the surface, may disturb the surface but do not explain the initial upward motion of bubbles within the liquid. None of these alternatives match the fundamental buoyant effect described by Archimedes principle. Common Pitfalls: Students sometimes focus on more complicated effects like turbulence, surface tension shapes, or bubble bursting and overlook the simple density argument. Another mistake is to think of the bubble as being somehow naturally attracted to the surface without considering forces. Remember that in fluid statics, density differences create pressure differences that translate into buoyant forces. Whenever you see a less dense region inside a more dense fluid, such as gas bubbles in water, you should immediately think of upthrust as the primary reason for upward motion. Final Answer: The correct choice is Buoyant force (upthrust) due to the air having lower density than the liquid, because this density difference produces a net upward force on the bubble that makes it rise through the liquid.

Question 5

In basic electrical engineering, a transformer is a device primarily used to convert which of the following types of electrical supply?

Accepted Answer

Introduction / Context: Transformers are fundamental components in power systems and electrical engineering. They are seen at power stations, on distribution poles, and inside many electronic devices. A transformer operates on the principle of electromagnetic induction and works only with alternating current. The main role of a transformer is to change voltage levels efficiently, either stepping voltage up for transmission or stepping it down for safe use in homes and industries. This question asks you to identify which type of electrical conversion a transformer is primarily designed to perform. Given Data / Assumptions: • The device in question is a transformer with primary and secondary coils wound on a magnetic core. • It is used in conventional AC power systems. • The options include conversions between direct current and alternating current, voltage changes, and energy conversion from mechanical to electrical. • Ideal transformer theory and standard textbook usage are assumed. Concept / Approach: A transformer operates when an alternating current flows through the primary coil, creating a changing magnetic flux in the core. This changing flux induces an alternating voltage in the secondary coil according to Faradays law of electromagnetic induction. The ratio of the number of turns in the coils determines whether the secondary voltage is higher or lower than the primary voltage. Importantly, transformers do not convert DC to AC or AC to DC; they require AC to function at all. They also do not directly convert mechanical energy into electrical energy, which is the job of generators. Therefore, the correct description of a transformer is a device that converts low voltage AC to high voltage AC or high voltage AC to low voltage AC, depending on the winding ratio. Step-by-Step Solution: Step 1: Recall that transformers are used in power transmission to step up voltages for long distance lines and step down voltages near consumers. Step 2: Recognise that these processes involve changing the magnitude of an alternating voltage, not changing AC to DC or DC to AC. Step 3: Understand that a transformer requires a time varying magnetic flux, which is produced by alternating current, to induce voltage in the secondary coil. Step 4: Note that DC, which does not vary with time, would not create the changing magnetic field needed for transformer action. Step 5: Compare the options and identify the one that explicitly states conversion between low voltage AC and high voltage AC, either stepping up or stepping down. Step 6: Select that option as it matches the standard function of a transformer. Verification / Alternative check: If you look at typical ratings on a distribution transformer, you might see values like 11 kV slash 415 V, indicating that the transformer takes 11 kilovolt AC on the primary side and provides 415 volt AC on the secondary side. No change from AC to DC occurs inside the transformer core itself. Devices such as rectifiers or power electronic converters are required to convert AC to DC or DC to AC. Generators, not transformers, are responsible for converting mechanical shaft power into electrical energy. These real world labels and roles confirm that transformers are AC voltage changing devices rather than current type changing or mechanical energy converting devices. Why Other Options Are Wrong: Option A, converting DC to AC, describes the function of an inverter, not a transformer. Option B, low voltage DC to high voltage DC, may be achieved using DC DC converters and switching circuits, but not directly by a simple transformer alone. Option D, converting mechanical energy into electrical energy, is the function of electrical generators or dynamos, not transformers. Because transformers require AC and mainly change its voltage level, these options do not accurately describe their primary purpose. Common Pitfalls: Students sometimes loosely say that a transformer changes electricity from high to low or vice versa without specifying that the current remains alternating. This can lead to confusion between transformers and rectifiers or inverters. Another pitfall is assuming that the large size of power transformers means they generate electricity themselves, when in reality they only transform voltages. Remembering the phrase transformer transforms AC voltage levels but does not create electricity helps keep these roles clear in your mind. Final Answer: The correct choice is Low voltage alternating current into high voltage alternating current (or vice versa), because a transformer is specifically designed to step up or step down AC voltages using electromagnetic induction, without changing AC into DC or converting mechanical energy into electrical energy.

Question 6

In the kinetic theory of gases, transport phenomena such as viscosity, thermal conduction and diffusion occur only in which thermodynamic state of a gas, and are these processes reversible or irreversible?

Accepted Answer

Introduction / Context: Transport phenomena in gases and liquids, such as viscosity, heat conduction and mass diffusion, are central topics in kinetic theory, fluid mechanics and thermodynamics. They describe how momentum, energy and particles move through a medium when gradients exist. For example, viscosity relates to momentum transport between layers of fluid moving at different velocities, while thermal conduction involves heat flow from hotter regions to colder regions. This question asks about the thermodynamic state under which such transport phenomena occur and whether they are reversible or irreversible processes in nature. Given Data / Assumptions: • We are considering a gas exhibiting viscosity, heat conduction and diffusion. • Transport phenomena require gradients of velocity, temperature or concentration. • Equilibrium implies no macroscopic gradients and no net flows. • Irreversibility is associated with entropy production in real processes. Concept / Approach: In thermodynamic equilibrium, all macroscopic properties of a system such as temperature, pressure and composition are uniform in space and remain constant in time. There are no net flows of heat, mass or momentum, and therefore transport phenomena vanish at equilibrium. Transport phenomena arise only when the system is in a non equilibrium state where gradients exist, such as temperature differences or velocity shear. These processes tend to reduce the gradients and drive the system toward equilibrium. They are inherently irreversible because they involve entropy production; heat spontaneously flows from hot to cold, and mixed gases do not spontaneously unmix. Thus, transport phenomena occur only in non equilibrium conditions and are irreversible in the thermodynamic sense. Step-by-Step Solution: Step 1: Recall that in equilibrium, temperature, pressure and composition are uniform, so there are no driving forces for heat flow, diffusion or viscous momentum transfer. Step 2: Recognise that viscosity requires velocity gradients between fluid layers; thermal conduction requires temperature gradients; diffusion requires concentration gradients. Step 3: Understand that these gradients define non equilibrium states because different parts of the system have different properties. Step 4: Note that as transport phenomena act, they smooth out the gradients and move the system toward equilibrium. Step 5: Identify that these processes produce entropy, making them irreversible; they do not spontaneously reverse direction without external work. Step 6: Conclude that the correct description is non equilibrium state with irreversible processes. Verification / Alternative check: Thermodynamics distinguishes between reversible and irreversible processes. Reversible processes are idealised, infinitely slow changes where the system passes through a continuous sequence of equilibrium states and produce no net entropy. Transport phenomena, however, are driven by finite gradients and involve finite rates of heat, mass and momentum transfer. These are textbook examples of irreversible processes that generate entropy. Kinetic theory also shows that collisions between molecules lead to relaxation toward equilibrium distributions, and the associated flows cannot be exactly reversed without detailed control of every microscopic motion. This theoretical context reinforces that transport phenomena occur in non equilibrium regimes and are irreversible. Why Other Options Are Wrong: Option B suggests that transport processes in non equilibrium states are reversible, which contradicts the fact that spontaneous transport increases entropy and cannot simply run backward without external intervention. Option C states that transport occurs in equilibrium states, which is incorrect because in equilibrium all net flows vanish. Option D combines equilibrium with reversible processes, but while reversible processes are indeed related to equilibrium paths, they do not describe actual transport phenomena like viscous flow, conduction and diffusion, which require non equilibrium conditions. Therefore, none of these alternatives correctly describes the nature of transport phenomena in gases. Common Pitfalls: Students sometimes confuse the idea that equations for transport coefficients may be derived assuming local equilibrium with the idea that transport itself happens at equilibrium. Local equilibrium means that small regions behave as if at equilibrium while overall gradients still exist. Another pitfall is to think that if a system eventually reaches equilibrium, the path it took must have been reversible, which is not true. Remember that real heat conduction, diffusion and viscous flow are always associated with entropy production and are inherently irreversible. They exist only when the system is out of equilibrium. Final Answer: The correct choice is Non equilibrium state, irreversible processes, because transport phenomena like viscosity, heat conduction and diffusion occur only when gradients are present in a non equilibrium gas, and they are intrinsically irreversible processes that generate entropy as the system relaxes toward equilibrium.

