Question 1

In everyday physics, we are able to walk on the earth without slipping mainly because of which force acting between our feet and the ground?

Accepted Answer

Introduction / Context: Walking is such a common activity that we rarely think about the physics behind it. However, every step you take is governed by basic principles of mechanics. The question asks which force allows us to walk on the surface of the earth without constantly slipping. This concept is fundamental in mechanics and is closely related to friction, contact forces, and motion. Understanding it helps explain not only walking, but also why tyres grip the road, why shoes are designed with tread patterns, and why people slip more easily on smooth ice or wet floors. Given Data / Assumptions: A person is walking on the surface of the earth. The person does not normally slip while walking on a rough or normal surface. Forces considered include gravitational force, frictional force, viscous force, and centrifugal force. We assume normal daily walking on the ground, not special cases like walking in deep water or on a rotating ride. Concept / Approach: When you walk, you push your foot backward against the ground. According to Newton's third law, the ground exerts an equal and opposite force on your foot. This forward reaction force provided by friction between your shoe and the ground is what propels you forward. If friction is too low, such as on ice, the backward push does not create enough reaction force, and you slip. Gravitational force simply keeps you on the ground but does not prevent slipping along the surface. Viscous force is important in fluids, and centrifugal force appears in rotating reference frames. Therefore, the correct force that prevents slipping is frictional force. Step-by-Step Solution: Step 1: Recognise that walking involves interaction between your feet and the ground.Step 2: When you attempt to walk, your foot applies a backward push on the ground.Step 3: The ground responds with a forward force on your foot due to friction between the surfaces.Step 4: This forward frictional force accelerates your body forward, allowing you to move without slipping.Step 5: If friction is absent or very small, as on ice, your foot would slide backward instead of gripping, and you would slip. Therefore, frictional force is essential. Verification / Alternative check: You can verify this idea by thinking about situations where friction is reduced. On a polished floor or icy road, it becomes hard to walk without slipping. People often use shoes with better grip or sprinkle sand on icy paths to increase friction. Vehicles use tyres with tread patterns to increase friction with the road. In all these examples, the key quantity being adjusted is friction. Gravity remains the same, yet slipping behaviour changes with friction, confirming that frictional force is the controlling factor in walking safely. Why Other Options Are Wrong: Gravitational force pulls you downward and keeps your feet in contact with the ground but does not stop horizontal slipping, so option A is incomplete. Viscous force acts in liquids and gases and is not the main factor when walking on solid ground, so option C is incorrect. Centrifugal force is a fictitious force observed in rotating frames, such as in a turning car, and is not responsible for ordinary walking on a straight path, so option D is also wrong. Only frictional force correctly explains walking without slipping. Common Pitfalls: Many learners quickly choose gravitational force because they know gravity keeps us on the ground. However, the question asks specifically about slipping, which is a tangential or sideways motion along the surface. It is easy to overlook the role of friction in providing horizontal grip. Always read words like slipping, rolling, and skidding carefully, because they usually point towards friction related concepts rather than gravity alone. Also, remember that multiple forces can act at once, but the question asks which particular force allows walking without slipping, so you must identify the most relevant one. Final Answer: Frictional force

Question 2

In a hydel power station, the rotational motion produced in the turbines is primarily due to which physical process acting on the flowing water?

Accepted Answer

Introduction / Context: Hydel power stations, also called hydroelectric power plants, generate electricity using the potential and kinetic energy of water. Unlike thermal power plants, which burn fuels, hydel stations rely on the natural water cycle and gravity. The question tests whether you clearly understand what actually drives the turbines in a hydel power station. Recognising the correct physical process is important for distinguishing between different types of power generation, such as thermal, hydro, and diesel power plants, which often appear together in general science exams. Given Data / Assumptions: The setup is a hydel (hydroelectric) power station. Water is stored at a higher level in a reservoir behind a dam. Turbines are located at a lower level. The question is about the motion of the turbines, not directly about electricity itself. Concept / Approach: In a hydel power station, water stored at a height has gravitational potential energy. When this water is released, it flows down through large pipes called penstocks. As it falls, its potential energy converts to kinetic energy. High speed jets of water strike the blades of turbines, causing them to rotate. This rotational mechanical energy is then converted to electrical energy by generators. No coal or diesel is burned in a pure hydroelectric plant, and steam is not the primary working fluid in this type of installation. Therefore, the correct physical process that produces turbine motion is the flow of water from a height under gravity. Step-by-Step Solution: Step 1: Identify that the power station mentioned is hydel, which implies the use of water and gravity.Step 2: Recall that water is stored in a reservoir at a higher level behind a dam.Step 3: When water is allowed to flow downward through penstocks, its potential energy converts into kinetic energy.Step 4: The fast flowing water jets strike turbine blades and transfer momentum, causing the turbines to rotate rapidly.Step 5: The rotating turbines drive generators, which convert mechanical energy into electrical energy, confirming that the key cause of turbine motion is the flow of water. Verification / Alternative check: Compare hydel plants with thermal power plants and diesel power stations. Thermal power plants burn coal to heat water, produce steam, and use that steam to rotate turbines. Diesel power plants burn diesel in an internal combustion engine that directly drives an alternator. In both cases, the working principle is based on burning fuel, not on the flow of water from a height. Since the question clearly states hydel power station, we must choose the option that involves flowing water rather than combustion, confirming that the flow of water from a height is correct. Why Other Options Are Wrong: Burning of coal in a furnace is typical of thermal power plants and is not a feature of pure hydroelectric stations, so option B is incorrect. Burning of diesel in an engine is associated with diesel generating sets and small diesel power stations, making option C wrong. Production of steam in a boiler is again a thermal power concept where water is converted to steam by heating, not the main mechanism in typical hydel systems, so option D is also wrong. Only option A correctly describes the process in hydel power stations. Common Pitfalls: Students sometimes confuse general power plant concepts and think that all large power stations use steam or burning of fuel. It is important to associate the term hydel or hydroelectric directly with flowing water. Another pitfall is to think too much about the final form of energy and ignore the immediate mechanical cause of turbine motion. Always break such questions into two parts: what is the energy source and what directly drives the turbine. In hydel plants, both answers point to water flowing from a height under the influence of gravity. Final Answer: Flow of water from a height

Question 3

In an electric room heater or electric cooking heater, the coil of high resistance wire that becomes hot and produces heat is commonly called what?

