Question 1

Which one of the following instruments is specifically designed to measure solar radiation (solar irradiance) at the Earth surface?

Accepted Answer

Introduction / Context: Solar radiation is an important parameter in fields such as meteorology, solar energy engineering, and climate science. Measuring how much solar energy reaches a surface helps in designing solar panels, studying weather patterns, and understanding energy balance. This question asks which instrument is used specifically for measuring solar radiation at the Earth surface. Given Data / Assumptions: We are concerned with instruments that measure physical quantities outdoors. The target quantity is solar radiation or solar irradiance. Other instruments listed measure different quantities like current or wind speed. Concept / Approach: A pyranometer is an instrument designed to measure the total hemispherical solar irradiance on a flat surface, usually expressed in watts per square metre. It uses a thermopile sensor and a dome to capture both direct and diffuse sunlight. By contrast, an ammeter measures electric current, a dynamometer can measure mechanical force or power, and an anemometer measures wind speed. Identifying each instrument and its purpose leads directly to the correct answer. Step-by-Step Solution: Step 1: Recall that an ammeter is connected in series in electrical circuits to measure current in amperes. Step 2: Recognise that a dynamometer in mechanical contexts measures force or torque, and in electrical contexts can be part of certain wattmeters. Step 3: Note that an anemometer is used in meteorology to measure wind speed and sometimes direction. Step 4: Identify the pyranometer as the standard instrument used on weather stations and solar farms to measure incoming solar radiation. Verification / Alternative check: Descriptions of solar energy measurement systems always mention pyranometers or related radiometers as the primary sensors. The data from pyranometers are used to calculate solar panel performance, determine solar resource maps, and study climate. No other instrument in the option list has this purpose, making the choice unambiguous. Why Other Options Are Wrong: Ammeter: Measures electric current, not radiation or energy from the Sun directly. Dynamometer: Measures mechanical or electrical power or force, not solar irradiance. Anemometer: Measures wind speed and sometimes wind direction, not solar radiation. Common Pitfalls: Students may confuse the names because many scientific instruments end with meter. It is helpful to associate the prefix pyro or pyra with heat and radiation, which relates to the function of a pyranometer. Another error is to think that an anemometer or another meteorological instrument might also measure radiation, but they are designed for different environmental variables. Final Answer: The instrument used to measure solar radiation at the Earth surface is a pyranometer.

Question 2

In elementary thermodynamics and gas laws, which of the following equations correctly represents Boyle's law for an ideal gas undergoing a change at constant temperature?

Accepted Answer

Introduction / Context: This question checks your understanding of the basic gas laws that together lead to the ideal gas equation. In school level physics and chemistry, you study individual relationships such as Boyle's law, Charles law and Gay Lussac law. Each describes how two state variables (pressure, volume and temperature) are related when the third is held constant. Recognising the correct mathematical form of Boyle's law is important because it is used repeatedly in numerical problems and also forms part of the derivation of the ideal gas law PV = nRT. Given Data / Assumptions: We are dealing with an ideal gas undergoing a change from an initial state (P1, V1, T1) to a final state (P2, V2, T2). Boyle's law applies when temperature is kept constant (isothermal process). The options show different possible relationships between pressure, volume and temperature. We assume the amount of gas (number of moles) remains constant during the process. Concept / Approach: Boyle's law states that for a fixed mass of an ideal gas at constant temperature, the pressure of the gas is inversely proportional to its volume. In symbols, P is proportional to 1 / V when temperature T is constant. This proportionality can be written in equation form as P * V = constant. When the gas changes from one state to another isothermally, this constant remains the same, so we obtain P1V1 = P2V2. The other equations involving temperature belong to different gas laws and do not represent Boyle's law. Step-by-Step Solution: Step 1: Recall the verbal statement of Boyle's law: at constant temperature, pressure is inversely proportional to volume for a fixed mass of gas. Step 2: Express this as P * V = constant when temperature is unchanged. Step 3: For two different isothermal states, write P1V1 = constant and P2V2 = constant. Step 4: Equate the two constants to obtain P1V1 = P2V2, which is the standard equation for Boyle's law. Step 5: Compare this with the answer choices and identify the option that matches exactly. Verification / Alternative check: A quick mental check is to think about what happens when you compress a gas slowly at constant temperature. If the volume is halved (V2 = V1 / 2), the equation P1V1 = P2V2 predicts that P2 = 2P1, meaning pressure doubles. This matches the idea of inverse proportionality from Boyle's law. Any equation involving T1 and T2 cannot represent Boyle's law because temperature must remain constant in an isothermal process. Thus, only P1V1 = P2V2 satisfies both the law and this simple test. Why Other Options Are Wrong: P1T1 = P2T2 suggests a direct relation between pressure and temperature, which is closer to Gay Lussac type behaviour, not Boyle's law. P1 / V1 = P2 / V2 implies pressure is directly proportional to volume, which contradicts the inverse relationship of Boyle's law. T1 / V1 = T2 / V2 mixes temperature and volume and relates to a different type of gas law, not Boyle's law at constant temperature. Common Pitfalls: Students often confuse the separate gas laws because they all involve P, V and T. A simple way to avoid this is to remember one key phrase: Boyle's law connects P and V with T constant, Charles law connects V and T with P constant and Gay Lussac law connects P and T with V constant. Matching these verbal statements to the correct formulas ensures that you do not pick an option just because it looks algebraically neat. Always check which variable is supposed to remain constant and whether the relationship is direct or inverse. Final Answer: The equation that correctly represents Boyle's law for an ideal gas at constant temperature is P1V1 = P2V2.

Question 3

In basic thermodynamics, which of the following substances will have the highest entropy under normal conditions among the options given?

Accepted Answer

Introduction / Context: This question tests your conceptual understanding of entropy, a central idea in thermodynamics. Entropy is often described as a measure of disorder, randomness or the number of possible microscopic arrangements of a system. In simple exam questions, you are usually asked to compare entropy for solids, liquids and gases qualitatively. Recognising that gases generally have higher entropy than liquids and solids at similar temperatures helps you quickly identify the correct option without performing detailed calculations. Given Data / Assumptions: We compare hydrogen gas, diamond (a solid), liquid nitrogen and liquid mercury. All substances are considered under normal laboratory conditions, not at extreme temperatures or pressures. Entropy is being compared qualitatively, not numerically. We assume standard physical states: hydrogen as a gas, nitrogen as a cryogenic liquid, diamond as a solid and mercury as a liquid metal. Concept / Approach: For a given substance or for similar conditions, the general rule is that gases have the highest entropy, liquids have intermediate entropy and solids have the lowest entropy. This is because gas molecules are free to move in a large volume and can occupy many more microscopic configurations than molecules locked in a solid lattice or constrained in a liquid. Among the options, hydrogen is a light diatomic gas at room temperature, whereas diamond is a rigid crystalline solid, and both nitrogen and mercury are liquids. Therefore, based on state alone, hydrogen gas must have the highest entropy. Step-by-Step Solution: Step 1: Classify each substance by physical state: hydrogen is a gas, diamond is a solid, liquid nitrogen is a cryogenic liquid and mercury is a liquid metal. Step 2: Recall the qualitative entropy order: solid < liquid < gas, where gas has the largest entropy for comparable conditions. Step 3: Compare hydrogen with the other options. As a gas, hydrogen molecules can move freely and occupy a much larger volume, giving many more possible microstates. Step 4: Recognise that diamond has a very ordered crystal structure, which corresponds to low entropy. Step 5: Understand that both liquid nitrogen and liquid mercury have higher entropy than solids but still less than gases because the molecules are more constrained than in a gas. Step 6: Conclude that hydrogen gas has the highest entropy among the given options. Verification / Alternative check: If you think in terms of molecular motion, hydrogen molecules move rapidly in all directions and can be present in a large volume, making the system highly disordered. In contrast, atoms in diamond vibrate around fixed positions in a rigid lattice, giving very few configurations. Liquid nitrogen and liquid mercury allow some flow but still maintain short range order. Standard thermodynamic tables also show that molar entropies of gases at standard conditions are significantly larger than those of liquids and solids, which confirms the qualitative reasoning. Why Other Options Are Wrong: Diamond has a very ordered crystalline structure, giving it one of the lowest entropies among the listed substances. Liquid nitrogen does have more entropy than many solids, but as a liquid it still has less entropy than a gas such as hydrogen at similar conditions. Mercury is a liquid metal with mobile atoms but still has fewer accessible microstates than a gas, so its entropy is lower than that of hydrogen gas. Common Pitfalls: Some learners mistakenly think that heavier substances always have higher entropy because they seem more complex. Others may confuse energy content with entropy. The key is to focus on the state of matter and the degree of freedom of the particles. Gases always win in terms of randomness and number of microstates. Remembering the simple ordering solid < liquid < gas is usually enough to answer qualitative entropy questions correctly in competitive exams. Final Answer: Among the given substances, the one with the highest entropy is Hydrogen gas.