Question 7

In electrical measurements, a sensitive galvanometer can be converted into a voltmeter suitable for measuring potential difference by connecting which type of resistance to it?

Accepted Answer

Introduction / Context: A galvanometer is a very sensitive instrument designed to detect and measure small currents. In practical circuits, however, we often need to measure voltage across components, not just tiny currents. By suitably modifying a galvanometer, it can be used as a voltmeter to measure potential difference across circuit elements. This question asks what kind of additional resistance must be connected, and in which configuration, to convert a galvanometer into a voltmeter correctly. Understanding this conversion is part of basic electrical instrumentation and circuit theory. Given Data / Assumptions: • The starting instrument is a sensitive galvanometer with a known full scale deflection current. • We wish to measure voltage (potential difference) without drawing large current from the circuit. • The options describe high or low resistances connected in series or parallel with the galvanometer. • We assume idealised conditions with linear galvanometer behaviour. Concept / Approach: A voltmeter should have a very high overall resistance so that it draws only a small current from the circuit, ensuring that it does not significantly disturb the voltage being measured. To convert a galvanometer to a voltmeter, we add a high resistance, called a series multiplier, in series with the galvanometer coil. This increases the effective resistance of the instrument so that even a relatively high applied voltage produces only a small current, within the galvanometer safe operating range. Connecting a low resistance or connecting resistances in parallel would change the behaviour in the wrong direction, either overloading the galvanometer or creating a device more like an ammeter or shunt. Step-by-Step Solution: Step 1: Recall that an ideal voltmeter must have very high resistance so that it draws minimal current from the circuit it is connected across. Step 2: Recognise that a galvanometer by itself has relatively low resistance and is sensitive to small currents. Step 3: To measure larger voltages safely, we must increase the total resistance seen by the applied voltage. Step 4: Adding a high resistance in series with the galvanometer increases the total resistance equal to galvanometer resistance plus series resistance. Step 5: For a given voltage, the current through the series combination remains small and within the galvanometer full scale range. Step 6: Conclude that a high series resistance transforms the galvanometer into a voltmeter with appropriate range and minimal circuit loading. Verification / Alternative check: Consider Ohms law, V = I * R. For a given voltage V to produce only a small current I through the instrument, the total resistance R must be large. If the galvanometer alone has resistance Rg and full scale current Ig, then to design a voltmeter of range V, the series resistance Rs must satisfy V = Ig * (Rg + Rs). Since V is large and Ig is small, Rs must be much larger than Rg, that is, a high resistance. Furthermore, placing this resistance in series ensures that the same small current passes through both the galvanometer and the resistor. These simple calculations confirm that the correct configuration is a high resistance in series. Why Other Options Are Wrong: Option A, a high resistance in parallel, would divert much of the current away from the galvanometer and complicate the relation between applied voltage and galvanometer deflection, making it unsuitable as a voltmeter arrangement. Option C, a low resistance in series, would not increase the total resistance sufficiently, allowing too much current and possibly damaging the galvanometer. Option D, a low resistance in parallel, is actually how a galvanometer is converted to an ammeter, not a voltmeter, because it allows most current to bypass the galvanometer through a shunt. These configurations therefore do not produce a proper voltmeter. Common Pitfalls: A frequent confusion is between the modifications needed for ammeters and voltmeters. For an ammeter, a galvanometer is shunted with a low resistance in parallel so that the overall resistance is low and large currents can be measured. For a voltmeter, the aim is the opposite: the instrument must draw very little current, so its resistance must be high. Remembering this contrast helps you quickly choose high series resistance for voltmeters and low parallel resistance for ammeters. Another pitfall is ignoring the effect of the measuring instrument on the circuit, which is crucial when designing accurate measurements. Final Answer: The correct choice is A high resistance in series with the galvanometer, because this configuration greatly increases the instrument resistance and allows it to measure potential difference with minimal current draw, effectively turning the galvanometer into a voltmeter.

Question 8

During gamma decay of an excited atomic nucleus, how do its mass number and atomic number change as a result of emitting a gamma photon?

Accepted Answer

Introduction / Context: Radioactive decay processes are commonly classified as alpha, beta and gamma decays. Each type of decay changes the nucleus in a different way. Alpha decay emits a helium nucleus and changes both mass number and atomic number. Beta decay changes a neutron into a proton or vice versa and alters the atomic number while keeping the mass number the same. Gamma decay, however, involves emission of high energy photons from an excited nucleus. This question asks you specifically how the mass number and atomic number of a nucleus are affected during gamma decay, which is important for interpreting nuclear reaction equations and decay schemes. Given Data / Assumptions: • The nucleus undergoes gamma decay, emitting a gamma photon. • Gamma radiation consists of high energy electromagnetic photons. • No nucleons (protons or neutrons) are emitted in pure gamma decay. • We assume the nucleus transitions from an excited state to a lower energy state of the same nuclide. Concept / Approach: Gamma decay occurs when a nucleus in an excited energy state releases excess energy by emitting a gamma photon. The internal arrangement of nucleons can change from a higher energy configuration to a lower one, but the numbers of protons and neutrons do not change. Since mass number A is the total number of protons plus neutrons, and atomic number Z is the number of protons, both A and Z remain unchanged in pure gamma emission. The process is analogous to an atom emitting a photon when an electron falls from an excited energy level to a lower one, which does not change the number of electrons or the nuclear charge. Therefore, gamma decay is an isomeric transition that alters only the energy of the nucleus, not its mass number or atomic number. Step-by-Step Solution: Step 1: Recall that alpha decay emits a helium nucleus containing two protons and two neutrons, reducing mass number by four and atomic number by two. Step 2: Recall that beta decay converts a neutron to a proton or a proton to a neutron, changing the atomic number by one while keeping mass number constant. Step 3: Recognise that gamma decay instead emits a photon, which is a quantum of electromagnetic energy with no rest mass and no charge. Step 4: Understand that during this emission, the nucleus transitions between energy levels but does not lose or gain any nucleons. Step 5: Conclude that the number of protons remains the same, so the atomic number Z is unchanged. Step 6: Since neither protons nor neutrons are added or removed, the mass number A also remains unchanged. Verification / Alternative check: Nuclear decay schemes often show an excited nucleus, indicated by an asterisk, transforming into the same nuclide without the asterisk plus a gamma photon. For example, cobalt 60 in an excited state decays to cobalt 60 in a lower energy state by emitting gamma rays, but the nuclide remains cobalt 60 with the same mass number and atomic number. This is distinct from alpha and beta changes that create different elements or isotopes. Nuclear reaction equations for gamma decay show identical nuclides on both sides, confirming that A and Z are unchanged. This observational evidence backs up the theoretical expectation based on the nature of photons. Why Other Options Are Wrong: Option A suggests that mass number changes while atomic number does not, which resembles alpha emission but not gamma decay. Option B suggests only atomic number changes, similar to beta decay processes where a neutron converts to a proton or vice versa. Option C suggests that both mass number and atomic number change, which may occur in combined processes but not in pure gamma emission. None of these patterns matches the specific role of gamma radiation, which carries away only energy, not nucleons or charge. Common Pitfalls: Students sometimes assume that all forms of radioactive decay must change the element or isotope, leading them to overlook the special role of gamma decay as an energy adjustment. Another pitfall is confusing the high energy of gamma photons with mass, even though photons have no rest mass and cannot carry away nucleon count. Remember to distinguish between mass number and energy; gamma decay reduces the energy content of the nucleus but not its nucleon count or charge. Keeping these distinctions clear helps in balancing nuclear equations correctly in exams. Final Answer: The correct choice is Neither the mass number nor the atomic number of the nucleus changes, because gamma decay involves emission of a photon that carries away only energy, leaving the number of protons and neutrons in the nucleus unchanged.