Accepted Answer

Introduction / Context: Electric room heaters and electric cooking heaters are very common household appliances. Inside them, a coiled wire heats up when current flows and then transfers heat to the surroundings. The question asks you to recall the correct technical term used for this heated coil. This term appears frequently in basic physics and general science, and it also helps in understanding other devices like electric irons, geysers, and toasters, which use the same principle of heating by electrical resistance. Given Data / Assumptions: The device is an electric room heater or electric cooking heater. There is a coil made of high resistance wire inside the appliance. When current flows through this coil, it becomes hot and produces heat energy. The question focuses on the name of this particular coil, not on the entire circuit or power supply. Concept / Approach: When an electric current passes through a wire with significant resistance, electrical energy is converted into heat energy. This is known as Joule heating or resistive heating. In heating appliances, a special alloy such as nichrome is used to make the coil because it has high resistance and can withstand high temperatures without melting. This coil is specifically called the heating element of the appliance. The rest of the wiring that connects the element to the power source forms the circuit, but the hot coil itself is the element. A filament is usually associated with electric bulbs, where a thin wire glows to produce light rather than primarily heat for cooking or room heating. Step-by-Step Solution: Step 1: Identify the key component described: a coil of wire that heats up when current flows.Step 2: Recall that the primary purpose of this component in heaters is to convert electrical energy into heat energy.Step 3: Recognise that such a component is technically called a heating element in electrical appliances.Step 4: Note that the term electrical circuit refers to the complete path of current flow, not just the coil.Step 5: Conclude that heating element is the correct term for the coil in a room or cooking heater. Verification / Alternative check: Think about other heating appliances such as an electric iron, geyser, toaster, or immersion rod. Instruction manuals and labels for these devices usually mention the power rating of the heating element. They rarely refer to a filament in this context. In contrast, electric bulbs have their working part described as a filament rather than an element. This clear distinction between filament for light and heating element for heat confirms that the correct answer for heaters is heating element. Additionally, technicians and electricians commonly use the term element when replacing or repairing these parts. Why Other Options Are Wrong: Electrical circuit refers to the complete loop through which current flows, including switches, wires, and the element, so option A is too general and not the specific coil. Filament of a bulb is associated with light producing devices such as incandescent lamps and does not correctly describe the heater coil, so option C is incorrect. Electric cells are devices like batteries that provide electrical energy and are not the part that becomes hot in a heater, so option D is also wrong. Heating element is the only precise term. Common Pitfalls: Students sometimes confuse the terms filament and element because both are wires that get hot. The key difference is the primary purpose: filaments are designed mainly to emit light, while elements are designed mainly to release heat. Another pitfall is thinking that any part carrying current is part of the element, but only the high resistance heating coil should be called the element. Paying attention to specific vocabulary used in textbooks and practical work helps avoid such confusion in examinations. Final Answer: Heating element

Question 4

In the study of friction, the magnitude of sliding friction between two solid surfaces is generally how related to the magnitude of static friction between the same surfaces?

Accepted Answer

Introduction / Context: Friction is the resistive force that opposes relative motion between two surfaces in contact. There are different types of friction, including static friction, which acts when surfaces are at rest relative to each other, and sliding or kinetic friction, which acts when surfaces are already moving. The question asks you to compare the magnitude of sliding friction with static friction for the same pair of surfaces. This comparison is very important in understanding why it is harder to start moving a heavy object than to keep it moving once it has started. Given Data / Assumptions: Two solid surfaces are in contact under a given normal reaction. Static friction acts just before motion begins. Sliding friction acts when one surface is moving relative to the other. We assume ordinary dry surfaces, not special lubricated or highly unusual materials. Concept / Approach: Static friction adjusts itself to match the applied force up to a maximum value called limiting friction. To start motion, the applied force must exceed this maximum static friction. Once motion begins, the frictional force drops slightly to a lower, almost constant value known as sliding or kinetic friction. For most pairs of surfaces, the coefficient of static friction is slightly higher than the coefficient of sliding friction. Therefore, the maximum static friction is greater than sliding friction. This is why it feels harder to set a heavy box in motion than to push it along the floor after it has started moving. Step-by-Step Solution: Step 1: Recall that static friction acts when there is no relative motion, and it increases with applied force up to a maximum value.Step 2: The maximum value of static friction is sometimes called limiting friction.Step 3: Once the applied force exceeds this limiting value, the object starts to move and sliding friction acts.Step 4: Experimental results show that the sliding frictional force is usually slightly less than the limiting static friction force for the same surfaces.Step 5: Therefore, sliding friction is smaller than static friction, making option D correct. Verification / Alternative check: You can verify this by thinking about practical experience. When you try to push a heavy cupboard, you feel a strong resistance at the start. After it begins to slide, maintaining the same speed seems easier, because the resistive force has decreased. Engineers also use different coefficients for static and kinetic friction in calculations. The typical relationship is that the coefficient of static friction is slightly greater than the coefficient of sliding friction, confirming that the sliding frictional force is smaller in magnitude than the maximum static friction. Why Other Options Are Wrong: Option A, stating that sliding friction is double the static friction, is incorrect and does not match experimental evidence. Option B claims both are exactly the same, which contradicts the established difference between limiting static friction and kinetic friction. Option C suggests that sliding friction is greater than static friction, which would make objects harder to move once they start, but this does not match everyday observation. Only option D correctly states that sliding friction is smaller than static friction for the same surfaces. Common Pitfalls: Some learners misread the question and compare sliding friction to normal static friction instead of the maximum static friction, which can cause confusion. Others mix up the direction of the inequality and think that motion should increase friction. Remember that nature often resists the start of motion more strongly than the continuation of motion. Keeping this physical interpretation in mind helps you recall that static friction is usually higher, and sliding friction is slightly reduced once motion begins. Final Answer: Smaller than the static friction

Question 5

In the International System of Units (SI), the kelvin (K) is used as the standard unit for measuring which physical quantity?

Accepted Answer

Introduction / Context: The International System of Units, or SI system, defines standard units for all basic physical quantities so that scientists and engineers around the world can communicate measurements consistently. The kelvin, symbol K, is one of the seven base SI units. The question checks whether you know which physical quantity is measured in kelvin. This is a fundamental fact that appears in physics, chemistry, and engineering, especially in topics related to thermodynamics, gases, and heat transfer. Given Data / Assumptions: The unit under discussion is kelvin, symbol K. Kelvin is known to be an SI base unit. Possible physical quantities listed are density, pressure, mass, and temperature. We assume standard SI usage as taught in school level physics and chemistry. Concept / Approach: Kelvin is the SI base unit of thermodynamic temperature. It is defined based on the triple point of water and, in modern definitions, related to fundamental physical constants. Temperature scales such as Celsius and Fahrenheit are more common in everyday life, but scientific work prefers kelvin because it starts from absolute zero, the theoretically lowest possible temperature. Density has SI units of kilogram per cubic meter. Pressure has SI units of pascal, which is newton per square meter. Mass has SI units of kilogram. Therefore, among the choices, temperature is the only physical quantity correctly associated with kelvin as its SI base unit. Step-by-Step Solution: Step 1: Recall the list of SI base units: metre, kilogram, second, ampere, kelvin, mole, and candela.Step 2: Associate each base unit with its corresponding physical quantity; kelvin is paired with thermodynamic temperature.Step 3: Identify that density is a derived quantity with units kg per m^3.Step 4: Recognise that pressure is also derived, with unit pascal, equal to newton per m^2.Step 5: Conclude that temperature is the correct physical quantity measured in kelvin. Verification / Alternative check: Check standard scientific formulas. The ideal gas law is written as pV = nRT, where temperature T must be expressed in kelvin for the equation to work correctly in SI units. Also, many thermodynamic relations use absolute temperature in kelvin because absolute zero is a natural reference point. Everyday Celsius readings are converted to kelvin by adding 273 approximately, which again confirms that kelvin is a temperature unit. No such association exists between kelvin and density, mass, or pressure, which makes temperature the only consistent choice. Why Other Options Are Wrong: Density is typically measured in kilograms per cubic meter or grams per cubic centimeter, not in kelvin, so option A is wrong. Pressure is measured in pascals, bars, or atmospheres in various contexts, but not in kelvin, so option B is incorrect. Mass is measured in kilograms, grams, or related units rather than kelvin, so option C is also wrong. Only option D, temperature, correctly matches the SI base unit kelvin. Common Pitfalls: Some students confuse kelvin with the Celsius scale because both appear in temperature related problems. However, Celsius is not an SI base unit, while kelvin is. Another pitfall is confusing base units and derived units. Remember that temperature is so fundamental that it has its own base unit, while quantities like density and pressure are combinations of mass, length, and time units. Keeping a mental list of SI base units and their quantities helps avoid errors in multiple choice questions about units. Final Answer: Temperature

Question 6

The meter installed in a vehicle that calculates and displays the total distance covered by the vehicle over time is commonly called what?