Question 4

In colloidal solutions, Brownian motion of the dispersed particles is mainly caused by which of the following microscopic processes?

Accepted Answer

Introduction / Context: This question tests your understanding of Brownian motion in colloids, a classic concept in physics and physical chemistry. Brownian motion refers to the continuous, random, zigzag motion of tiny particles suspended in a fluid. It provides strong evidence for the existence of molecules and their incessant thermal motion. In colloidal systems, Brownian motion contributes to stability by preventing particles from settling under gravity. Knowing the true microscopic cause of this motion helps you connect kinetic theory to observable macroscopic behaviour. Given Data / Assumptions: We are dealing with a colloidal solution having a dispersed phase (colloidal particles) and a dispersion medium (fluid). Brownian motion is the erratic movement of the dispersed particles seen under a microscope. The options mention coagulation, flashes of light and molecular collisions. We assume normal temperatures where molecules have significant thermal energy. Concept / Approach: According to kinetic theory, molecules of a fluid are in constant random motion due to their thermal energy. In a colloid, the dispersed particles are much larger than individual molecules but are still small enough to be strongly affected by collisions. When molecules of the dispersion medium bombard the dispersed particles from all sides in an uneven way, the resulting net force on each particle fluctuates rapidly in magnitude and direction. This produces the observed irregular path known as Brownian motion. Coagulation and light scattering are related phenomena in colloids but do not directly cause this zigzag movement. Step-by-Step Solution: Step 1: Recall that Brownian motion is the random, continuous motion of small suspended particles observed under a microscope. Step 2: Recognise that this motion persists even in the absence of external stirring, so it must come from internal molecular activity. Step 3: According to kinetic theory, molecules of the dispersion medium are constantly moving and colliding due to thermal energy. Step 4: Each colloidal particle is bombarded unevenly by these molecules, causing constant changes in the net force on the particle. Step 5: This irregular bombardment makes the particle perform a zigzag path, which is observed as Brownian motion, confirming that molecular collisions in the dispersion medium are the cause. Verification / Alternative check: If the temperature is increased, molecules of the dispersion medium move faster, and Brownian motion becomes more vigorous. If the viscosity of the medium is increased, the motion slows down, which also matches the idea of molecular bombardment. These trends would not follow naturally from explanations based on coagulation or light flashes. Brownian motion was historically used as strong evidence for molecular theory when direct observation of molecules was not yet possible, further supporting the collision based explanation. Why Other Options Are Wrong: Coagulation of particles is a separate process where colloidal particles aggregate and settle; it does not create the characteristic continuous zigzag motion. Erratic flashes of light are related to scattering or Tyndall effect, where light is scattered by the particles; they are an optical effect and not the mechanical cause of motion. All of the above is incorrect because only molecular collisions of the dispersion medium with dispersed particles directly produce Brownian motion. Common Pitfalls: Some learners confuse Brownian motion with optical phenomena like the Tyndall effect or with instability processes like coagulation. Another misunderstanding is to think that gravity or external vibrations are responsible. To avoid these errors, remember that Brownian motion is fundamentally a kinetic phenomenon caused by molecular impacts. Whenever you see a question mentioning random motion of colloidal particles, link it directly to collisions by the surrounding fluid molecules. Final Answer: Brownian motion in colloids is mainly caused by molecules of the dispersion medium colliding with dispersed particles.

Question 5

Under dry conditions, the electrical resistance of the human body between two contact points is typically of the order of which of the following values?

Accepted Answer

Introduction / Context: This question checks your awareness of electrical safety and the approximate resistance of the human body in dry conditions. Understanding how much current can flow through the body when it comes into contact with live electrical parts is critical for assessing shock hazards. In general, the human body offers relatively high resistance when the skin is dry and intact, but this resistance can drop drastically when the skin is wet or damaged. Competitive exams often ask for an order of magnitude estimate rather than an exact value. Given Data / Assumptions: The human body is assumed to have dry, intact skin at the points of contact. We consider the resistance between two hands or between a hand and a foot in a typical shock scenario. The answer choices range from tens of ohms to hundreds of thousands of ohms. We are interested in the order of magnitude, not a precise measured resistance. Concept / Approach: In electrical safety literature, the resistance of the human body in dry conditions is generally estimated to be tens of thousands to around one hundred thousand ohm or more. The outer layer of skin, particularly when dry, acts as a good insulator. When the skin is wet or broken, resistance can drop significantly, allowing dangerous currents to flow even at low voltages. For dry conditions, values around 100000 ohm are commonly used for simple calculations. Comparing this with the options given, the closest and most appropriate order of magnitude is about 100000 ohm. Step-by-Step Solution: Step 1: Recall that dry human skin offers significant resistance to current flow, typically in the kilo-ohm to hundreds of kilo-ohm range. Step 2: Recognise that a resistance as low as 10 ohm would make the body almost like a metallic conductor, which is unrealistic for dry skin. Step 3: Understand that 1000 ohm or 10000 ohm may be reasonable in wet or compromised conditions but are lower than typical dry skin estimates. Step 4: Note that many standard references use a value around 100000 ohm for dry body resistance in safety calculations. Step 5: Select about 100000 ohm as the best approximate value for dry conditions. Verification / Alternative check: To appreciate the importance of this resistance, consider a domestic supply of 230 volt. If body resistance is taken as 100000 ohm, the current I = V / R is about 2.3 milliampere, which is noticeable but usually not lethal in a brief contact. If the resistance dropped to 1000 ohm, the current would become 230 milliampere, which can be extremely dangerous. This comparison explains why electrical shocks are much more hazardous when the skin is wet and why the dry resistance is taken to be relatively high in safety discussions. Why Other Options Are Wrong: About 10 ohm is far too low and would imply almost metallic conductivity, which does not match the insulating nature of dry skin. About 1000 ohm is closer to typical values for very wet skin or internal body tissues but not for a complete dry body path through the skin. About 10000 ohm is still lower than the commonly quoted dry skin resistance and is more typical of partially wet or compromised conditions. Common Pitfalls: Students sometimes underestimate body resistance because they focus only on dangerous current values and not on the insulating role of skin. Others may confuse wet and dry conditions. For exam purposes, it is helpful to remember that dry body resistance is of the order of 100000 ohm, while wet conditions can reduce resistance by an order of magnitude or more. This distinction underlines the importance of keeping electrical equipment and surroundings dry. Final Answer: Under dry conditions, the electrical resistance of the human body is typically of the order of about 100000 ohm.

Question 6

In a perfect vacuum, all electromagnetic waves share which common property regardless of their frequency or wavelength?