Question 9

What is the frequency of an ideal direct current (DC) signal in an electrical circuit?

Accepted Answer

Introduction / Context: In electrical engineering and basic physics, we distinguish between direct current and alternating current. Alternating current varies periodically in magnitude and reverses direction, while direct current maintains a constant value and direction over time. Frequency is defined as the number of complete cycles of a repeating waveform per second and is measured in hertz. This question asks for the frequency of an ideal direct current signal, which is a simple conceptual test of your understanding of what frequency means in relation to time varying signals. Given Data / Assumptions: • Direct current is assumed to be perfectly steady over time, without fluctuations. • Frequency is defined only for signals that repeat in cycles. • We consider ideal DC, not ripple contaminated or switched DC. • Standard power system AC frequencies like 50 Hz and 60 Hz are included among the options. Concept / Approach: Frequency applies to periodic signals, such as sinusoidal alternating currents, square waves or any waveform that repeats a pattern over time. For alternating current in power systems, the waveform completes 50 or 60 cycles per second, leading to frequencies of 50 Hz or 60 Hz. An ideal direct current, by contrast, has a constant magnitude and does not vary with time. Because there is no oscillation, no cycles are completed in a given time interval. This means its frequency is zero hertz. You can think of DC as the zero frequency component of more general signals in Fourier analysis, reinforcing that its frequency is zero rather than any finite value. Step-by-Step Solution: Step 1: Recall that frequency is defined as number of cycles of a repeating waveform per second. Step 2: Understand that alternating currents and voltages have waveforms that change sign and shape periodically, so they have non zero frequencies. Step 3: Note that an ideal direct current is a constant value represented by a straight line when plotted against time. Step 4: Recognise that a constant signal does not complete any cycles because it never changes its value. Step 5: Conclude that the number of cycles per second is zero and therefore the frequency of ideal DC is zero hertz. Step 6: Select zero hertz from the options as the correct answer. Verification / Alternative check: From a mathematical point of view, any time varying signal can be decomposed into sinusoidal components with different frequencies. In such a representation, a constant or DC component corresponds to the term with frequency equal to zero. In phasor analysis and AC circuit theory, DC conditions are often described as the limiting case when frequency tends to zero. Practical DC supplies may have small ripple components due to imperfect rectification, but the idealised definition of direct current ignores these and treats DC as constant. Thus, both physical and mathematical viewpoints agree that DC has zero frequency. Why Other Options Are Wrong: Option B, 50 Hz, is the nominal mains frequency in many countries for AC power networks, not for DC. Option C, 60 Hz, is the mains frequency in some other countries, again associated with AC systems. Option D, 100 Hz, can occur as a ripple frequency in rectified AC circuits but is still an AC component, not pure DC. These non zero frequencies all describe alternating phenomena with repeated cycles; they cannot represent an ideal steady direct current. Common Pitfalls: Students sometimes confuse DC power supplies that are derived from AC mains, which may have ripple at twice the mains frequency, with perfectly steady DC. If they focus on the ripple frequency, they might mistakenly assign a non zero frequency to DC. Another pitfall is to think that because DC circuits can be switched on and off, they must have some frequency. Frequency, however, refers to periodic variation in a continuous signal, not occasional switching. Always remember that an ideal DC line on a time graph is flat, and a flat line has no cycles, so its frequency is zero hertz. Final Answer: The correct choice is Zero hertz, because an ideal direct current does not vary with time and therefore completes no cycles per second, giving it a frequency of zero.

Question 10

In a bipolar junction transistor used in electronics, which region has the least concentration of impurity doping compared with the other two regions?

Accepted Answer

Introduction / Context: Bipolar junction transistors are key components in analog and digital electronics. They consist of three regions, called the emitter, base and collector, with different doping levels and functions. The performance of a transistor in amplification and switching depends strongly on how these regions are doped and constructed. This question focuses on which of these three regions has the lowest impurity concentration in a typical transistor design. Understanding the relative doping levels is important for explaining current flow, carrier injection and transistor gain. Given Data / Assumptions: • We are considering a conventional bipolar junction transistor, either NPN or PNP. • The transistor has three regions: emitter, base and collector. • Each region is doped with impurities to create n type or p type semiconductor material. • Standard transistor design practices are assumed, as found in basic electronics texts. Concept / Approach: In a bipolar junction transistor, the emitter is heavily doped to inject a large number of charge carriers (electrons in an NPN transistor or holes in a PNP transistor) into the base. The base region is made very thin and lightly doped so that only a small fraction of these carriers recombine in the base, and most are swept into the collector. The collector region is moderately doped and physically larger, designed to collect carriers and withstand higher voltages. Because the base must allow efficient carrier transport while maintaining good control, it is intentionally doped less heavily than the emitter and collector. Therefore, among the three regions, the base has the least impurity concentration. Step-by-Step Solution: Step 1: Recall that the emitter provides carriers and should inject them efficiently into the base, so it is heavily doped. Step 2: Recognise that the base acts as a control region and should be thin and lightly doped, allowing most carriers from the emitter to pass through without recombining. Step 3: Understand that the collector region is designed to collect carriers and handle voltage; it is doped moderately, usually less than the emitter but more than the base. Step 4: Compare the doping levels: emitter has the highest, collector is intermediate, base has the lowest. Step 5: Identify from the options which region is described as having the least impurity concentration. Step 6: Choose the base region as the least doped part of the transistor. Verification / Alternative check: Textbooks on semiconductor devices describe the typical doping profile of bipolar junction transistors as emitter heavily doped, base lightly doped, collector moderately doped. Circuit designers often emphasise that the thin, lightly doped base region is essential for high current gain, because most carriers injected from the emitter should reach the collector instead of recombining in the base. Device cross section diagrams show a narrow base sandwiched between emitter and collector with a lower doping concentration. These consistent design details across many sources confirm that the base has the least impurity concentration. Why Other Options Are Wrong: Option A, stating that all three regions have equal impurity concentration, contradicts actual transistor design, which relies on different doping levels to achieve desired behaviour. Option B, suggesting the emitter has the least doping, is incorrect because the emitter must be heavily doped to inject carriers effectively. Option C, claiming the collector has the least doping, is also wrong; while the collector may be less doped than the emitter, it is typically more doped than the base to handle power and voltage requirements. Therefore, only the base region being the least doped matches standard transistor structure. Common Pitfalls: Students sometimes memorise that the emitter is heavily doped but forget to compare all three regions, leading to confusion about the collector and base. Another pitfall is assuming that collector doping must be the lowest because it deals with high voltages; in reality, the collector requires a balance between breakdown voltage and current handling. Remember the pattern emitter heavily doped, base lightly doped, collector moderately doped to quickly answer similar questions about transistor structure and doping profiles. Final Answer: The correct choice is The base has the least concentration of impurity, because in a bipolar junction transistor the base region is intentionally made thin and lightly doped compared with the heavily doped emitter and the moderately doped collector.