Accepted Answer

Introduction / Context: Modern vehicles have several instruments on the dashboard to provide useful information to the driver. Two important ones are the speedometer and the odometer. The speedometer shows how fast the vehicle is moving at a given instant, while the odometer records the total distance traveled. The question asks you to identify the name of the meter that measures distance, which is a common point of confusion in basic general science and everyday knowledge. Given Data / Assumptions: A vehicle is fitted with a meter to measure distance covered. The meter keeps a cumulative record over time, not just an instantaneous reading. Possible instruments mentioned include speedometer, odometer, thermometer, and a generic kilometre counter. We assume a typical car, bike, or similar road vehicle. Concept / Approach: The word odometer comes from Greek roots that mean way or path and measure. It is specifically designed to count how many kilometres or miles a vehicle has traveled during its lifetime. The reading usually appears as a rolling number display or a digital display on the dashboard. In contrast, the speedometer, as the name suggests, measures speed, which is distance per unit time, not total distance. A thermometer measures temperature and is unrelated to vehicle distance measurement. A so called kilometre counter is not the standard technical term, even though it describes the function informally. The correct scientific term used in vehicles is odometer. Step-by-Step Solution: Step 1: Identify that the quantity being measured is total distance covered by the vehicle.Step 2: Recall that in vehicles, speed is shown by the speedometer, while distance is shown by a different instrument.Step 3: Remember that the distance measuring device is called an odometer, often located within or near the speedometer dial.Step 4: Note that thermometer is used for temperature and does not relate to distance.Step 5: Conclude that the correct term for the meter that calculates distance covered is odometer. Verification / Alternative check: If you look at a vehicle dashboard, you will see a display labelled with total kilometres or miles, often called the odometer reading. Service schedules, resale valuations, and fuel efficiency calculations are all based on the odometer reading. The speedometer, on the other hand, has a moving needle or digital value that continuously changes as the speed varies. Since the question emphasises the meter that calculates distance covered, checking the dashboard terminology confirms that odometer is the appropriate word. Why Other Options Are Wrong: Speedometer measures the instantaneous speed of the vehicle in km per hour or miles per hour, not the cumulative distance, so option A is wrong. Thermometer measures temperature and has no direct link with distance measurement, so option C is clearly incorrect. Kilometre counter describes the function in everyday language but is not the standard scientific or technical term used in textbooks and exams, so option D is not preferred. Odometer is the correct technical name for this instrument. Common Pitfalls: Students often quickly choose speedometer because they associate it strongly with vehicle instruments. However, reading the phrase distance covered carefully should remind you that speed and distance are different physical quantities. Another pitfall is thinking that any term containing kilometre must be correct, but exams usually test recognition of standard terms like odometer. To avoid confusion, remember this pair: speedometer for speed and odometer for distance, both usually placed together on the vehicle dashboard. Final Answer: Odometer

Question 7

In Indian polity, the Fundamental Duties of citizens were incorporated into the Constitution of India by which Constitutional Amendment Act?

Accepted Answer

Introduction / Context: Fundamental Duties are an important part of the Indian Constitution and define the moral obligations of citizens toward the nation and society. They were not present in the original Constitution adopted in 1950. Instead, they were added later by a specific Constitutional Amendment. The question checks your knowledge of which Amendment Act introduced these duties, a standard fact in Indian polity and general knowledge examinations, especially for civil services and competitive tests. Given Data / Assumptions: The subject is Fundamental Duties in the Constitution of India. The options list the Forty-fourth, Forty-second, Twenty-sixth, and Twenty-fifth Amendment Acts. We assume the standard chronology of Constitutional Amendments as recognised in Indian polity. The task is to identify the specific Amendment Act that inserted Fundamental Duties. Concept / Approach: Fundamental Duties were added to the Constitution of India in Part IV A on the recommendations of the Swaran Singh Committee. This change took place during the period of internal Emergency in the mid 1970s. The relevant amendment is the Forty-second Constitutional Amendment Act of 1976, often called the Mini Constitution because it made many important changes. This amendment inserted Article 51A, which initially listed ten Fundamental Duties for citizens, later increased to eleven. The other amendments in the options dealt with different issues, such as curbing the right to property or modifying emergency provisions. Step-by-Step Solution: Step 1: Recall that Fundamental Duties were introduced in the mid 1970s, not at the time of the original Constitution.Step 2: Remember that the Swaran Singh Committee recommended including these duties.Step 3: Associate these changes with the Forty-second Constitutional Amendment Act of 1976.Step 4: Note that Article 51A in Part IV A was inserted by this Amendment, listing Fundamental Duties.Step 5: Conclude that the Forty-second Amendment Act is the correct answer. Verification / Alternative check: You can cross check by remembering that the Forty-fourth Amendment Act of 1978 mainly dealt with restoring some democratic features after the Emergency, altering provisions related to fundamental rights and emergency powers, but it did not introduce Fundamental Duties. The Twenty-sixth Amendment Act of 1971 abolished privy purses of former rulers, and the Twenty-fifth Amendment Act of 1971 curtailed the right to property. None of these amendments are linked to Fundamental Duties. Only the Forty-second Amendment Act is consistently mentioned in standard polity textbooks as the amendment that introduced Fundamental Duties, which confirms the answer. Why Other Options Are Wrong: The Forty-fourth Amendment Act reversed some provisions of the Forty-second Amendment and modified emergency rules, but it did not create Fundamental Duties, so option A is incorrect. The Twenty-sixth Amendment Act abolished privy purses and privileges of former princes, which is unrelated to duties of citizens, making option C wrong. The Twenty-fifth Amendment Act focused on property rights and nationalisation, not on moral duties, so option D is also incorrect. Only option B, the Forty-second Amendment Act, is associated with the insertion of Article 51A and Fundamental Duties. Common Pitfalls: Candidates sometimes confuse the Forty-second and Forty-fourth Amendments because both are very important and close in time. Another pitfall is remembering that the Forty-fourth Amendment undid some changes brought by the Forty-second and mistakenly assuming it also brought the duties. To avoid this, link Fundamental Duties specifically with Article 51A and the Forty-second Amendment Act of 1976, and remember the Forty-fourth Amendment as a corrective measure that focused on rights and emergency provisions rather than duties. Final Answer: Forty-second Amendment Act

Question 8

In physics, physical quantities that have only magnitude and no associated direction are known by which term?