Accepted Answer

Introduction / Context: This question tests your understanding of basic properties of electromagnetic waves in a vacuum. Electromagnetic waves include radio waves, microwaves, infrared, visible light, ultraviolet, X rays and gamma rays. Although they differ greatly in frequency and wavelength, they share some fundamental characteristics. Knowing which property is common to all of them in vacuum is essential for understanding optics, communication technologies and modern physics. Given Data / Assumptions: We are considering electromagnetic waves propagating in a perfect vacuum, not in a material medium like glass or air. Examples include radio waves, light and X rays. The options suggest equal velocity, wavelength, frequency or all properties being identical. We assume standard physics where speed of light in vacuum is a universal constant. Concept / Approach: According to Maxwell theory and experimental evidence, all electromagnetic waves travel through vacuum at the same speed, commonly denoted by c, which is approximately 3 * 10^8 metres per second. What distinguishes one type of electromagnetic radiation from another is its frequency and wavelength. These are related by the equation c = lambda * f, where lambda is wavelength and f is frequency. If c is fixed, increasing the frequency must decrease the wavelength, and vice versa. Therefore, while velocity is common to all electromagnetic waves in vacuum, wavelength and frequency vary widely across the spectrum. Step-by-Step Solution: Step 1: Recall that electromagnetic waves are self propagating oscillations of electric and magnetic fields and do not require a material medium. Step 2: Recognise that in vacuum, all such waves propagate at the universal speed c, approximately 3 * 10^8 metres per second. Step 3: Remember the relation c = lambda * f. Since c is constant in vacuum, lambda and f must vary inversely to each other. Step 4: Compare radio waves and X rays. They obviously have different wavelengths and frequencies but both travel at the same speed in space. Step 5: Conclude that the only common property among all electromagnetic waves in vacuum is their velocity, not wavelength or frequency. Verification / Alternative check: A practical check comes from astronomy and satellite communication. Signals from radio telescopes and visible light from distant stars reach us in times consistent with a single propagation speed. If different kinds of electromagnetic waves had different velocities, pulses containing multiple frequencies would spread out dramatically over astronomical distances, which is not observed in vacuum. This supports the idea that the speed is the same for all electromagnetic waves in empty space, while their wavelengths and frequencies may differ. Why Other Options Are Wrong: They all have the same wavelength is incorrect because the defining difference between radio waves, visible light and X rays is their wavelength. They all have the same frequency is also wrong since frequency varies across the electromagnetic spectrum over many orders of magnitude. All of the above cannot be true because wavelength and frequency are clearly not identical for all electromagnetic waves, even though the speed is. Common Pitfalls: Students sometimes confuse the idea of a common speed with other properties and mistakenly think that properties like wavelength are fixed. Another confusion is between velocity in vacuum and velocity in materials such as glass, where different frequencies may travel at slightly different speeds due to dispersion. For exam purposes, always remember the simple statement: in vacuum, all electromagnetic waves travel with the same velocity c, while wavelength and frequency differ. Final Answer: In a vacuum, all electromagnetic waves have the same velocity.

Question 7

In environmental physics and public health, continuous sound levels become hazardous noise pollution for human beings at sound intensities above approximately what decibel (dB) level?

Accepted Answer

Introduction / Context: This question evaluates your awareness of noise pollution and the decibel scale used to measure sound intensity levels. Noise pollution is not just about loud, momentary sounds; long term exposure to moderately high levels of noise can damage hearing, raise stress and affect overall health. Public health guidelines specify approximate decibel levels beyond which noise is considered harmful, especially for continuous exposure. Knowing these thresholds is useful for both exams and daily life decisions, such as using ear protection in noisy environments. Given Data / Assumptions: We are comparing different continuous sound levels expressed in decibel (dB). We focus on levels where sound becomes hazardous noise pollution for normal human ears. Options range from low environmental levels (30 dB) to very high levels (120 dB). We assume typical guidelines where prolonged exposure is considered. Concept / Approach: The decibel scale is logarithmic, so each increase of 10 dB represents a tenfold increase in sound intensity. Soft conversation may be around 50 to 60 dB, busy traffic can be around 70 to 85 dB, and rock concerts or sirens may reach 110 dB or more. Health agencies often indicate that long term exposure to sound above roughly 80 to 85 dB can lead to hearing damage and is treated as noise pollution. Very short term exposure to extremely loud sounds such as 120 dB is immediately painful, but the standard exam answer for the threshold of hazardous noise in ordinary settings is usually above 80 dB. Step-by-Step Solution: Step 1: Recognise that 30 dB corresponds roughly to a quiet library or soft whisper, which is not hazardous. Step 2: Note that 50 dB is comparable to normal conversation, which again is not typically considered dangerous noise. Step 3: Understand that 80 dB to 90 dB levels correspond to heavy traffic, loud workplace machinery or busy urban environments, where long term exposure can damage hearing. Step 4: Remember that many safety guidelines treat levels above about 80 dB as potentially harmful if exposure is prolonged, marking the onset of hazardous noise pollution. Step 5: Recognise that 120 dB is an extremely loud and painful level, but the exam threshold for when ordinary environmental sound becomes noise pollution is most commonly taken as above 80 dB. Verification / Alternative check: As a simple check, think about common experiences. People generally do not complain about noise pollution in places with 30 or 50 dB sound levels; these are typical for quiet residential areas or offices. Complaints and health concerns rise in places like factories, traffic junctions or loud music venues, where sound levels often exceed 80 dB. Hearing protection rules in workplaces also tend to become strict in this range. Thus, above 80 dB fits both practical experience and textbook guidance. Why Other Options Are Wrong: Above 120 dB describes extremely loud and painful sounds but is much higher than the commonly accepted threshold for hazardous noise pollution in everyday environments. Above 50 dB is too low, since normal speech and many harmless daily activities produce this level of sound. Above 30 dB is also far too low, covering almost all ordinary environments; calling this hazardous would not match scientific or regulatory practice. Common Pitfalls: Students sometimes focus only on the most dramatic number (120 dB) and assume that is the threshold for danger. While such levels are indeed very dangerous, the exam phrase noise pollution usually refers to continuous or frequent exposure levels around 80 dB and above. Another mistake is to ignore that the scale is logarithmic, meaning a change of 30 dB represents a huge difference in intensity. Remembering that normal conversation is around 50 to 60 dB and that harmful environmental noise starts above about 80 dB will guide you to the correct answer. Final Answer: Sound becomes hazardous noise pollution for humans at levels above about 80 dB when exposure is prolonged.

Question 8

The bright, changing colours seen in a very thin soap film or an oil film on water are mainly due to which physical phenomenon of light?