Question 11

The pressure exerted by a column of mercury 760 mm high at 0 degree Celsius is commonly defined as which standard unit of pressure?

Accepted Answer

Introduction / Context: Standard reference values for pressure are widely used in physics, chemistry, engineering and meteorology. One classical way of defining a standard pressure is using a mercury barometer, which balances atmospheric pressure with the pressure due to a vertical column of mercury. Historically, a specific height of mercury in a barometer tube has been used to define one atmosphere of pressure. This question asks which named pressure unit corresponds to the pressure exerted by a 760 mm column of mercury at 0 degree Celsius, a value that appears frequently in gas laws and thermodynamics problems. Given Data / Assumptions: • Height of the mercury column is 760 mm (0.76 m). • Temperature of the mercury column is 0 degree Celsius. • The arrangement is a standard mercury barometer used to measure atmospheric pressure. • The options include pascal, atmosphere, bar and poise. Concept / Approach: The classical definition of one atmosphere of pressure is based on the height of a mercury column in a barometer at sea level under standard conditions. One atmosphere is defined as the pressure exerted by a column of mercury 760 mm (or 76 cm) high at 0 degree Celsius under standard gravity. In SI units, this pressure is approximately 1.013 × 10^5 pascals. The pascal is the SI unit for pressure, defined as one newton per square metre. The bar is a metric unit equal to 10^5 pascals, close to one atmosphere but not exactly the same by definition. Poise is a unit of dynamic viscosity, not pressure. Therefore, the name associated directly with 760 mm of mercury is one atmosphere, not pascal, bar or poise. Step-by-Step Solution: Step 1: Recall that pressure due to a fluid column is given by p = rho * g * h. Step 2: Understand that a mercury barometer balances atmospheric pressure with the pressure from a column of mercury. Step 3: Note from standard conditions that at sea level, atmospheric pressure supports a column of mercury approximately 760 mm high at 0 degree Celsius. Step 4: Recognise that this condition has historically been used to define one atmosphere of pressure. Step 5: Compare the given description with the options and identify one atmosphere as the matching named unit. Step 6: Confirm that other units listed serve different roles or approximate values and are not defined directly by 760 mm of mercury. Verification / Alternative check: Looking at gas law problems and tables for standard temperature and pressure, you will often see standard pressure given as 1 atm = 760 mm of mercury = 1.013 × 10^5 pascals. The bar, defined as exactly 10^5 pascals, is numerically close but slightly different from 1 atm. The pascal itself is a very small unit, and many practical pressures are expressed in kilopascals or bars. Poise appears in fluid mechanics as a unit of viscosity, not pressure. These equivalences and definitions confirm that 760 mm of mercury at 0 degree Celsius is associated specifically with one atmosphere of pressure. Why Other Options Are Wrong: Option A, 1 pascal, is far smaller than atmospheric pressure; 1 atm is over one hundred thousand pascals. Option C, 1 bar, is close to one atmosphere but defined independently as exactly 10^5 pascals, not by the 760 mm mercury column. Option D, 1 poise, measures dynamic viscosity in the cgs system and has no direct relation to pressure. None of these descriptions matches the classical definition based on a 760 mm mercury column. Common Pitfalls: Students sometimes mix up atmosphere, bar and pascal because their numerical values are similar. Another common error is to assume that any unit used for gases must be atmosphere, without checking the precise definition. It is helpful to remember the key identity 1 atm = 760 mm Hg and to keep separate in your mind the SI standard unit pascal and the convenient near equivalent bar. Distinguishing these clearly prevents unit conversion mistakes in gas law calculations and fluid mechanics problems. Final Answer: The correct choice is 1 atmosphere, because by definition one atmosphere of pressure is the pressure exerted by a 760 mm column of mercury at 0 degree Celsius under standard gravity.

Question 12

Which one of the following physical quantities has the same dimensions as Planck constant h in dimensional analysis?

Accepted Answer

Introduction / Context: Planck constant h is a fundamental constant in quantum mechanics. It appears in relations such as E = h * f for photons and p = h / lambda for de Broglie waves. Understanding the dimensions of h helps in interpreting these equations and recognising which physical quantities are of the same dimensional type. The question asks you to identify which listed quantity has the same dimensions as Planck constant. This is a classic dimensional analysis problem that reinforces the connection between quantum constants and mechanical quantities like action and angular momentum. Given Data / Assumptions: • Planck constant h has known dimensions that can be derived from photon energy formulas. • The options include linear momentum, angular momentum, force and potential energy. • We use base dimensions mass, length and time. • We assume standard definitions of each mechanical quantity listed. Concept / Approach: From the photon formula E = h * f, where E is energy and f is frequency, we can determine the dimensions of h. Energy has dimensions of mass times length squared divided by time squared, and frequency has dimensions of one over time. Therefore h has dimensions of energy divided by frequency, which simplifies to mass times length squared divided by time. Angular momentum also has dimensions mass times length squared divided by time, since it is defined as moment of momentum (position vector cross linear momentum). Linear momentum, force and potential energy each have different dimensions: momentum is mass times length divided by time, force is mass times length divided by time squared, and energy is mass times length squared divided by time squared. Hence, angular momentum is the quantity whose dimensions match those of Planck constant. Step-by-Step Solution: Step 1: Start from the relation E = h * f and write the dimensions of energy E as M * L^2 * T^-2. Step 2: Write the dimensions of frequency f as T^-1. Step 3: Solve for the dimensions of h: [h] = [E] / [f] = (M * L^2 * T^-2) / (T^-1) = M * L^2 * T^-1. Step 4: Compare this with the dimensions of angular momentum L, which is r cross p, so dimensions are (L) * (M * L * T^-1) = M * L^2 * T^-1. Step 5: Note that linear momentum p has dimensions M * L * T^-1, which do not match. Step 6: Note that force and potential energy both involve T^-2, not T^-1, so they do not match either. Verification / Alternative check: An alternative way is to observe that in quantum mechanics, Planck constant h and its reduced form h slash 2 pi often appear multiplying phase angles, and are associated with action, which is the time integral of the Lagrangian. Action has the same dimensions as angular momentum (mass times length squared divided by time). This connection is why h is sometimes described as a quantum of action. In atomic physics, orbital angular momentum of electrons is quantised in units of h slash 2 pi, again emphasising that h and angular momentum share the same dimensional nature. These theoretical considerations support the dimensional analysis result. Why Other Options Are Wrong: Option A, linear momentum, has one less factor of length in its dimensions compared with h; it is M * L * T^-1, not M * L^2 * T^-1. Option C, force, has dimensions M * L * T^-2, with an extra factor of T^-1 compared with h. Option D, potential energy, shares the same dimensions as energy, M * L^2 * T^-2, again differing by an extra T^-1 factor. These differences show that momentum, force and energy are not dimensionally equivalent to Planck constant, which instead matches angular momentum and action. Common Pitfalls: A frequent mistake is to assume that because h appears in energy and momentum formulas, it must have the same dimensions as energy or momentum. Without doing explicit dimensional analysis, this intuition can easily mislead. Another pitfall is to treat h as dimensionless because it often appears in exponentials or phase factors; in fact, expressions such as S / h inside exponentials are dimensionless because S and h both share the same dimensions. Always write down base dimensions systematically when comparing quantities to avoid such confusion. Final Answer: The correct choice is Angular momentum, because Planck constant h has dimensions M * L^2 * T^-1, which are the same as those of angular momentum and action, not those of linear momentum, force or potential energy.