Accepted Answer

Introduction / Context: When studying mechanics and other branches of physics, it is essential to classify physical quantities into scalars and vectors. This classification tells you whether a quantity is completely described by a single number or whether it also has a direction associated with it. The question asks for the correct description of physical quantities that possess only magnitude but no direction. Understanding this distinction is vital for solving problems correctly, especially when adding or subtracting quantities like forces, velocities, and distances. Given Data / Assumptions: Physical quantities can be described by magnitude and possibly direction. The question focuses on quantities with only magnitude. Options mix up magnitude, direction, speed, and velocity terms. We assume the standard physics definitions of scalar and vector quantities. Concept / Approach: A scalar quantity is defined as one that is completely described by a single real number, called its magnitude. Examples include mass, temperature, time, distance, and energy. These do not require direction for their specification. A vector quantity, on the other hand, is described by both magnitude and direction, for example displacement, velocity, acceleration, and force. Therefore, quantities with only magnitude and no direction are scalar quantities. The language used in the correct option should capture this definition succinctly, mentioning magnitude and explicitly stating the absence of direction. Step-by-Step Solution: Step 1: Recall that scalars have magnitude only, while vectors have both magnitude and direction.Step 2: Identify the option that states magnitude only and no direction, which matches the scalar definition.Step 3: Check that the other options either specify direction only or confuse speed and velocity.Step 4: Confirm that magnitude only and no direction is the correct descriptive phrase for scalar quantities.Step 5: Choose that option as the answer. Verification / Alternative check: Consider some examples. Mass of 5 kilograms, time of 10 seconds, and temperature of 30 degrees Celsius all make sense without any directional information. These are clearly scalar quantities. In contrast, a velocity of 10 metres per second north requires a direction to be complete. If you remove direction from velocity, you are left with speed, which is scalar. This exercise shows that the essential property of scalars is magnitude only, which confirms the correctness of the option stating magnitude only and no direction. Why Other Options Are Wrong: Option A, direction only and no magnitude, is physically meaningless because a physical quantity without any size cannot be measured or used in equations. Option C, speed and velocity together, is a mix of a scalar and a vector and does not describe a category of quantities. Option D, velocity only and no speed, confuses the definitions, since velocity inherently has both magnitude and direction and cannot exist without magnitude. Only option B correctly captures the property of scalar quantities as having magnitude only and no direction. Common Pitfalls: Students sometimes confuse speed and velocity, thinking both are vectors, when actually speed is scalar and velocity is vector. Another mistake is to assume that all quantities in motion, like distance and displacement, are vectors, but distance is scalar. Always ask yourself whether specifying a number alone is enough to describe the quantity. If yes, it is likely scalar. If a direction must be added for the description to make sense, the quantity is vector. This habit helps you avoid errors in classification questions like this. Final Answer: Magnitude only and no direction

Question 9

In basic electrical engineering, an ideal transformer can perform several operations on an alternating current (AC) supply. Which of the following is an operation that a transformer cannot perform?

Accepted Answer

Introduction / Context: Transformers are widely used devices in electrical power systems. They operate only on alternating current and are used to change voltage and current levels efficiently. The question asks which operation a transformer cannot perform, assuming an ideal transformer. This tests your understanding of energy conservation and the basic relationships between voltage, current, and power in a transformer. Knowing what a transformer can and cannot do is essential for understanding power transmission and distribution networks. Given Data / Assumptions: The transformer considered is ideal, meaning no losses. The input is an alternating current supply. Possible operations include stepping up voltage, stepping up current, stepping up power, and stepping down voltage. We assume standard transformer equations that conserve power in the ideal case. Concept / Approach: In an ideal transformer, input power equals output power, ignoring losses. Power is given by P = V * I for AC magnitudes. If the transformer steps up voltage, then current decreases proportionally so that the product V * I remains the same. Similarly, if it steps up current, the voltage decreases. Therefore, a transformer can step up or step down voltage and current, but it cannot increase the total power; it merely transfers power from the primary to the secondary side. Real transformers have small losses, so output power is slightly less than input power, but they still cannot increase power. Thus, the operation of stepping up AC power is not possible for a transformer. Step-by-Step Solution: Step 1: Recall the transformer power relation for an ideal transformer: input power P1 = output power P2.Step 2: Express power as P = V * I, where V is voltage and I is current.Step 3: If voltage is stepped up, current must be stepped down so that V1 * I1 = V2 * I2.Step 4: If current is stepped up, voltage must be stepped down to keep total power the same.Step 5: Since a transformer conserves power and cannot create extra energy, it cannot step up AC power; therefore, that operation is impossible. Verification / Alternative check: You can verify this by looking at power ratings on actual transformers. A transformer rated at a certain kilovolt ampere value on the primary side has the same or slightly lower kilovolt ampere rating on the secondary side. Engineers design transformers to avoid exceeding this rating. The device can convert high voltage low current to low voltage high current or vice versa, but the overall apparent power remains constant, minus small losses. This observation confirms that a transformer does not have the ability to increase power. It simply transforms voltage and current levels efficiently. Why Other Options Are Wrong: Step up AC voltage is a basic and very common function of transformers used in power transmission, especially at generating stations, so option A is clearly a valid transformer operation. Step up AC current is also possible by using a step down transformer, which reduces voltage and increases current, so option B is feasible. Step down AC voltage is the function of distribution transformers that provide safe voltages for domestic and industrial use, so option D is correct as an operation. Only option C, step up AC power, contradicts the principle of energy conservation in an ideal transformer and is therefore impossible. Common Pitfalls: Students sometimes confuse increasing voltage or current with increasing power. It is important to remember that power depends on both voltage and current. Increasing one while decreasing the other can keep power constant. Another mistake is to assume that because losses exist, a transformer might increase power on one side, but real devices actually have output power slightly less than input power. Keeping the principle of conservation of energy in mind will guide you to reject any claim that a passive device like a transformer can increase total power on its own. Final Answer: Step up AC power

Question 10

In electromagnetism, the standard SI unit used to measure the physical quantity magnetic field strength (H) is which of the following?

Accepted Answer

Introduction / Context: Magnetic field strength, often denoted by H, is a fundamental quantity in electromagnetism that describes the intensity of a magnetic field due to currents and magnets. Like all physical quantities, it has an associated SI unit. The question asks you to identify this unit from a list of options, many of which are units for other physical quantities. Recognising units correctly is an important skill in physics because it helps check equations and understand the nature of different quantities. Given Data / Assumptions: The quantity in focus is magnetic field strength H. We are working in the SI system of units. Options include joule per meter, newton per meter, kelvin per meter, and ampere per meter. Each of these combinations is associated with different physical meanings. Concept / Approach: In the SI system, magnetic field strength H has the unit ampere per meter, written as A per m. It is related to magnetising force and current distribution around conductors. Magnetic flux density B, a related but different quantity, has the SI unit tesla. Joule per meter could represent energy per unit distance, newton per meter can appear in contexts like surface tension or spring constants, and kelvin per meter appears in temperature gradients. None of these correspond to magnetic field strength. Therefore, ampere per meter is the correct unit for H in electromagnetism. Step-by-Step Solution: Step 1: Recall that magnetic field strength H is related to current and geometry of conductors.Step 2: Remember that in SI units, H is measured in ampere per meter.Step 3: Note that joule per meter is a unit of energy per distance, not related to magnetising force.Step 4: Recognise that newton per meter is linked to force per length, and kelvin per meter to temperature gradient.Step 5: Conclude that ampere per meter is the only correct unit for magnetic field strength H. Verification / Alternative check: Consider the simple formula H = I / (2 * pi * r) for the magnetic field strength around a long straight conductor, where I is current in amperes and r is distance in meters. The right side of the equation has units ampere divided by meter, which matches the unit of H. This dimensional check confirms that ampere per meter is the correct SI unit. No similar relationship would lead to joule per meter or kelvin per meter for H, which further validates the answer. Why Other Options Are Wrong: Joule per meter is related to energy per unit length and is not used as the SI unit for magnetic field strength, so option A is incorrect. Newton per meter appears in contexts like spring constant in Hooke's law or surface tension, but again it does not describe magnetic field strength, so option B is wrong. Kelvin per meter represents a temperature gradient and is used in heat transfer problems, not magnetism, so option C is also incorrect. Only option D, ampere per meter, correctly represents the SI unit of H. Common Pitfalls: One common mistake is confusing magnetic field strength H with magnetic flux density B and thinking of the tesla unit. While tesla is indeed a magnetic unit, it belongs to B, not H. Another pitfall is choosing units that seem mathematically reasonable without considering dimensional analysis. Training yourself to quickly check equations and units can help you avoid such confusion. When in doubt, relate the quantity back to its defining formula, as this often reveals the correct unit through dimensional reasoning. Final Answer: Ampere per meter

Question 11

If ice cubes floating on water in an open vessel are allowed to melt completely, what will happen to the water level in the vessel once all the ice has melted?