Accepted Answer

Introduction / Context: This question comes from the wave optics part of physics and focuses on the beautiful colours seen in thin films, such as soap bubbles or thin oil layers on water. These colourful patterns cannot be explained by simple refraction alone. Instead, they are classic examples used to illustrate interference of light. Understanding which phenomenon is responsible helps you connect visual everyday experiences with abstract wave concepts like path difference and phase difference. Given Data / Assumptions: We are looking at thin transparent films such as soap films or oil films. The films show bright, iridescent colours that change with viewing angle or film thickness. The options include scattering, dispersion, diffraction and interference of light. We assume white light illumination, which contains multiple wavelengths. Concept / Approach: When light is incident on a thin transparent film, part of the light is reflected from the top surface and part enters the film, reflects from the bottom surface and emerges again. These two reflected rays have travelled paths of slightly different lengths and may also undergo phase changes at the boundaries. Depending on the thickness of the film and the wavelength of light, the two rays can interfere constructively or destructively. For some wavelengths, constructive interference increases brightness; for others, destructive interference reduces intensity. This wavelength dependent interference produces the observed coloured patterns. This is known as thin film interference. Step-by-Step Solution: Step 1: Consider light striking the top surface of a thin film; part of the wave is reflected and part is transmitted into the film. Step 2: The transmitted part reaches the lower boundary, reflects and returns to the top surface, where it emerges and overlaps with the first reflected wave. Step 3: The two reflected waves have a path difference that depends on the thickness of the film, the refractive index and the angle of incidence. Step 4: For certain wavelengths, this path difference leads to constructive interference, producing bright colours; for others, it leads to destructive interference, creating dark regions. Step 5: Since white light contains many wavelengths, the pattern of constructive and destructive interference varies by colour, giving rise to the characteristic iridescent appearance of thin films. Verification / Alternative check: If scattering were the main cause, one would expect a more uniform bluish tint as seen in the sky rather than sharply defined colour bands. Dispersion alone, as in a prism, separates colours but does not create alternating bright and dark regions. Diffraction produces spreading and patterns around apertures or edges rather than in uniform thin layers. The combination of multiple reflections and wavelength dependent constructive and destructive superposition matches precisely the definition of interference, confirming that interference of light is responsible. Why Other Options Are Wrong: Scattering explains phenomena like the blue colour of the sky and the whiteness of clouds but not the regular coloured bands in a thin film. Dispersion is the splitting of light into its component colours when passing through a medium like a prism; it does not alone cause the alternating bright and dark bands characteristic of thin film colours. Diffraction involves the bending and spreading of waves around obstacles or through slits, usually producing fringes near apertures, not the large smooth colour patches seen on soap films. Common Pitfalls: Many students see colours and immediately think of dispersion, forgetting that dispersion by itself does not produce the pattern of bright and dark regions that depends sensitively on thickness. Another mistake is to mix up diffraction and interference because they both involve wave behaviour. A simple memory aid is that thin films and soap bubbles are classic textbook examples of interference, specifically thin film interference. When you see a question about colour in thin films, interference should be your first thought. Final Answer: The colour of a thin film such as a soap film is mainly due to interference of light in the film.

Question 9

According to modern physics, the wave like (de Broglie) properties of electrons are directly used in which of the following practical applications?

Accepted Answer

Introduction / Context: This question checks your understanding of de Broglie wave concept and how wave like behaviour of matter particles, especially electrons, is used in real world devices. Quantum mechanics states that particles such as electrons exhibit both wave and particle characteristics. In particular, the wavelength associated with electrons can be very small, allowing them to act as probes with much finer resolving power than visible light. Knowing which application exploits this property helps you connect abstract theory with concrete technology. Given Data / Assumptions: Electrons are treated as matter waves with de Broglie wavelength. Options mention magnifying objects, lasers and weather reports. We assume ordinary lasers that use electronic transitions, not special electron beam devices. Weather reports typically use satellite and radar data, not electron wave properties. Concept / Approach: The de Broglie wavelength of a particle is given by lambda = h / p, where h is Planck constant and p is momentum. For electrons accelerated through suitable voltages, this wavelength can be comparable to or smaller than interatomic distances. Waves with such short wavelengths can resolve extremely small structures, far beyond the limit of optical microscopes that use visible light. Electron microscopes use focused beams of electrons and exploit their wave nature to form high resolution images. Lasers primarily rely on stimulated emission of photons in atomic or semiconductor systems, not directly on electron de Broglie waves, and weather reporting uses macroscopic electromagnetic waves like radar, not electron matter waves. Step-by-Step Solution: Step 1: Recall that de Broglie proposed that particles such as electrons have an associated wavelength lambda = h / p. Step 2: Recognise that for fast electrons, lambda is very small, allowing the wave to probe structures on atomic and molecular scales. Step 3: Understand that an electron microscope uses a beam of electrons, shaped and focused similarly to how lenses focus light, to form images of very tiny objects. Step 4: Note that the resolving power of a microscope is proportional to 1 / lambda, meaning shorter wavelength gives better resolution, which is the key advantage of electron waves. Step 5: Conclude that magnifying extremely small objects in electron microscopes is the application that directly uses the wave like properties of electrons. Verification / Alternative check: Think about what lasers and weather systems rely on. Lasers are based on stimulated emission and population inversion in atoms or semiconductors, producing coherent photon beams. While electrons are involved in pumping processes, the key wave used is the light wave, not the de Broglie wave of electrons. Weather reports involve use of visible and infrared satellite images and radar signals in the radio or microwave region, again electromagnetic waves, not electron matter waves. Only electron microscopes specifically benefit from electron diffraction and interference, which are direct consequences of their wave nature. Why Other Options Are Wrong: Producing coherent light in ordinary lasers mainly uses photon properties and atomic energy levels; de Broglie wavelength of electrons is not the core operating principle. Weather reports rely on large scale meteorological measurements, satellites and radar, not on the microscopic wave properties of electrons. All of the above is incorrect because only electron microscopy, among the listed options, directly exploits electron wave behaviour for imaging. Common Pitfalls: Students sometimes see the word laser and associate it vaguely with all advanced quantum effects, including de Broglie waves, leading them to choose lasers incorrectly. Another error is to assume that any technology involving electronics must automatically rely on electron wave properties. To avoid confusion, remember that electron microscopes specifically use electron diffraction and interference, while lasers and weather systems are based on electromagnetic waves at much larger scales. Final Answer: The wave like properties of electrons are practically used in magnifying extremely small objects in electron microscopes.

Question 10

In basic mechanics, the rate at which the velocity of a body changes with time is called what physical quantity?

Accepted Answer

Introduction / Context: This question tests a fundamental definition from kinematics, the part of mechanics that deals with motion. Many exam problems involve interpreting motion using position, velocity and acceleration. Each quantity describes a different rate of change. Being able to correctly match verbal descriptions, such as rate of change of velocity, to the corresponding term is essential for solving numerical problems and understanding motion graphs. Given Data / Assumptions: We are asked about the rate at which velocity changes with time. Options include speed, displacement, acceleration and jerk. We assume motion in a straight line, although the definition is general. All quantities are standard terms from introductory physics. Concept / Approach: Velocity is the rate of change of displacement with respect to time. Acceleration, in turn, is the rate of change of velocity with respect to time. Mathematically, acceleration a can be written as a = change in velocity / change in time for average acceleration, or as the time derivative of velocity for instantaneous acceleration. Jerk is a more advanced concept defined as the rate of change of acceleration with time. Speed is the magnitude of velocity, not a rate of change of velocity. Therefore, the correct term for the rate at which velocity changes is acceleration. Step-by-Step Solution: Step 1: Recall that displacement describes change in position, and velocity describes the rate at which displacement changes with time. Step 2: Identify that acceleration is defined as the rate at which velocity changes with time, both in magnitude and direction. Step 3: Recognise that speed is simply the magnitude of velocity and does not indicate how velocity is changing with time. Step 4: Remember that jerk is the rate of change of acceleration with time, one level higher in differentiation than acceleration. Step 5: Based on these definitions, conclude that acceleration is the quantity that matches the given description. Verification / Alternative check: A practical way to verify is to think about a car. When the driver presses the accelerator pedal, the car speed increases; we say the car is accelerating because its velocity is changing with time. If the car moves at a constant speed in a straight line, there is no acceleration, because velocity is not changing. In advanced discussions, sudden changes in acceleration, such as those experienced in a roller coaster, may be described using jerk, but that is not what is being asked here. These everyday examples confirm that the correct term for rate of change of velocity is acceleration. Why Other Options Are Wrong: Speed is the magnitude of velocity and describes how fast an object moves, not how quickly its velocity changes. Displacement is the change in position from one point to another and is not a rate of change of velocity. Jerk is defined as the rate of change of acceleration, not the rate of change of velocity, and therefore does not match the description given. Common Pitfalls: Students sometimes confuse speed with acceleration, especially when they hear phrases like increasing speed. It is important to distinguish between how fast you are moving (speed or velocity) and how quickly that speed is changing (acceleration). Similarly, the term jerk may seem attractive because it is less familiar, but its definition involves acceleration, not velocity. To avoid confusion, remember the chain: displacement, velocity as its time rate of change and acceleration as the time rate of change of velocity. Final Answer: The physical quantity defined as the rate at which velocity changes over time is Acceleration.