Question 13

Which one of the following statements about sound waves and human hearing is not correct?

Accepted Answer

Introduction / Context: The physics of sound involves not only the behaviour of sound waves in media but also how the human ear and brain perceive those waves. Several important facts are commonly taught at school level: the ear responds roughly logarithmically to intensity, superposition applies to waves, and longitudinal sound waves can travel through various media. This question presents four statements about sound and hearing and asks you to identify which one is not correct. It tests your knowledge of psychoacoustics, wave superposition and propagation in different media. Given Data / Assumptions: • Statement A describes a logarithmic response of hearing to sound intensity. • Statement B claims ear sensitivity does not vary with frequency. • Statement C states the principle of superposition for waves. • Statement D concerns longitudinal waves in solids, liquids and gases. • We assume normal human hearing and standard wave theory. Concept / Approach: The loudness of a sound as perceived by the ear does not increase linearly with physical intensity; instead, it follows an approximately logarithmic relation. This is reflected in the decibel scale, which is logarithmic in intensity. The ear is also not equally sensitive to all frequencies. It is most sensitive to mid range frequencies, roughly around 2 kHz to 4 kHz, and much less sensitive to very low or very high frequencies. Therefore, a statement claiming that ear sensitivity does not vary with frequency must be incorrect. The principle of superposition is a fundamental rule for waves in linear media: the net displacement is simply the algebraic sum of individual displacements. Finally, longitudinal sound waves can propagate through solids, liquids and gases because in all these media particles can oscillate back and forth along the direction of wave travel, transmitting compressions and rarefactions. Step-by-Step Solution: Step 1: Evaluate statement A. It says ear response to increasing intensity is approximately logarithmic. This matches the decibel loudness scale and is correct. Step 2: Evaluate statement B. It claims that ear sensitivity does not vary with frequency, suggesting equal sensitivity at all frequencies. Step 3: Recall that audiograms of human hearing show much better sensitivity in the mid frequency range and reduced sensitivity at very low and very high frequencies. Step 4: Conclude that statement B is false, because sensitivity is highly frequency dependent. Step 5: Evaluate statement C, which restates the principle of superposition for waves in a linear medium; this is a standard and correct result of wave theory. Step 6: Evaluate statement D, noting that sound waves can indeed travel in solids, liquids and gases as longitudinal waves, making this statement correct. Verification / Alternative check: You can verify statement B by considering everyday experience and scientific hearing tests. Very low frequency sounds, such as below 20 hertz, are difficult or impossible to hear, and high frequency sounds above about 20 kHz are also inaudible. Even within the audible range, equal loudness contours show that the ear needs higher intensity at low and high frequencies to perceive the same loudness as at mid frequencies. This clearly demonstrates frequency dependent sensitivity. On the other hand, the use of logarithmic decibel scales for measuring sound levels supports statement A. Wave superposition is confirmed in phenomena such as interference and beats, validating statement C. Observations of sound propagation in solids as longitudinal waves, such as in ultrasound imaging of materials, confirm statement D. Why Other Options Are Wrong: Option A is correct because the ear compresses a wide range of intensities into a manageable range of perceived loudness, approximately logarithmic in intensity. Option C correctly expresses the superposition principle, which is fundamental to the analysis of wave interference and standing waves. Option D accurately states that longitudinal waves can propagate through solids, liquids and gases, since particles in all these media can oscillate along the direction of travel. These statements are consistent with established physical theory and experiment, so they cannot be the incorrect one asked for in the question. Common Pitfalls: Learners sometimes assume that because the ear can detect a wide range of frequencies, it must be equally sensitive to all of them. This is not true; the threshold of hearing and perceived loudness vary significantly with frequency. Another pitfall is misinterpreting the decibel scale as linear, when in fact it is logarithmic and reflects the logarithmic nature of hearing. Confusion about wave propagation in different media can also lead to doubts about statement D, but remember that longitudinal waves only require that particles can be compressed and rarefied along the direction of motion, which is possible in solids, liquids and gases. Keeping these distinctions clear helps correctly identify the false statement. Final Answer: The correct choice is The sensitivity of the human ear does not vary with the frequency of the sound., because in reality ear sensitivity depends strongly on frequency, and this statement contradicts both experimental data and everyday experience.

Question 14

In geometrical optics, during total internal reflection, light travels from which type of medium to which other type, and what happens to its intensity in the ideal case?

Accepted Answer

Introduction / Context: Total internal reflection is a striking optical phenomenon that explains how light can be trapped inside glass fibres, raindrops and gemstones. It occurs when a light ray tries to pass from a medium with higher refractive index to a medium with lower refractive index at an angle of incidence greater than a certain critical angle. Instead of refracting into the second medium, the light is completely reflected back into the first medium. This question asks from which type of medium to which other type light must travel for total internal reflection to occur, and what happens to the intensity of the reflected light in the ideal case. Given Data / Assumptions: • We are dealing with the interface between two transparent media. • One medium is optically denser (higher refractive index) and the other is optically rarer (lower refractive index). • The angle of incidence in the denser medium exceeds the critical angle. • Idealised conditions ignore absorption and scattering losses. Concept / Approach: Total internal reflection has two necessary conditions. First, light must travel from an optically denser medium to an optically rarer medium, for example from glass to air or from water to air. Second, the angle of incidence in the denser medium must be greater than the critical angle, which is defined as the angle of incidence for which the angle of refraction in the rarer medium becomes 90 degrees. Under these conditions, Snell law indicates that no refracted ray exists and all of the incident energy is reflected back into the denser medium. In the ideal limit, the reflection is total and there is no loss of intensity on reflection, unlike ordinary reflection where some energy is transmitted or absorbed. Step-by-Step Solution: Step 1: Identify that total internal reflection can happen only when the light tries to go from a medium of higher refractive index to one of lower refractive index. Step 2: Recognise that this means light must travel from a denser medium to a rarer medium, such as glass to air or water to air. Step 3: Recall that at the critical angle, the refracted ray would travel along the interface. Step 4: Note that for angles of incidence greater than the critical angle, Snell law has no real refraction solution, and instead the wave is completely reflected back into the denser medium. Step 5: Understand that in the ideal case, all of the incident light energy is reflected, so the intensity of the reflected ray equals the intensity of the incident ray. Step 6: Conclude that the correct description is denser to rarer medium with no loss of intensity in the idealised situation. Verification / Alternative check: Practical examples support this description. Optical fibres rely on total internal reflection of light inside a core with higher refractive index surrounded by cladding of lower refractive index; light is guided along the fibre with very low loss over long distances. Similarly, sparkling effects in diamonds arise because light is trapped by repeated total internal reflections inside the denser diamond and only escapes after multiple reflections. In theoretical treatments, reflectivity at angles above the critical angle is taken as 100 percent for ideal, non absorbing media. These examples confirm that the direction is from denser to rarer and that intensity is preserved in the ideal case. Why Other Options Are Wrong: Option A and option C describe light travelling from a rarer to a denser medium, which is the ordinary refraction scenario where total internal reflection cannot occur. In such cases, light bends toward the normal and part of it may be reflected, but not totally. Option C and option D mention loss of intensity, which applies to partial reflection and transmission, not to ideal total internal reflection where reflectivity is unity. Real materials do have slight absorption, but the phenomenon is defined in terms of complete reflection of the wave. Therefore, only the option describing travel from denser to rarer medium with no loss of intensity matches the textbook definition. Common Pitfalls: A common confusion is to think of total internal reflection as just a stronger version of normal reflection at any interface, regardless of direction. Students may forget that the direction from optically denser to optically rarer medium is essential. Another pitfall is to assume that some fractional transmission must always occur; in the special case of total internal reflection, the transmitted wave is replaced by an evanescent field that does not carry energy far into the rarer medium. Remembering the two conditions denser to rarer and angle greater than critical makes it easier to identify when total internal reflection can happen and how it behaves. Final Answer: The correct choice is From a denser to a rarer medium and it occurs with no loss of intensity, because total internal reflection only occurs for light travelling from higher to lower refractive index at angles greater than the critical angle, and in the ideal case all the incident light energy is reflected back into the denser medium.