Accepted Answer

Introduction / Context: This question explores an interesting application of the principle of flotation and density. Ice is less dense than liquid water, which is why it floats. Many learners wonder whether the water level in a glass or vessel changes when floating ice melts. Understanding this situation helps to clarify Archimedes principle and the idea of displaced volume, and it also gives an intuitive explanation of why floating sea ice melting does not directly raise sea levels. Given Data / Assumptions: Ice cubes are floating on the surface of water in a vessel. The ice is completely above and below the water line as in a normal floating situation. The system is open to the atmosphere and no water is added or removed. The ice is allowed to melt completely without any evaporation losses being considered. Concept / Approach: When a body floats, it displaces a volume of liquid whose weight equals the weight of the body, as stated by Archimedes principle. Ice has a lower density than liquid water, so a portion of its volume remains above the surface, but the submerged part displaces water. The weight of this displaced water is equal to the weight of the ice. When the ice melts, it turns into liquid water of the same mass. This melted water exactly occupies the volume that was previously displaced, because mass is conserved and the density is now that of liquid water. As a result, the overall water level in the vessel remains unchanged after melting. Step-by-Step Solution: Step 1: Recognise that floating ice displaces a volume of water whose weight equals the weight of the ice.Step 2: Let the mass of a piece of ice be m; its weight is m * g, where g is acceleration due to gravity.Step 3: The displaced water has mass m as well, because its weight m * g equals the weight of the ice.Step 4: When the ice melts, it becomes water of mass m and density equal to that of the surrounding water.Step 5: This melted water occupies exactly the same volume as the water that was displaced earlier, so the water level remains the same. Verification / Alternative check: You can perform a simple experiment by placing a few ice cubes in a glass of water, marking the initial water level on the glass, and then waiting for the ice to melt at room temperature. You will observe that the final water level matches the initial mark closely if evaporation is negligible. This experimental check confirms the theoretical prediction. The reasoning also explains why melting of floating sea ice, such as icebergs, does not directly raise ocean levels, although other forms of ice melt, like land based glaciers, do have an effect. Why Other Options Are Wrong: If the water level increased, as in option A, it would imply that the melted water occupies more volume than the water originally displaced by the ice, which contradicts the equality of weights and densities. Option C, which suggests the level first rises and then falls, has no physical basis in this simple system. Option D, stating that the water level decreases, would imply that the melted water occupies less volume than was originally displaced, which is also incorrect. Only option B, that the water level does not change, is consistent with Archimedes principle and practical observation. Common Pitfalls: Many learners mistakenly think that melting ice always raises water levels because they focus on the extra water being added without considering displacement. Others wrongly apply intuition from adding ice to an empty container instead of to water. The key idea is that a floating object already causes a displacement equal to its weight. When it melts and becomes part of the liquid, the volume it occupies is exactly the same as the volume of water it previously displaced. Keeping this mass and displacement reasoning in mind helps avoid confusion in similar buoyancy problems. Final Answer: The water level does not change

Question 12

In gravitational physics, if the value of acceleration due to gravity at the surface of the earth is g, what is its value at a point located at a height of 2R above the surface (that is, at a distance 3R from the centre of the earth), where R is the…

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Introduction / Context: The acceleration due to gravity g at the surface of the earth has a standard approximate value of 9.8 metres per second squared. However, this value changes as we move away from the surface. The question asks you to determine how g changes at a specific height above the surface, expressed in terms of the earth's radius R. This is a typical conceptual numerical question in gravitation, testing your understanding of the inverse square dependence of gravitational field on distance from the centre of a spherical mass. Given Data / Assumptions: Acceleration due to gravity at the surface of the earth is g. Radius of the earth is R. We consider a point at a height of 2R above the surface, so its distance from the centre is 3R. The earth is treated as a spherically symmetric mass distribution. Concept / Approach: Outside a spherically symmetric body like the earth, the gravitational field behaves as if all mass were concentrated at the centre. The acceleration due to gravity at a distance r from the centre is given by g(r) = G * M / r^2, where G is the universal gravitational constant and M is the mass of the earth. At the surface, r = R and g = G * M / R^2. At a distance r = 3R, the new acceleration g(3R) becomes G * M / (3R)^2. By taking the ratio of g(3R) to g, we can find the new value in terms of g without knowing numerical values of G or M. Step-by-Step Solution: Step 1: Write the expression for acceleration due to gravity at distance r: g(r) = G * M / r^2.Step 2: At the surface, r = R, so g = G * M / R^2.Step 3: At height 2R above the surface, the distance from the centre is r = R + 2R = 3R.Step 4: The new acceleration is g(3R) = G * M / (3R)^2 = G * M / (9R^2).Step 5: Divide g(3R) by g to find the ratio: g(3R) / g = (G * M / 9R^2) / (G * M / R^2) = 1/9, so g(3R) = g / 9. Verification / Alternative check: You can check the trend qualitatively. As we move farther from the earth, the acceleration due to gravity must decrease. The distance has become three times the radius, so the inverse square law suggests the acceleration becomes one ninth of its surface value. This matches the derived result g/9. Since we expect a significant decrease but not negative or zero value at this moderate height, the answer g/9 is reasonable and consistent with physical intuition and the mathematical formula. Why Other Options Are Wrong: Option A, g/3, corresponds to an inverse first power dependence on distance, which is not correct for gravitational field; gravity follows a 1/r^2 law, not 1/r. Option B, g/4, would correspond to doubling the distance, not tripling it. Option D, g/2, again does not fit the inverse square relationship and would predict too weak a decrease. Only option C, g/9, is consistent with the factor of three increase in distance and the inverse square law, since 3^2 is 9. Common Pitfalls: Students often confuse height above the surface with distance from the centre, forgetting to add the radius of the earth. Another common mistake is applying a linear 1/r dependence rather than 1/r^2, leading to wrong factors such as g/3. To avoid these errors, always express distances from the centre of the earth and substitute carefully into the formula g(r) = G * M / r^2. Doing a quick sanity check on whether gravity should decrease strongly or slowly as distance changes also helps confirm the result. Final Answer: g/9

Question 13

In basic electricity, electrical energy in a conductor is fundamentally associated with the motion or behaviour of which type of subatomic particles?