Question 11

In pollution control technology, an electrostatic precipitator installed in an industrial plant is primarily used to control which type of environmental pollution?

Accepted Answer

Introduction / Context: This question tests your understanding of a common device used to control industrial pollution: the electrostatic precipitator. Large power plants, cement factories, steel plants and similar industries often release fine particulate matter in their flue gases. If not controlled, these particles cause serious air pollution and health problems. Knowing which type of pollution electrostatic precipitators target helps you link basic physics of electrostatics with real world environmental protection measures. Given Data / Assumptions: An electrostatic precipitator is installed in an industrial setting. It uses electric fields to remove particles from gas streams. The options include water, noise, air pollution and solid waste. We assume a typical flue gas cleaning application in chimneys or exhaust ducts. Concept / Approach: An electrostatic precipitator works by charging fine dust and smoke particles as they pass through a region of strong electric field between high voltage electrodes and collecting plates. The charged particles are attracted to oppositely charged plates and get deposited on them, leaving the cleaned gas to exit the chimney. This process directly targets airborne particulate pollutants such as ash, soot and smoke in exhaust gases and significantly reduces air pollution. It does not treat water, reduce noise or manage solid waste on land, although it indirectly reduces solid particulate emissions into the atmosphere. Step-by-Step Solution: Step 1: Recall that electrostatic precipitators are usually mounted in the exhaust system of power plants and industrial furnaces. Step 2: Recognise that hot flue gases leaving a boiler contain fine solid particles like fly ash and dust. Step 3: Understand that in an electrostatic precipitator, these particles are electrically charged and then attracted to collecting plates, effectively removing them from the gas stream. Step 4: The cleaned gas, with most of the particulates removed, is then released into the atmosphere through the chimney, reducing air pollution. Step 5: Since the device acts on gases and suspended particles, it directly addresses air pollution rather than water, noise or land based solid waste. Verification / Alternative check: To verify, think about what types of pollution the other options refer to. Water pollution control uses treatment plants, filters, sedimentation tanks and chemical treatments, not electrostatic precipitators. Noise pollution is managed by silencers, acoustic barriers and insulation. Solid waste on land is handled by landfills, recycling and incinerators. Electrostatic precipitators specifically appear in diagrams of thermal power plant chimneys and are always mentioned in the context of reducing smoke and dust emissions into the air, confirming that they are air pollution control devices. Why Other Options Are Wrong: Water pollution is not treated by electrostatic precipitators; these devices are designed for gases, not liquids. Noise pollution is unrelated to charged dust removal; electrostatic precipitators do not significantly reduce sound levels. Solid waste on land is handled by different methods such as landfills and recycling facilities, not by passing waste through electric fields in chimneys. Common Pitfalls: Students sometimes confuse the term precipitator with precipitation in water treatment and incorrectly link it to water pollution. Others may not recall the exact positioning of the device in power plant diagrams. To avoid confusion, remember that electrostatic precipitators are located in the path of exhaust gases and rely on electrostatics to pull out suspended particles, which clearly identifies them as instruments for controlling air pollution. Final Answer: An electrostatic precipitator is primarily used to control air pollution by removing particulate matter from exhaust gases.

Question 12

In lighting and photometry, the SI unit called lux (lx) is used to measure which of the following quantities?

Accepted Answer

Introduction / Context: This question tests your understanding of basic photometric units used in lighting engineering and physics. Modern lighting design uses several related quantities: luminous flux, luminous intensity, illuminance and luminance. Each has its own SI unit and physical meaning. Lux is a common unit seen in lighting specifications for rooms, offices and sports grounds. Knowing exactly what lux measures helps you interpret these specifications correctly and answer general knowledge questions. Given Data / Assumptions: The unit mentioned is lux (symbol lx). The options refer to luminous flux, luminous efficiency, intensity of illumination and luminous intensity. We assume standard SI definitions used in photometry. Lighting levels in rooms are often specified in lux. Concept / Approach: Illuminance, or intensity of illumination on a surface, is defined as the luminous flux incident per unit area. Its SI unit is lux, where 1 lux equals 1 lumen per square metre. Luminous flux is the total perceived power of light emitted by a source and is measured in lumen. Luminous intensity describes light power emitted in a particular direction and is measured in candela. Luminous efficiency is a dimensionless ratio relating light output to power input. Therefore, whenever you see lux, you are dealing with illuminance on a surface, not with flux or source intensity alone. Step-by-Step Solution: Step 1: Recall that illuminance E is defined as E = luminous flux / area, and describes how bright a surface appears due to incident light. Step 2: Note that the SI unit of luminous flux is lumen, and that of area is square metre. Step 3: Thus, the SI unit of illuminance is lumen per square metre, given a special name lux (lx). Step 4: Recognise that luminous intensity I refers to light power per unit solid angle and has the SI unit candela (cd), not lux. Step 5: Conclude that lux measures intensity of illumination (illuminance) on a surface and select the corresponding option. Verification / Alternative check: As a quick check, consider typical lighting recommendations: for reading, a desk surface may require around 300 to 500 lux; for detailed technical drawing, even higher values are suggested. These recommendations refer to how brightly the working surface is illuminated, not to the total light emitted by the bulb or to its luminous intensity in a specific direction. Lighting designers calculate illuminance in lux when planning the number and placement of fixtures in a room, which confirms that lux is the unit of illuminance. Why Other Options Are Wrong: Luminous flux is measured in lumen, not in lux; it describes total light output of a source, not the light falling on a surface. Luminous efficiency is a ratio comparing light output to power input and does not use lux as its SI unit. Luminous intensity of a source is measured in candela, which is related but distinct from illuminance measured in lux. Common Pitfalls: Students often confuse luminous intensity and illuminance because both involve brightness concepts. A helpful way to separate them is to remember that candela describes the source itself, while lux describes the effect of that source on a surface. Also, the word efficiency may tempt some to choose luminous efficiency, but lux always refers to illuminance. Keeping these distinctions clear will help you avoid mistakes in exam questions related to photometry. Final Answer: Lux (lx) is the SI unit of intensity of illumination (illuminance) on a surface.

Question 13

Historically, who is generally credited with developing one of the earliest practical thermometers (often called a thermoscope) before the Celsius, Fahrenheit and Kelvin scales?