Question 15

For a photon whose momentum is 10 MeV/c, where c is the speed of light, what is the energy of this photon expressed in MeV?

Accepted Answer

Introduction / Context: In relativistic physics and quantum mechanics, photons are massless particles of light that carry energy and momentum. For photons, energy and momentum are related by a simple formula involving the speed of light. While massive particles obey the full relativistic energy momentum relation, photons satisfy E = p * c, where E is energy, p is momentum and c is the speed of light. This question gives the momentum of a photon in convenient units and asks you to find its energy in mega electron volts, which is a common energy unit in nuclear and particle physics. Given Data / Assumptions: • The particle is a photon, so its rest mass is zero. • Photon momentum p is given as 10 MeV/c. • c represents the speed of light in vacuum. • We are asked to express energy E in units of MeV. Concept / Approach: For photons, the energy momentum relation simplifies to E = p * c because the rest mass term in the general relativistic formula is zero. If momentum p is expressed in units of MeV/c, multiplying by c simply cancels c in the denominator, leaving energy in MeV. This is why high energy physics often quotes momentum in MeV/c or GeV/c, making conversion to energy straightforward for massless particles. Therefore, to find the photon energy, we substitute p = 10 MeV/c into E = p * c, giving E = (10 MeV/c) * c = 10 MeV. Step-by-Step Solution: Step 1: Write down the photon energy momentum relation: E = p * c. Step 2: Note that the given momentum is p = 10 MeV/c, already expressed in mixed units convenient for high energy physics. Step 3: Substitute p into the formula: E = (10 MeV/c) * c. Step 4: Recognise that the factor c in the numerator cancels the c in the denominator. Step 5: After cancellation, we obtain E = 10 MeV. Step 6: Conclude that the photon energy is 10 MeV, matching option A. Verification / Alternative check: If you prefer to think in SI units, you could convert 10 MeV to joules and compute p in kilogram metre per second, but this is unnecessary because the question uses natural high energy units. The key point is that MeV/c multiplied by c yields MeV. In many particle physics problems, energy and momentum are treated in these units specifically to simplify such calculations. Additionally, the general relativistic relation E^2 = p^2 * c^2 + m^2 * c^4 reduces to E = p * c when m = 0 for photons, confirming the starting formula. Both the short and long methods lead to the same result of 10 MeV. Why Other Options Are Wrong: Option B, 100 MeV, corresponds to mistakenly multiplying by an extra factor of ten, perhaps by misreading the given units or squaring c unnecessarily. Option C, 1 MeV, would result from dividing by ten instead of simply cancelling c. Option D, 0.1 MeV, represents an even larger error. None of these alternatives correctly apply the simple relation E = p * c when p is given in MeV/c. The units themselves signal that multiplication by c only cancels the denominator and does not introduce any additional numerical factor. Common Pitfalls: A common mistake is to overcomplicate the problem by trying to use the full relativistic energy equation, including rest mass, without noticing that photons have zero rest mass. Another pitfall is misunderstanding the meaning of MeV/c units and treating c as a simple number like 3 × 10^8 without respecting unit cancellation. Remember that in high energy physics, using MeV, MeV/c and MeV/c^2 as units is designed to simplify calculations. When you see p in MeV/c for a photon, multiplying by c immediately gives you E in MeV with no extra numerical factor. Final Answer: The correct choice is 10 MeV, because for a photon with momentum 10 MeV/c, the energy is E = p * c = (10 MeV/c) * c = 10 MeV.

Question 16

Which of the following magnetic properties is never exhibited by liquids and gases under ordinary laboratory conditions?

Accepted Answer

Introduction / Context: Materials can respond to magnetic fields in several different ways depending on their atomic and electronic structure. Three commonly discussed magnetic behaviours are diamagnetism, paramagnetism and ferromagnetism. Many solids and some fluids exhibit diamagnetic or paramagnetic responses, and ferromagnetism is seen in familiar materials like iron, cobalt and nickel. This question focuses on which of these behaviours is never observed in liquids and gases under ordinary laboratory conditions, highlighting the special nature of ferromagnetism compared with other magnetic responses. Given Data / Assumptions: • We are considering liquids and gases, not solid materials. • The options list diamagnetic, paramagnetic, ferromagnetic and electromagnetic properties. • Ordinary laboratory conditions mean room temperature and pressure without extreme fields. • We use standard definitions of magnetic behaviour. Concept / Approach: Diamagnetism is a universal but usually weak property of all materials, including liquids and gases, arising from induced currents that oppose applied magnetic fields. Paramagnetism occurs when materials contain unpaired electrons whose magnetic moments tend to align weakly with an external field; many ionic solutions and gases of paramagnetic molecules show this behaviour. Ferromagnetism, however, requires strong cooperative alignment of atomic magnetic moments in domains and is associated with long range ordering in crystalline solids. This kind of domain structure is not supported in normal liquids and gases because the thermal motion and lack of fixed lattice positions prevent stable domain formation. Therefore, liquids and gases can and do exhibit diamagnetic and paramagnetic behaviour, but they do not show ferromagnetism under usual conditions. Step-by-Step Solution: Step 1: Recall that diamagnetism arises from induced currents that weakly oppose an applied field and is present in all materials to some extent, including fluids. Step 2: Recognise that paramagnetism occurs in materials with unpaired electrons and can be observed in some gases and liquid solutions of paramagnetic ions. Step 3: Understand that ferromagnetism involves strong interactions between neighbouring atomic moments leading to spontaneous alignment in domains, which requires a solid crystalline lattice. Step 4: Note that in liquids and gases, atoms and molecules move freely, and the structural stability needed for ferromagnetic domain formation is absent. Step 5: Conclude that ferromagnetic order cannot be maintained in liquids and gases under ordinary conditions. Step 6: Choose ferromagnetic behaviour as the magnetic property never exhibited by liquids and gases in usual laboratory environments. Verification / Alternative check: Experimental evidence supports this reasoning. Susceptibility measurements show that many gases, such as oxygen, exhibit paramagnetism, while others like nitrogen show diamagnetism. A number of liquid solutions display dia or paramagnetic responses depending on their solute ions. However, ferromagnetism is observed in solid phases like iron, cobalt, nickel and certain alloys, which have well defined crystal structures. When such solids are melted into liquids, their ferromagnetic ordering disappears, and they behave as paramagnets or diamagnets depending on temperature. High temperature destroys ferromagnetic order even in solids, showing how sensitive it is to structural order and thermal agitation. These facts confirm that ferromagnetism is not present in fluids under ordinary conditions. Why Other Options Are Wrong: Option A, diamagnetic behaviour, can occur in liquids and gases because all materials exhibit diamagnetism to some degree. Option B, paramagnetic behaviour, is also exhibited by certain gases like oxygen and by many solutions of paramagnetic salts. Option D, electromagnetic behaviour, is a vague term but generally refers to interaction with electric and magnetic fields, which liquids and gases certainly can have, for example in plasmas or conducting fluids. The only specified behaviour that truly cannot occur in ordinary fluid phases is ferromagnetism, which requires solid like order. Common Pitfalls: Students sometimes think that because ferromagnetism is the most familiar form of magnetism, it must occur in any material with unpaired electrons, including liquids and gases. They may not appreciate how crucial fixed lattice positions and exchange interactions are for sustaining ferromagnetic domains. Another pitfall is to overlook the subtle but measurable dia and paramagnetic responses in fluids because these effects are weaker and less obvious than solid ferromagnetism. Remembering that ferromagnetism is tied to solid crystal structure helps you quickly eliminate it as a property of liquids and gases in normal conditions. Final Answer: The correct choice is Ferromagnetic behaviour, because liquids and gases can show diamagnetic or paramagnetic responses but do not exhibit ferromagnetism under ordinary laboratory conditions due to the lack of stable domain structures and strong long range ordering.