Accepted Answer

Introduction / Context: Electricity is one of the most familiar forms of energy in daily life, powering lights, appliances, and electronic devices. At the microscopic level, electrical energy and current in metallic conductors are linked to the movement of charged particles. The question asks which type of subatomic particles are primarily responsible for electrical energy and current in ordinary conductors. Understanding this helps connect atomic structure with macroscopic electrical phenomena and clarifies the roles of protons, neutrons, and electrons. Given Data / Assumptions: We are considering electrical energy and current in typical solid conductors such as metals. Options include protons, neutrons, electrons, and all of the above. We assume basic atomic structure: protons and neutrons in the nucleus, electrons in shells around the nucleus. The conductor is neutral overall, but charge carriers can move. Concept / Approach: In metallic conductors, atoms have valence electrons that are loosely bound. These electrons can move freely through the lattice, forming an electron gas. When a potential difference is applied between the ends of a conductor, these free electrons drift toward the positive terminal, creating an electric current. Protons and neutrons are located in the nucleus and are bound strongly; they do not move through the conductor to produce current under normal conditions. Therefore, the flow of electrons is what carries electrical energy along a wire. Hence, electrons are the correct answer. Step-by-Step Solution: Step 1: Recall the structure of an atom: protons and neutrons in the nucleus, electrons in surrounding shells.Step 2: Recognise that metals have free or conduction electrons that are not tightly bound to any single atom.Step 3: When a voltage is applied, these free electrons acquire a net drift velocity, forming an electric current.Step 4: Note that protons and neutrons remain fixed in the lattice and do not move through the metal to produce current.Step 5: Conclude that electrical energy and current in conductors are associated mainly with moving electrons, so electrons is the correct choice. Verification / Alternative check: Look at basic circuit theory and experimental observations. The conventional direction of current is taken as from positive to negative, but the actual charge carriers in metals are electrons moving from negative to positive. Devices like cathode ray tubes and electron beams explicitly show streams of electrons. Semiconductor physics also focuses on electrons and holes, but even there, holes represent the absence of electrons rather than moving protons. These facts consistently point to electrons as the primary charge carriers in ordinary electrical circuits. Why Other Options Are Wrong: Protons are positively charged particles but are confined to atomic nuclei in solid conductors and do not drift through the lattice under normal electrical conditions, so option A is incorrect. Neutrons are electrically neutral and do not contribute to electric current in a conductor, making option B wrong. Option D, all of the above, would imply that protons, neutrons, and electrons all move to create electrical energy, which is not true for standard circuits. Only electrons, option C, are responsible for current in ordinary metallic conductors. Common Pitfalls: Some learners mistakenly think that protons move inside wires because current direction is from positive to negative. However, conventional current direction is a historical choice that does not match the actual direction of electron motion. Another pitfall is to generalise from special plasma or ionised gas situations where positive ions may move, but such cases are not typical for solid metal wires. For school level physics and general circuits, always remember that electrons are the charge carriers responsible for electrical energy flow in conductors. Final Answer: Electrons

Question 14

The orbital velocity required to place a spacecraft in a circular orbit around the moon is how related to the orbital velocity required to place it in a similar circular orbit around the earth?

Accepted Answer

Introduction / Context: Orbital velocity is the minimum horizontal speed required for a satellite or spacecraft to remain in a stable circular orbit around a celestial body, such as the earth or the moon. It depends on the mass and radius of the central body. The question asks you to compare the orbital velocity around the moon with that around the earth. This comparison tests your understanding of gravitational strength and orbital mechanics, which are important topics in space science and physics. Given Data / Assumptions: We consider circular orbits close to the surfaces of the earth and the moon for comparison. Orbital velocity depends on mass M of the central body and radius r of the orbit. The mass of the moon is much smaller than the mass of the earth. The radius of the moon is also smaller than the radius of the earth. Concept / Approach: For a satellite in a circular orbit of radius r around a body of mass M, the orbital velocity v is given by v = sqrt(G * M / r), where G is the gravitational constant. For the earth, M is large and r is relatively large. For the moon, both M and r are smaller, but the mass difference is much more significant than the radius difference. The result is that the gravitational pull of the moon is weaker, and hence the orbital velocity required around the moon is lower than that around the earth. Therefore, the correct qualitative comparison is that orbital velocity around the moon is less than that around the earth. Step-by-Step Solution: Step 1: Recall the formula for orbital velocity: v = sqrt(G * M / r).Step 2: For the earth, call the orbital velocity v_e = sqrt(G * M_e / r_e).Step 3: For the moon, call the orbital velocity v_m = sqrt(G * M_m / r_m).Step 4: Note that M_m is much less than M_e, while r_m is smaller than r_e, but the mass difference dominates.Step 5: Since the gravitational field of the moon is weaker, v_m is less than v_e, so the orbital velocity around the moon is lower than around the earth. Verification / Alternative check: Numerically, the approximate first cosmic velocity near the earth's surface is about 7.9 kilometres per second. The corresponding orbital velocity near the moon is significantly smaller, roughly around 1.7 kilometres per second. These values are consistent with the reasoning from the formula and show clearly that the orbital velocity around the moon is less. Therefore, the qualitative statement that the required velocity around the moon is less than that around the earth is correct and matches both theory and real space mission data. Why Other Options Are Wrong: Option B, greater than the orbital velocity around the earth, would suggest that the moon's gravity is stronger than the earth's, which is false. Option C, equal to the orbital velocity around the earth, ignores the large difference in mass and size between the two bodies and contradicts the formula. Option D, greater than or equal to, also implies the possibility of equal or greater value, which is not supported by gravitational data. Only option A correctly states that the orbital velocity around the moon is less than that around the earth. Common Pitfalls: A common mistake is to assume that a smaller body requires higher speed to stay in orbit because its radius is smaller. However, the dominant factor in gravitational attraction is mass. Another pitfall is ignoring the formula and relying solely on guesswork. To avoid such errors, always recall that orbital velocity depends on both mass and radius through v = sqrt(G * M / r), and for the moon the significantly lower mass ensures a lower orbital velocity compared to the earth. Final Answer: Less than the orbital velocity around the earth

Question 15

A bucket of water is hung from a spring balance. A solid piece of iron is suspended in the water using a thread so that it is fully immersed but does not touch the sides or the bottom of the bucket. How does the reading of the spring balance change …

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Introduction / Context: This question illustrates the application of Archimedes principle and Newton's third law in a slightly indirect situation. A bucket of water is suspended from a spring balance, and then an iron piece is immersed in the water without touching the bucket itself. Many students are unsure whether the reading on the spring balance will increase, decrease, or remain the same. Understanding this scenario requires careful analysis of forces, buoyancy, and the interaction between the iron piece, water, and bucket. Given Data / Assumptions: A bucket filled with water is hung from a spring balance. The initial reading shows the weight of the bucket plus water. An iron piece is suspended by a separate thread and fully immersed in the water. The iron piece does not touch the sides or the bottom of the bucket. Concept / Approach: When the iron piece is immersed in water, it experiences an upward buoyant force equal to the weight of the displaced water, according to Archimedes principle. For this buoyant force to exist, the water must exert an upward force on the iron piece. By Newton's third law, the iron piece exerts an equal and opposite downward force on the water. This additional downward force is transmitted to the bucket and then to the spring balance. Therefore, the total downward force acting on the spring balance increases by an amount equal to the buoyant force, causing the reading to increase. The depth of immersion does not change the magnitude of buoyant force as long as the volume of displaced water remains the same. Step-by-Step Solution: Step 1: Initially, the spring balance reading equals the weight of the bucket plus the water.Step 2: When the iron piece is submerged, it displaces a certain volume of water and experiences an upward buoyant force F_b.Step 3: The water exerts this upward force on the iron piece; by Newton's third law, the iron piece exerts a downward force of equal magnitude F_b on the water.Step 4: This downward force on the water is transmitted to the bucket and then to the spring balance, effectively increasing the total load.Step 5: Therefore, the spring balance reading becomes the original weight plus the additional downward force F_b, so the reading increases. Verification / Alternative check: Imagine replacing the immersed iron piece and thread with an equivalent extra volume of water that exactly equals the displaced water. The system would then simply have more water in the bucket, clearly increasing its weight and, thus, the spring balance reading. Since the effect of the immersed body is to make the water support part of its weight via buoyancy, the net effect on the bucket is the same as adding that much weight of water. This analogy confirms that the reading must increase when the iron piece is immersed. Why Other Options Are Wrong: Option B suggests that the reading decreases, which would require a reduction in the total downward force, contradicting the fact that the water now has to support part of the iron's weight. Option C states no change, which would mean the downward reaction from the water on the iron is ignored, violating Newton's third law. Option D claims that the reading varies with depth, but for a fully immersed object of fixed volume, the buoyant force is constant and independent of depth, so the additional load is fixed. Only option A stating that the reading increases is consistent with fluid mechanics and dynamics. Common Pitfalls: One common mistake is to think only about the weight of the iron being supported by the thread and to ignore the action reaction pair between the water and the iron piece. Another pitfall is to assume that buoyant force somehow reduces the total load on the spring balance, when in fact it represents an extra downward force transmitted through the water to the bucket. To avoid confusion, always draw free body diagrams for each object separately and carefully apply Newton's third law and Archimedes principle. Final Answer: The reading of the spring balance increases