Accepted Answer

Introduction / Context: This general knowledge question comes from the history of science and focuses on the development of early thermometers. Today we are familiar with temperature scales such as Celsius, Fahrenheit and Kelvin, but these scales were created after the basic instrument itself had already been invented. The earliest practical thermometer like devices, called thermoscopes, appeared in the early seventeenth century. Knowing which scientist first developed such an instrument helps you place later innovations by Celsius and Fahrenheit in the correct historical context. Given Data / Assumptions: The question asks about the earliest thermometer or thermoscope, not about modern standard scales. Options list Celsius, Fahrenheit, Kelvin and Galileo. We assume the historical account where Galileo designed an early air based thermoscope. Later scientists refined the design and introduced calibrated scales. Concept / Approach: Galileo Galilei, the famous Italian scientist, is generally credited with inventing one of the earliest thermoscopes around the beginning of the seventeenth century. His device used the expansion and contraction of air in a glass bulb connected to a tube dipping into water. Changes in air temperature caused the water level in the tube to rise or fall, giving a qualitative indication of temperature changes. Anders Celsius later proposed the Celsius scale, Daniel Fahrenheit introduced the Fahrenheit scale with a mercury thermometer and Lord Kelvin proposed the absolute temperature scale. Therefore, the earliest thermometer like device is associated with Galileo, not with the later scale makers. Step-by-Step Solution: Step 1: Recall that Galileo lived from the late sixteenth to the early seventeenth century and made many contributions to physics and astronomy. Step 2: Remember that he devised an early thermoscope, which was sensitive to temperature changes but not yet calibrated in degrees. Step 3: Understand that Anders Celsius, in the eighteenth century, created a temperature scale based on the freezing and boiling points of water, but did not invent the first thermometer itself. Step 4: Recognise that Daniel Fahrenheit refined the mercury thermometer and introduced the Fahrenheit scale, again later than Galileo. Step 5: Note that Lord Kelvin introduced the absolute temperature scale used in thermodynamics, building on earlier work rather than creating the first instrument. Step 6: Conclude that Galileo Galilei is the correct answer to the question about the earliest thermometer development. Verification / Alternative check: Historical summaries of temperature measurement usually begin with Galileo's thermoscope around 1600, followed by developments by Santorio, Fahrenheit and Celsius. Textbooks describe how the initial devices were uncalibrated and sensitive to both temperature and pressure changes, but they represent the first serious attempts to measure temperature. Only later were sealed liquids like mercury used to create more accurate and repeatable thermometers with standard scales. This timeline confirms that Galileo comes before Celsius, Fahrenheit and Kelvin in the story of thermometer invention. Why Other Options Are Wrong: Anders Celsius is known for proposing the Celsius scale, not for inventing the first thermometer. Daniel Fahrenheit designed a mercury thermometer and the Fahrenheit scale, which are improvements on earlier designs, not the very first instrument. Lord Kelvin introduced the absolute temperature scale important in thermodynamics, but he did so long after practical thermometers already existed. Common Pitfalls: Students often confuse the inventor of the first thermometer with the people who created well known temperature scales. Seeing familiar names like Celsius and Fahrenheit in the options, they may choose them simply because they associate those names with temperature. To avoid this mistake, remember that the idea of measuring temperature with an instrument predates the establishment of standard scales, and Galileo is widely credited with the earliest thermoscope. Final Answer: One of the earliest practical thermometers (thermoscopes) was developed by Galileo Galilei.

Question 14

According to the International System of Units (SI), which of the following is the SI base unit of temperature used in science and engineering?

Accepted Answer

Introduction / Context: This question checks your knowledge of the SI system of units, which is the standard in science and engineering. Temperature can be measured in several scales in everyday life, such as Celsius and Fahrenheit, but only one of them is the official SI base unit. Correctly identifying this unit is important for understanding thermodynamics, heat transfer and many scientific formulae where absolute temperature appears, such as the ideal gas law PV = nRT. Given Data / Assumptions: The options include Kelvin, degree Celsius, degree Fahrenheit and joule. We are asked about the SI base unit of temperature. We assume current SI definitions where base units are clearly listed. Joule is known as the SI unit of energy, not temperature. Concept / Approach: The SI system defines seven base units, including the kelvin for thermodynamic temperature. Kelvin measures absolute temperature starting from absolute zero, the theoretical point where all classical thermal motion ceases. While degree Celsius is widely used for everyday weather and laboratory work, it is derived from the Kelvin scale by a simple offset: T in Celsius equals T in Kelvin minus 273.15. Degree Fahrenheit is used in some countries for weather reporting but is not part of SI. Joule is the SI unit of energy, not temperature. Therefore, the correct SI base unit of temperature is kelvin. Step-by-Step Solution: Step 1: Recall the list of SI base units, which includes the metre, kilogram, second, ampere, kelvin, mole and candela. Step 2: Identify that kelvin is specifically designated as the base unit for thermodynamic temperature. Step 3: Recognise that degree Celsius is a commonly used scale, but it is derived from the Kelvin scale and is not itself the SI base unit. Step 4: Note that degree Fahrenheit is used mainly in some countries for weather and cooking and is not part of the SI system. Step 5: Understand that joule is the SI unit of energy and appears in equations for work and heat, not as a unit of temperature. Step 6: Conclude that the correct answer is kelvin. Verification / Alternative check: If you look at fundamental equations such as the ideal gas law PV = nRT, the temperature T must be in Kelvin for R (the gas constant) to have its standard SI value. Similarly, in thermodynamic formulas involving entropy and free energy, temperature is always considered in Kelvin. Reference charts of SI base units also explicitly list the symbol K as the unit of thermodynamic temperature. These consistent uses confirm that kelvin is the official SI unit of temperature. Why Other Options Are Wrong: Degree Celsius is not an SI base unit; it is derived from the Kelvin scale by a fixed offset, even though it is widely used in daily life. Degree Fahrenheit is a non SI scale associated with older measurement systems and is not used in scientific SI based work. Joule is the SI unit of energy, not temperature, and appears in contexts involving work and heat rather than in temperature measurement directly. Common Pitfalls: Students may mistakenly choose degree Celsius because they are more familiar with it and see it used in many laboratory thermometers. To avoid this, remember that the SI system focuses on absolute physical quantities, and Kelvin is the only base temperature unit. Also recall that when performing calculations involving thermodynamic formulas, you almost always convert temperatures to Kelvin, not to Celsius or Fahrenheit. Final Answer: The SI base unit of temperature is the Kelvin (symbol K).

Question 15

In nuclear reactor terminology, when a reactor is said to have become "critical", what does this condition most accurately mean?

Accepted Answer

Introduction / Context: This question tests basic understanding of nuclear reactor operation and the meaning of the technical term critical. In everyday language, the word critical can sound dangerous, but in nuclear engineering it has a specific, more neutral meaning related to the balance of nuclear reactions. Understanding this distinction is important to correctly interpret news reports and exam questions involving nuclear power and safety. Given Data / Assumptions: The context is a nuclear reactor used for power generation or research. The term critical refers to a particular state of the chain reaction. Options describe malfunction, high radiation, shutdown and controlled energy production. We assume standard reactor physics where neutron population behaviour defines the state. Concept / Approach: In a nuclear reactor, fission of heavy nuclei such as uranium 235 produces neutrons that can cause further fissions, leading to a chain reaction. The reactor state is described as subcritical, critical or supercritical depending on whether the neutron population decreases, remains constant or increases with time. A critical reactor is one in which each fission event, on average, causes exactly one more fission event. This means the neutron population and power output are steady and the reactor is producing controlled energy at a constant level. This is the normal, desired operating condition, not necessarily an emergency state. Step-by-Step Solution: Step 1: Recall that in a subcritical state, the chain reaction dies out because each generation of neutrons produces fewer fissions than the previous one. Step 2: Recognise that in a critical state, the effective multiplication factor is equal to one, so each generation of neutrons produces the same number of fissions as the previous generation. Step 3: Understand that in this condition, reactor power remains steady and the system can be used to produce controlled energy safely. Step 4: Note that supercritical conditions involve an increasing neutron population and rising power, which are used for startup but must be carefully controlled. Step 5: Compare these definitions with the options and identify that the correct description is a stable, self sustaining chain reaction for controlled energy production. Verification / Alternative check: News reports often say that a new reactor has gone critical as a milestone in its commissioning. This does not mean an accident has occurred; it means the reactor has successfully achieved its first self sustaining chain reaction at low power, an important step before full power operation. Safety systems ensure that the reactor remains near critical with fine adjustments using control rods and coolant flow. These real world descriptions match the technical meaning of critical as a normal, desired operating state rather than a dangerous one. Why Other Options Are Wrong: Stopping due to malfunctioning describes a shutdown or trip, not the critical state of the chain reaction. Emitting dangerous radiation refers to an accident or severe malfunction; criticality by itself does not imply excessive radiation beyond design. Being shut down to avoid explosion is also incorrect; shutdown corresponds to moving away from criticality, not achieving it. Common Pitfalls: Because the word critical sounds alarming in everyday language, many students wrongly associate it with danger or near explosion. This misunderstanding leads them to choose options that mention malfunction or shutdown. To avoid this, remember that in reactor physics critical simply means that the chain reaction is self sustaining and stable. It is actually the goal of normal, safe operation, not a sign of immediate danger. Final Answer: A nuclear reactor is said to have become critical when it has achieved a stable, self sustaining chain reaction and is ready to produce controlled energy.