Question 17

The statement that light travels in straight lines (rectilinear propagation of light) holds true only when the medium through which light travels has what nature?

Accepted Answer

Introduction / Context: One of the basic laws taught in geometrical optics is the rectilinear propagation of light, which states that light travels in straight lines. This principle is used in explaining the formation of shadows, pinhole images and the apparent straight line paths of light rays in simple media. However, this statement is an approximation that holds only under certain conditions. When the optical properties of the medium vary in space, light rays can bend gradually rather than travel in straight lines. This question asks you to identify the conditions on the medium for which the statement that light travels in straight lines remains valid. Given Data / Assumptions: • The medium refers to the material through which light propagates. • The options describe mediums with constant (homogeneous) or varying (inhomogeneous) refractive indices. • Transparency is required for light to propagate without strong scattering or absorption. • We focus on macroscopic ray paths rather than microscopic scattering events. Concept / Approach: Rectilinear propagation of light is strictly valid in a homogeneous and isotropic medium that is transparent, meaning its refractive index is the same everywhere and in all directions, and it does not scatter light significantly. In such a medium, Snell law applied locally tells us that light rays go straight until they encounter a boundary with a different refractive index. In inhomogeneous media where the refractive index changes with position, such as in the atmosphere with temperature gradients, rays can bend continuously, producing mirages and other effects. In layered media made of slabs, rays change direction at each interface. Thus, the simple statement that light travels in straight lines holds only for homogeneous, transparent media. Step-by-Step Solution: Step 1: Recall that a homogeneous medium has the same refractive index at every point. Step 2: Understand that in such a medium, there are no internal boundaries or gradients to bend light rays. Step 3: Recognise that transparency means light can travel through without severe scattering or absorption, preserving well defined ray paths. Step 4: Note that in inhomogeneous media where refractive index varies, such as hot air near a road, light rays refract gradually and curve rather than remaining straight. Step 5: Note that in layered slabs of different refractive indices, rays travel straight within each layer but bend at interfaces, so overall paths are kinked, not strictly straight. Step 6: Conclude that the rectilinear propagation law strictly holds only in a homogeneous and transparent medium. Verification / Alternative check: Everyday examples support this interpretation. In calm, clear air, shadows of objects have sharp edges, consistent with straight line travel of light. In contrast, near hot surfaces such as asphalt roads on sunny days, shimmering and distorted images arise because temperature gradients cause gradual refractive index changes that bend light rays, violating simple straight line propagation. Optical fibres deliberately use refractive index gradients or core claddings to bend light paths. These phenomena show that when the medium is not homogeneous, light does not follow straight lines in the simple sense assumed in geometrical optics. Therefore, the requirement for homogeneity and transparency is essential for rectilinear propagation to hold as an approximation. Why Other Options Are Wrong: Option A, a medium of variable refractive index, explicitly contradicts the condition needed for straight line travel; such variation causes rays to curve. Option B, slabs of different refractive indices, leads to multiple refractions at interfaces, producing broken or zigzag paths rather than a single straight path. Option D, an inhomogeneous but transparent medium, again allows light to propagate but with curved or distorted rays due to spatial changes in index. These media do not satisfy the strict condition that light travels in straight lines throughout the region, so they are not consistent with the simple rectilinear propagation statement. Common Pitfalls: Students sometimes overgeneralise the idea of rectilinear propagation and assume that light always travels in straight lines no matter what the medium is like. This can lead to confusion when learning about atmospheric refraction, mirages and graded index fibres. Another pitfall is to ignore the role of transparency and think that any homogeneous medium, even a strongly scattering one, would preserve straight rays, when in practice scattering destroys the simple ray model. Remember that rectilinear propagation is a useful approximation that applies to clear, homogeneous media, and that deviations from these conditions naturally lead to bending or diffusion of light rays. Final Answer: The correct choice is A homogeneous and transparent composition, because light travels in straight lines only in media with uniform refractive index and sufficient transparency, whereas variations in refractive index or layered structures cause rays to bend or change direction.

Question 18

Two solid pieces, one made of gold and the other of silver, are chosen so that when they are completely immersed in water they show equal apparent weights. Given that the density of gold is greater than the density of silver, what can be said about …

Accepted Answer

Introduction / Context: This question combines ideas from fluid statics, buoyancy and density. When an object is immersed in a fluid such as water, it appears to weigh less than in air because the fluid exerts an upward buoyant force equal to the weight of the displaced fluid. The apparent weight is the true weight minus the buoyant force. Here, two different materials gold and silver are compared under the condition that their apparent weights in water are equal. Since gold is denser than silver, the volumes required to give the same apparent weight will differ, and this affects their true weights in air. The question asks which piece will weigh more in air, given equal apparent weights in water. Given Data / Assumptions: • One solid piece is made of gold, the other of silver. • When completely immersed in water, both show equal apparent weights. • Density of gold is greater than density of silver. • Water has the same density for both experiments and is at rest. • Shapes of the pieces do not matter; only volumes and densities are relevant. Concept / Approach: The apparent weight W_app of an immersed object is given by W_app = W_air minus B, where W_air is its true weight in air and B is the buoyant force. The buoyant force equals the weight of displaced water, B = rho_water * V * g, where V is the volume of the object. The true weight in air is W_air = rho_object * V * g. The condition of equal apparent weights means that for gold and silver, W_air gold minus B_gold equals W_air silver minus B_silver. Using densities, this becomes (rho_gold minus rho_water) * V_gold * g equals (rho_silver minus rho_water) * V_silver * g. Because rho_gold is greater than rho_silver, the volume of the gold piece must be smaller than the volume of the silver piece to keep the products equal. When we then compute the true weight in air, W_air = rho_object * V * g, the piece with larger volume of less dense material can end up heavier in air. Step-by-Step Solution: Step 1: Write apparent weight expressions: W_app gold = (rho_gold * V_gold * g) minus (rho_water * V_gold * g) and W_app silver = (rho_silver * V_silver * g) minus (rho_water * V_silver * g). Step 2: Factor common terms to get W_app gold = (rho_gold minus rho_water) * V_gold * g and W_app silver = (rho_silver minus rho_water) * V_silver * g. Step 3: Given that apparent weights are equal, set these equal: (rho_gold minus rho_water) * V_gold = (rho_silver minus rho_water) * V_silver. Step 4: Because rho_gold is greater than rho_silver, the factor (rho_gold minus rho_water) is greater than (rho_silver minus rho_water). Step 5: For the products to be equal, V_gold must be smaller than V_silver, since the larger density difference on the gold side is multiplied by a smaller volume. Step 6: Compute true weights in air: W_air gold = rho_gold * V_gold * g and W_air silver = rho_silver * V_silver * g, and compare them using the relationships between densities and volumes. Verification / Alternative check: From the equality condition, V_gold divided by V_silver equals (rho_silver minus rho_water) divided by (rho_gold minus rho_water), which is less than one because rho_gold is greater than rho_silver. Thus V_gold is less than V_silver. Now compare true weights: W_air gold minus W_air silver equals rho_water * g times (V_gold minus V_silver), obtained by manipulating the expressions and the equality of apparent weights. Since V_gold is less than V_silver, V_gold minus V_silver is negative, so W_air gold minus W_air silver is negative. This means W_air silver is greater than W_air gold, so the silver piece weighs more in air, even though both appear equal in water. This algebraic check confirms the qualitative reasoning. Why Other Options Are Wrong: Option A, stating that gold weighs more in air, might seem intuitive because gold is denser and often associated with heaviness, but it neglects the fact that the volumes have been adjusted to equalise apparent weight in water. Option C, that both weigh equal in air, would be true only if their volumes and densities happened to produce equal true weights, which conflicts with the equal apparent weight condition and the density difference. Option D, claiming the weighing depends only on mass and cannot be predicted, ignores the precise relationship between mass, volume, density and buoyant force; with density information given, a definite prediction is possible. Only option B matches the correct physical analysis. Common Pitfalls: A common mistake is to think only in terms of density and forget that equal apparent weights in water imply different volumes and masses. Many students automatically assume that the denser material must always be heavier, even when the volumes are chosen specifically to adjust buoyant effects. Another pitfall is to ignore buoyant force when comparing immersed weights, treating all objects as if they were weighed in air. Remember that apparent weight depends on both object density and displaced fluid weight; when these are carefully balanced, surprising comparisons like a less dense material weighing more in air can arise. Carefully using formulas for buoyancy and apparent weight helps avoid these intuitive traps. Final Answer: The correct choice is The silver piece will weigh more in air, because to have equal apparent weights in water, the less dense silver piece must have a larger volume, and when true weights in air are compared, this larger volume of silver leads to a greater actual weight than the smaller, denser gold piece.