Question 16

In atmospheric physics and nuclear chemistry, radiocarbon (carbon-14) present in the atmosphere is continually produced mainly as a result of which process?

Accepted Answer

Introduction / Context: Radiocarbon, or carbon 14, is a radioactive isotope of carbon that is very important in archaeology and geology because it is used for radiocarbon dating of organic materials. The presence of carbon 14 in the atmosphere is not permanent; it is continuously produced by nuclear reactions. The question asks you to identify the main process responsible for the formation of radiocarbon in the upper atmosphere. Understanding this process links concepts from cosmic rays, nuclear reactions, and environmental science. Given Data / Assumptions: Radiocarbon refers to carbon 14, an isotope of carbon. Its production occurs in the atmosphere. Options include collisions involving neutrons and nitrogen, ultraviolet light acting on oxygen, solar radiation acting on carbon dioxide, and lightning discharges. We consider established nuclear reaction pathways in the upper atmosphere. Concept / Approach: High energy cosmic rays from space strike the earth's atmosphere and produce cascades of secondary particles, including fast neutrons. These neutrons can collide with nitrogen 14 nuclei, which are abundant in the atmosphere. A typical reaction is nitrogen 14 plus a neutron giving carbon 14 and a proton. In equation form, N 14 + n gives C 14 + p. This nuclear reaction converts nitrogen into radiocarbon. The newly formed carbon 14 combines with oxygen to form carbon dioxide containing carbon 14, which then mixes into the global carbon cycle. Ultraviolet light mainly affects ozone and oxygen chemistry, not carbon 14 production. Lightning and general solar radiation do not produce radiocarbon in the primary and continuous way that neutron nitrogen collisions do. Step-by-Step Solution: Step 1: Recognise that cosmic rays generate high energy neutrons in the upper atmosphere.Step 2: Note that the atmosphere contains a large amount of nitrogen 14, which can interact with these neutrons.Step 3: The key nuclear reaction is N 14 plus a neutron gives C 14 plus a proton.Step 4: This reaction directly converts nitrogen nuclei into radiocarbon, carbon 14.Step 5: Therefore, radiocarbon is produced mainly by collisions between fast neutrons and nitrogen nuclei in the atmosphere. Verification / Alternative check: Standard descriptions of radiocarbon dating in physics and chemistry textbooks explicitly mention the neutron nitrogen reaction as the source of carbon 14. The rate of production depends on cosmic ray intensity and atmospheric composition, which explains variations in radiocarbon levels over time. Other processes involving ultraviolet light or lightning are important for different atmospheric phenomena, such as ozone formation or nitrogen oxides, but they are not identified as the main source of carbon 14. This agreement between multiple educational sources and the nuclear reaction pathway confirms the correctness of the chosen process. Why Other Options Are Wrong: Ultraviolet light from the sun mainly causes photodissociation of oxygen and ozone, leading to ozone layer chemistry, but does not create carbon 14, so option B is incorrect. Solar radiation acting directly on carbon dioxide does not change stable carbon isotopes into carbon 14 in any dominant way under normal atmospheric conditions, so option C is wrong. Lightning discharges can produce nitrogen oxides and other chemical species but are not the primary source of radiocarbon, so option D is also incorrect. Only option A correctly identifies the collision between fast neutrons and nitrogen nuclei as the main mechanism for radiocarbon production. Common Pitfalls: Students may be tempted to pick options that mention solar or ultraviolet radiation because they seem naturally connected to atmospheric processes. Another pitfall is to overestimate the role of lightning in nuclear reactions, even though its main effects are chemical rather than nuclear. To avoid such errors, remember that producing a different isotope usually requires nuclear processes, which are more likely to involve high energy particles like neutrons rather than ordinary photons or electric discharges. Associating radiocarbon production with cosmic ray neutrons and nitrogen 14 helps fix the correct concept in memory. Final Answer: Collision between fast neutrons and nitrogen nuclei present in the atmosphere

Question 17

In measurement science, the radian is the SI unit used to measure which of the following physical quantities?

Accepted Answer

Introduction / Context: Angles are fundamental in geometry, trigonometry, and physics, and there are several ways to measure them. Degrees are common in everyday usage, but in mathematics and physics the radian is the preferred unit because it simplifies many formulas. The question checks whether you know which physical quantity is measured in radians according to the SI system. Recognising this helps you interpret trigonometric functions, wave equations, and rotational motion correctly. Given Data / Assumptions: The unit in question is the radian. Options for the measured quantity include temperature, intensity of flame, angle, and solid angle. We assume the standard SI usage where radians appear in formulas involving circular motion and periodic functions. Solid angle has its own unit, which is related but different. Concept / Approach: A radian is defined based on the geometry of a circle. One radian is the angle subtended at the centre of a circle by an arc whose length equals the radius of the circle. Mathematically, a full circle corresponds to 2 * pi radians, which is equal to 360 degrees. Therefore, radian is a unit of plane angle, not of temperature or intensity. Solid angle, which measures the size of an object as seen from a point in three dimensional space, is measured in steradians, not radians. Thus, among the given options, angle is the correct physical quantity measured in radians. Step-by-Step Solution: Step 1: Recall that radian is defined using arc length and radius in a circle.Step 2: Recognise that this definition clearly refers to a plane angle at the centre of the circle.Step 3: Note that temperature is measured in kelvin or degrees Celsius, not in radians.Step 4: Understand that intensity of flame could be measured in power units or brightness units, not radians.Step 5: Identify that solid angle uses steradian as its SI unit, leaving angle as the only correct choice for radian. Verification / Alternative check: Consider standard mathematical formulas such as the trigonometric expansions sin x and cos x, where x is in radians for the series to hold directly. In physics, formulas for simple harmonic motion, wave propagation, and rotational kinematics also use angular quantities in radians. For example, in circular motion, the relation between arc length s, radius r, and angle theta is s = r * theta, where theta must be in radians. No similar equations use radians for temperature or intensity. Solid angle equations, such as those involving flux passing through an area, specifically use steradians. These examples confirm that radians are units of plane angle. Why Other Options Are Wrong: Temperature is a measure of hotness or coldness and has SI unit kelvin, so option A is incorrect. Intensity of flame is related to energy output or brightness and does not use radians as a unit, making option B wrong. Solid angle has its own SI unit called steradian, which is different from radian, so option D is incorrect. Only option C, angle, correctly matches the radian as a unit of measurement. Common Pitfalls: Students sometimes confuse radians and steradians because they sound similar. Remember that a radian describes a plane angle in two dimensional geometry, while a steradian describes a solid angle in three dimensional space. Another mistake is to rely only on degrees and think of radians as an abstract concept. Practising conversions between degrees and radians and using formulas where radians appear naturally will solidify your understanding that radians measure ordinary plane angles. Final Answer: Angle

Question 18

When a bullet is fired from a gun, what happens to the gun itself according to the law of conservation of momentum?