Question 16

In a household cream separator, fat (cream) is separated from milk mainly due to the action of which of the following forces?

Accepted Answer

Introduction / Context: This question relates to mechanics and everyday applications of circular motion. Cream separators are commonly used in dairies and rural homes to separate lighter fat from heavier skim milk. The device spins milk rapidly, causing the components to move differently according to their densities. Understanding which force is effectively responsible for this separation helps you connect rotational dynamics with a familiar household process. Given Data / Assumptions: Milk is a mixture of fat globules and watery skim milk. The cream separator spins milk at high speed in a circular motion. The options mention cohesive, gravitational, centripetal and centrifugal forces. We adopt the common rotating frame description where centrifugal force acts outward on rotating masses. Concept / Approach: When a liquid mixture like milk is rotated rapidly in a separator, the components experience an outward apparent force in the rotating frame known as centrifugal force. Heavier components (higher density) tend to move outward more strongly and concentrate near the outer wall, while lighter components remain closer to the axis. In the case of milk, the denser skim milk moves outward, while the less dense fat globules concentrate nearer the centre, allowing separation. In an inertial frame, this effect is described by centripetal acceleration and the different radial positions at equilibrium, but in practical and exam language, the separation is usually attributed to centrifugal force. Cohesive forces and gravity play roles but are not the main cause of rapid separation in a rotating cream separator. Step-by-Step Solution: Step 1: Recognise that the cream separator bowl or drum rotates at high speed, causing the milk to move in circular paths. Step 2: In the rotating frame, each small element of the liquid experiences an outward centrifugal force proportional to its mass and the square of the rotational speed, acting away from the axis. Step 3: Because skim milk is slightly denser than fat, the heavier portion tends to move further outward under this apparent force. Step 4: The lighter fat globules remain closer to the axis, forming a separate layer (cream) that can be collected. Step 5: Therefore, the separation of fat from milk in a cream separator is mainly due to centrifugal force produced by rapid rotation. Verification / Alternative check: If the milk were simply left standing under gravity without rotation, cream would eventually rise to the top due to density differences, but this process would be slow. The cream separator speeds up separation dramatically by imposing strong radial acceleration, which is much larger than gravitational acceleration. This confirms that gravitational force alone is not the main driver in the machine. Cohesive forces keep the liquid together but do not separate components. In technical descriptions, the mechanism is consistently described as centrifugal separation. Why Other Options Are Wrong: Cohesive force refers to attraction between molecules of the same substance and does not selectively separate fat from milk. Gravitational force alone can cause slow separation when milk stands still, but the cream separator specifically uses rapid rotation to enhance the process. Centripetal force is the inward force required to maintain circular motion; in the rotating frame, the separation is described more directly by the outward centrifugal effect on components of different densities. Common Pitfalls: Some students confuse centripetal and centrifugal forces, forgetting that centripetal is an inward real force while centrifugal is an outward apparent force in rotating frames. Exam questions about cream separators and centrifuges typically use the word centrifugal because it directly describes the outward tendency that leads to separation. Remembering this association between centrifuges, cream separators and centrifugal force will help you answer such questions quickly and accurately. Final Answer: Fat is separated from milk in a cream separator mainly due to centrifugal force in rapid rotation.

Question 17

In projectile motion, a body that is given an initial velocity and then allowed to move only under the effect of gravitational acceleration is called what?

Accepted Answer

Introduction / Context: This question tests your understanding of the basic definition of projectile motion in mechanics. Many problems in physics involve objects that are thrown, kicked or launched and then move under the influence of gravity alone, ignoring air resistance. These motions follow characteristic curved paths known as trajectories. Knowing the correct term for such a body helps you classify problems and apply the correct kinematic equations in two dimensions. Given Data / Assumptions: A body is given an initial velocity at some angle or along some direction. After launch, the only significant force acting on the body is the gravitational force of the Earth. Air resistance and other forces are neglected for simplicity. The options include projectile, satellite, pendulum bob and free electron. Concept / Approach: A projectile is defined as any object that is projected into space with an initial velocity and then allowed to move under the influence of gravity alone. Its path is determined by its initial speed, launch angle and gravitational acceleration. Typical examples include a thrown ball, a bullet fired horizontally (ignoring air drag) or a stone launched from a catapult. Satellites, while also under gravity, are in continuous free fall around the Earth and involve orbital mechanics rather than simple projectile motion. A pendulum bob is constrained by a string, and a free electron in a conductor is influenced by electric fields, not purely by gravity. Therefore, the correct term is projectile. Step-by-Step Solution: Step 1: Recall that projectile motion occurs when an object is launched and then moves freely under gravity, with no propulsion and negligible air resistance. Step 2: Recognise that this situation matches the description given: initial velocity followed by motion only under gravity. Step 3: Identify that the standard term for such a body is projectile, and that its trajectory is typically a parabola near the Earth surface. Step 4: Compare this with satellites, which undergo orbital motion, and pendulums, which are constrained by strings and experience tension forces as well as gravity. Step 5: Note that a free electron in a conductor primarily responds to electric fields and collisions, not just gravitational acceleration. Step 6: Conclude that the correct answer is projectile. Verification / Alternative check: In textbook chapters on projectile motion, the first definition usually states something like: any object thrown into space with an initial velocity and then moving under the action of gravity alone is called a projectile. Diagrams of cannon shells, balls and arrows are then used to illustrate curved trajectories. Satellites are treated separately under gravitation and planetary motion, where the curvature of the Earth and orbital speed are crucial. This clear separation in teaching materials confirms that the term you need in this context is projectile. Why Other Options Are Wrong: A satellite in orbit is under gravity but moves fast enough to keep missing the Earth, leading to orbital motion rather than the simple projectile path assumed in elementary problems. A simple pendulum bob is attached to a string and experiences tension as well as gravity, so its motion is not free projectile motion. A free electron in a conductor is strongly influenced by electric fields and collisions with atoms, making its dynamics very different from a purely gravitational projectile. Common Pitfalls: Some learners may overgeneralise and think that any object under gravity is a projectile, including satellites and pendulum bobs. However, in exam level mechanics, the term projectile is reserved for objects that move freely under gravity with no constraint forces like tension or continuous propulsion. Remembering examples like a thrown ball or a kicked football will help solidify the correct definition. Final Answer: A body given an initial velocity and then allowed to move only under gravity is called a projectile.

Question 18

When a small steel ball is gently placed on the surface of liquid mercury, it does not sink. What is the main reason for this behaviour?