Question 19

In the electromagnetic spectrum, the wavelengths corresponding to ultraviolet, visible and infrared radiations are represented by λ_uv, λ_vis and λ_ir respectively. Which of the following gives the correct increasing order (from shortest to longest)…

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Introduction / Context: The electromagnetic spectrum is arranged according to wavelength or frequency and includes many familiar regions such as ultraviolet (UV), visible and infrared (IR) radiation. Competitive exams frequently test whether you know the relative positions of these regions. This question specifically asks you to identify the correct relationship between the wavelengths of ultraviolet, visible and infrared light, using the symbols λ_uv, λ_vis and λ_ir. Given Data / Assumptions: λ_uv denotes wavelength of ultraviolet radiation. λ_vis denotes wavelength of visible light. λ_ir denotes wavelength of infrared radiation. We assume standard ordering of the electromagnetic spectrum in free space. Concept / Approach: In the electromagnetic spectrum, as we go from gamma rays to radio waves, wavelength increases and frequency decreases. Ultraviolet radiation lies just beyond the violet end of visible light and therefore has shorter wavelengths than visible light. Infrared radiation lies beyond the red end of visible light and therefore has longer wavelengths than visible light. Thus, the correct order of increasing wavelength must place ultraviolet first (shortest), then visible, then infrared (longest). Step-by-Step Solution: Step 1: Recall that UV lies on the higher energy, shorter wavelength side of visible light. Step 2: Recall that IR lies on the lower energy, longer wavelength side of visible light. Step 3: Therefore, by wavelength, UV < visible < infrared. Step 4: Translate this into symbols: λ_uv < λ_vis < λ_ir. Verification / Alternative check: Typical wavelength ranges confirm this ordering. Ultraviolet light has wavelengths roughly from 10 nm to 400 nm, visible light from about 400 nm to 700 nm, and infrared from about 700 nm to 1 mm. These numerical ranges clearly show that ultraviolet has the smallest wavelengths and infrared has the largest, with visible in between, matching λ_uv < λ_vis < λ_ir. Why Other Options Are Wrong: Option A: λ_uv < λ_ir < λ_vis incorrectly places infrared before visible, suggesting that visible has longer wavelength than infrared, which contradicts the spectrum. Option B: λ_uv > λ_vis > λ_ir reverses the true order and claims ultraviolet has the longest wavelength. Option C: λ_uv > λ_ir > λ_vis is also reversed and incorrect in every comparison. Common Pitfalls: A common mistake is to remember the order by frequency rather than wavelength and then forget that higher frequency means shorter wavelength. Another confusion is mixing infrared and ultraviolet simply because both are outside the visible range. To avoid errors, always remember the phrase ultraviolet on the short side, infrared on the long side of visible light. Final Answer: The correct relationship is λ_uv < λ_vis < λ_ir.

Question 20

An electron and a proton, both starting from rest, are accelerated through the same potential difference of 100 kV. Their final speeds are denoted by V_e for the electron and V_p for the proton. Which of the following relations between V_e and V_p i…

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Introduction / Context: Charged particles like electrons and protons gain kinetic energy when they are accelerated through an electric potential difference. This question compares the speeds of an electron and a proton that both start from rest and are accelerated through the same potential difference of 100 kV. The key idea is to relate the gain in kinetic energy to the particle mass and then compare the resulting speeds. Given Data / Assumptions: Electron charge magnitude and proton charge magnitude are equal to e. Both particles start from rest. Both experience the same potential difference V = 100 kV. Relativistic effects are neglected for the purpose of this conceptual question. Concept / Approach: When a charge q moves through a potential difference V, the work done by the electric field on the charge is q * V. This work appears as kinetic energy if the particle starts from rest. Therefore, the kinetic energy gained by each particle is KE = q * V. Since the electron and proton have the same charge magnitude e, they gain the same kinetic energy. The speed v of a particle with mass m and kinetic energy KE is given by KE = (1 / 2) * m * v^2. For the same KE, a lighter particle must have a higher speed than a heavier particle. Step-by-Step Solution: Step 1: For either particle, set q = e and V = 100 kV; then KE = e * V. Step 2: For the electron, KE_e = (1 / 2) * m_e * V_e^2 = e * V. Step 3: For the proton, KE_p = (1 / 2) * m_p * V_p^2 = e * V. Step 4: Since m_p is much greater than m_e, to give the same kinetic energy, V_p must be much smaller than V_e. Thus V_e > V_p. Verification / Alternative check: We can compare the speeds symbolically by equating e * V = (1 / 2) * m * v^2 and solving for v to get v = sqrt(2 * e * V / m). For the electron and proton, the numerator 2 * e * V is the same, but the denominator mass m differs by a factor of about 1836 (the mass ratio of proton to electron). Thus V_e / V_p = sqrt(m_p / m_e) which is much greater than 1, confirming that the electron is much faster. Why Other Options Are Wrong: Option B: V_e < V_p contradicts the dependence v proportional to 1 / sqrt(m). A heavier particle cannot be faster if both receive the same kinetic energy. Option C: V_e = V_p would require the masses to be equal, which is not true for electrons and protons. Option D: The speeds can be compared using basic formulas, so it is not true that they cannot be determined. Common Pitfalls: Students sometimes focus on the fact that q and V are the same and incorrectly assume that the speeds must also be the same. The crucial missing step in that reasoning is the role of mass in the kinetic energy expression. Remember that equal kinetic energies do not imply equal speeds when the masses are different. Lighter particles move faster for the same kinetic energy. Final Answer: The correct relation is V_e > V_p.

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