Accepted Answer

Introduction / Context: This question combines ideas from fluid mechanics and properties of materials. Mercury is a very dense liquid metal, and steel is a dense solid. Intuitively, many people expect solids to sink in liquids, but whether an object sinks or floats actually depends on density and buoyancy, not on whether the object is solid or liquid. Observing that a steel ball can float on mercury under appropriate conditions is a striking demonstration of Archimedes principle. Understanding why this happens helps you connect density concepts with real examples. Given Data / Assumptions: A small steel ball is placed gently on the surface of mercury. Mercury is a dense liquid with a density much greater than that of water. Steel also has significant density, greater than water, but less than mercury. We assume the ball is not dropped violently, so splashing effects are avoided. Concept / Approach: According to Archimedes principle, a body immersed in a fluid experiences an upward buoyant force equal to the weight of the fluid displaced. If the fluid is very dense, a relatively small volume of fluid is enough to support a given weight. Mercury has a density around 13.6 times that of water, while steel is roughly 7 to 8 times as dense as water. Because mercury is denser than steel, a steel ball can displace a volume of mercury whose weight equals the weight of the ball before it completely submerges. As a result, the ball floats partially immersed. Surface tension effects can contribute for very small objects, but the primary reason in this classic example is the density difference and buoyant force. Step-by-Step Solution: Step 1: Recall that whether an object sinks or floats depends on the comparison between its density and the density of the fluid. Step 2: Note that steel is denser than water and therefore sinks in water, but mercury is much denser than both water and steel. Step 3: When the steel ball is placed on mercury, it displaces some mercury. The buoyant force depends on the weight of displaced mercury. Step 4: Because mercury is very dense, even a modest displacement provides a buoyant force that can balance the weight of the steel ball. Step 5: At equilibrium, the upward buoyant force equals the weight of the ball, so it floats instead of sinking, demonstrating that mercury density exceeds steel density. Verification / Alternative check: Imagine replacing mercury with water while keeping the same steel ball. Steel has greater density than water, so in water the ball displaces a volume whose weight is less than the ball's own weight before it becomes fully submerged, and thus it sinks. The different outcomes in mercury versus water illustrate that it is the density of the fluid compared to the density of the solid that matters. Reference tables of densities confirm that mercury has higher density than steel, matching the explanation and supporting the correct option. Why Other Options Are Wrong: Surface tension of mercury does affect very small particles, but for a steel ball of ordinary size, buoyancy from high density is the main cause of floating in textbook treatments. Mercury is not a semisolid; it is a true liquid metal at room temperature with well defined fluid properties. High viscosity would only slow movement; it does not by itself prevent sinking in the long run if density conditions favour sinking. Common Pitfalls: Students sometimes attribute all floating effects on liquids like mercury to surface tension, especially if they have seen small objects resting on surfaces without sinking. While surface tension can be significant for microscopic particles, density and buoyancy are the dominant factors for larger objects like steel balls in typical classroom demonstrations. To avoid confusion, always check density comparisons first when explaining why objects float or sink. Final Answer: A steel ball placed on mercury does not sink mainly because the density of mercury is greater than the density of steel, so buoyant force can balance its weight.

Question 19

If a musical band is played on the surface of the Moon, how will the sound be perceived by an observer some distance away from it?

Accepted Answer

Introduction / Context: This question tests your understanding of how sound propagates and the difference between sound waves and electromagnetic waves. The Moon has essentially no atmosphere, which makes it a convenient example for explaining why sound cannot travel in a vacuum. Many space related questions use this idea to reinforce the concept that sound needs a material medium, while light and radio waves do not. Given Data / Assumptions: A musical band is playing on the lunar surface. An observer is located at some distance from the band. The Moon is assumed to have negligible atmosphere, effectively a vacuum. We consider ordinary sound produced by instruments and loudspeakers, not radio transmissions. Concept / Approach: Sound waves are mechanical waves that require a material medium such as air, water or solids to propagate. They consist of compressions and rarefactions in the medium. In a vacuum, there are no particles to transmit these pressure variations, so sound cannot travel from one point to another. The Moon lacks a significant atmosphere; the space between the band and the observer is essentially vacuum. Therefore, even though the instruments produce vibrations, these vibrations cannot propagate as sound waves through the vacuum, and an observer at any distance will not hear the music. Light from the scene would still be visible, but the sound would be absent. Step-by-Step Solution: Step 1: Recall that sound is a longitudinal mechanical wave requiring a material medium for propagation. Step 2: Recognise that the Moon has no substantial atmosphere and the space between objects on its surface is practically vacuum. Step 3: Understand that in vacuum there are no air molecules to compress and expand, so mechanical sound waves cannot travel. Step 4: Conclude that the vibrations of the musical instruments will not reach the ears of an observer through the vacuum; only direct contact through the solid ground could carry vibrations, but not ordinary airborne sound. Step 5: Therefore, the correct description is that the sound will not be heard at any distance away from the band through the lunar environment. Verification / Alternative check: A common demonstration in physics classrooms uses an electric bell or buzzer inside a glass jar. As the air is gradually pumped out, the sound becomes weaker and eventually disappears even though the bell is still vibrating. This shows that removing the medium prevents sound propagation. The Moon represents an extreme case where no air is present to begin with. Astronauts on the Moon communicate using radio, not by shouting, which further confirms that sound cannot travel through the lunar vacuum. Why Other Options Are Wrong: Reverberation requires reflections of sound in a medium such as air or within enclosed spaces; without a medium on the Moon, no reverberation occurs. Hearing the sound faintly assumes that some air is present; in reality, the Moon has essentially no atmosphere to carry the sound. Hearing the band up to 10 km away is impossible in vacuum, as sound cannot travel even a small distance without a medium. Common Pitfalls: Some learners incorrectly generalise from light and radio wave behaviour and assume that all waves, including sound, can travel through space. Another confusion comes from movies where explosions in space are accompanied by loud sounds, which is physically inaccurate. To avoid such misconceptions, always remember the key distinction: sound requires matter to propagate, whereas electromagnetic waves such as light and radio can travel through vacuum. Final Answer: If a band is played on the Moon, the sound will not be heard at any distance at all through the lunar vacuum.

Question 20

In kinetic theory, heat energy contained in a substance is primarily associated with which of the following forms of molecular motion?

Accepted Answer

Introduction / Context: This question examines your understanding of heat from the viewpoint of kinetic theory of matter. In thermodynamics, heat is a form of energy transfer, while in kinetic theory we connect internal energy and temperature with motions of molecules. The distinction between random microscopic motion and any large scale ordered motion is important. Recognising that heat is linked to random molecular motion helps clarify what happens when a body is heated or cooled. Given Data / Assumptions: We are talking about molecules in a solid, liquid or gas. Heat is considered as energy related to molecular motion. Options distinguish between random and orderly motion. Macroscopic ordered motion might include bulk flow, rotation or translation of the entire body. Concept / Approach: In kinetic theory, temperature is related to the average kinetic energy of random molecular motion within a substance. When we talk about heat content or thermal energy, we usually refer to this microscopic disordered motion: molecules moving, vibrating and rotating in all directions. Ordered motion, such as the whole body moving in one direction, corresponds to macroscopic kinetic energy, not heat. For example, a hot gas at rest in a container has high internal energy due to random motion even though there is no bulk movement. Therefore, heat energy is associated primarily with the kinetic energy of random motion of molecules. Step-by-Step Solution: Step 1: Recall that molecules in a substance are constantly in motion, even in solids where they vibrate about fixed positions. Step 2: Recognise that this motion is random in direction and magnitude at the microscopic level, with no overall preferred direction. Step 3: Understand that temperature is a measure of the average kinetic energy of this random microscopic motion. Step 4: When heat is supplied to a body at constant phase, the main effect is to increase the kinetic energy of this random motion, which raises the temperature. Step 5: Conclude that heat energy is most directly associated with the kinetic energy of random, disordered molecular motion, not with large scale ordered motion. Verification / Alternative check: Consider a block of metal that is moving across a table with some speed but at a low temperature. Its molecules have significant ordered motion because the whole block is moving, yet we do not say it is hot. If the block is heated while it is at rest, its temperature and heat content increase due to more vigorous random vibrations of the atoms, even though there is no bulk motion. Similarly, a gas confined in a container at high temperature can have intense random motion of molecules while the container itself does not move. These examples show that heat relates to random microscopic motion, not to macroscopic uniform motion. Why Other Options Are Wrong: Kinetic energy of perfectly orderly motion of molecules corresponds to bulk motion, such as translation of the whole body, which is mechanical kinetic energy, not heat. Total kinetic energy of both random and orderly motion mixes internal thermal energy with bulk kinetic energy; in thermodynamics, heat is associated specifically with random internal motion. None of these is incorrect because the first option correctly describes the connection between heat and random molecular kinetic energy. Common Pitfalls: Students sometimes blur the distinction between internal energy and total mechanical energy. They may think that if a body is moving fast, it must contain more heat, which is not necessarily true. To avoid this confusion, remember that internal energy and heat are connected to disordered microscopic motion, while macroscopic kinetic energy relates to ordered motion of the whole object. Focusing on random molecular motion will guide you to the correct interpretation in exam questions. Final Answer: Heat energy in a substance is primarily associated with the kinetic energy of random motion of molecules.

Physics Questions