Accepted Answer

Introduction / Context: This question illustrates the law of conservation of linear momentum using the familiar example of firing a bullet from a gun. Before firing, both gun and bullet are at rest, so the total momentum of the system is zero. After firing, the bullet moves forward with high speed, and the gun responds in a characteristic way. Understanding this behaviour is essential in mechanics because it connects force, mass, and motion and explains why firearms produce a backward kick or recoil when used. Given Data / Assumptions: The system consists of a gun and a bullet. Initially, both gun and bullet are at rest, so total momentum is zero. On firing, the bullet is projected forward with a certain velocity. No external horizontal forces act on the system during the short firing interval. Concept / Approach: The law of conservation of linear momentum states that if no external force acts on a system, the total momentum of the system remains constant. In the horizontal direction for a gun bullet system, external forces such as friction can be neglected during the brief firing event. Therefore, the momentum gained by the bullet must be balanced by an equal and opposite momentum of the gun. Because the gun has a much larger mass than the bullet, it moves backward with a smaller velocity, but the product of mass and velocity (momentum) has equal magnitude and opposite direction to the bullet's momentum. Step-by-Step Solution: Step 1: Initially, gun and bullet are at rest, so total momentum is zero.Step 2: When the gun is fired, the bullet of mass m_b moves forward with velocity v_b, so its momentum is m_b * v_b.Step 3: To conserve momentum, the gun of mass m_g must acquire momentum of equal magnitude but opposite direction.Step 4: Let the gun recoil velocity be v_g backward; then m_g * v_g = m_b * v_b in magnitude.Step 5: The gun therefore recoils backward with momentum equal in magnitude but opposite in direction to that of the bullet, satisfying momentum conservation. Verification / Alternative check: Observations from real life confirm this analysis. When a shotgun or rifle is fired, the shooter feels a backward kick. Heavier guns have smaller recoil velocities, while lighter guns or higher powered cartridges produce stronger recoil effects. This matches the relationship m_g * v_g = m_b * v_b. In addition, recoil damping systems and shoulder supports are used to manage this backward motion. If the gun did not move, total momentum after firing would not be zero, contradicting the conservation law. Therefore, a backward recoil is both theoretically and practically necessary. Why Other Options Are Wrong: Option A, stating that the gun moves slightly forward, contradicts the requirement that momentum must be opposite to that of the bullet. Option B, saying the gun jumps only upward, might describe some observed motion due to the shooter's grip and gravity, but the primary reaction is backward, not purely upward. Option C, claiming that the gun does not move, violates conservation of momentum because the bullet acquires forward momentum that must be balanced. Only option D correctly states that the gun recoils backward with the same magnitude of momentum as the bullet but in the opposite direction. Common Pitfalls: Some learners think of the gun as fixed and assume that only the bullet moves, ignoring the fact that the gun is free to recoil. Another pitfall is to confuse velocity with momentum and forget that a large mass with small velocity can have the same momentum as a small mass with large velocity. Remember to apply conservation laws to the entire system and not only to one part. Doing so will guide you to recognise recoil as an unavoidable consequence of momentum conservation in firearms and similar systems like rockets and jet propulsion. Final Answer: The gun recoils backward with the same momentum as the bullet in the opposite direction

Question 19

For highly accurate scientific work in laboratories, temperatures are most often measured using which type of thermometer?

Accepted Answer

Introduction / Context: Temperature measurement is essential in scientific experiments, industrial processes, and everyday applications. However, different thermometers are suitable for different levels of precision and temperature ranges. The question asks which type of thermometer is commonly used for accurate scientific work, especially in laboratories where reliability and precision are crucial. Understanding the advantages of different thermometer types helps in choosing the right instrument for a given measurement task. Given Data / Assumptions: The context is accurate scientific temperature measurement. Options include mercury thermometers, alcohol thermometers, platinum resistance thermometers, and thermoelectric thermometers. We assume the need for high precision, stability, and repeatability over a wide range. We focus on standard laboratory practice rather than casual domestic use. Concept / Approach: Platinum resistance thermometers, also called platinum resistance temperature detectors, are widely regarded as standards for precise temperature measurement. They are based on the predictable change in electrical resistance of a platinum wire with temperature. Platinum is chemically stable and has a nearly linear resistance temperature relationship over a wide range. This allows for very accurate calibration and highly reproducible measurements. Mercury thermometers are useful and fairly accurate, but they are limited by the freezing point of mercury and safety concerns. Alcohol thermometers are used for low temperatures but are less precise. Thermoelectric thermometers, or thermocouples, are robust and cover very wide ranges but are generally less accurate than platinum resistance thermometers in precision laboratory work. Step-by-Step Solution: Step 1: Identify that the question emphasises accurate scientific work, not just ordinary temperature readings.Step 2: Recall that platinum resistance thermometers are used as standard instruments in many metrology laboratories.Step 3: Note that these devices measure temperature by monitoring the resistance of a platinum element, which changes reliably with temperature.Step 4: Compare this with mercury and alcohol thermometers, which rely on expansion of liquids and have practical limitations.Step 5: Conclude that platinum resistance thermometers are the most suitable for precise scientific measurements. Verification / Alternative check: National standards laboratories and calibration centres often refer temperatures to platinum resistance thermometers, especially near the triple point of water and other fixed points used to define temperature scales. Instrumentation handbooks also highlight platinum resistance sensors for high accuracy industrial control. While thermocouples are common in industry because of their durability and wide range, they usually require complex compensation and are not as inherently precise as platinum resistance sensors. These practical references confirm that platinum resistance thermometers are the preferred choice when accuracy is the primary concern. Why Other Options Are Wrong: Mercury thermometers, option A, provide good accuracy for many purposes but are limited by the physical properties of mercury and pose safety issues due to toxicity, so they are not the first choice for highest precision work. Alcohol thermometers, option B, are suited for low temperature measurements but are less accurate and less linear than platinum resistance sensors. Thermoelectric thermometers, option D, are robust and cover wide ranges but usually have lower accuracy and require careful calibration; they are better for industrial rather than reference standards. Therefore, only option C, platinum resistance thermometers, matches the requirement of accurate scientific measurement. Common Pitfalls: Students often choose mercury thermometers because they have seen them commonly in laboratories and clinics. However, common use does not always imply highest accuracy. Another pitfall is to assume that complex sounding devices like thermoelectric thermometers must be more accurate, when in reality their strength is range and robustness, not ultimate precision. To avoid confusion, remember that platinum resistance thermometers have become the standard for precision temperature measurement and form the basis for many temperature scales in modern metrology. Final Answer: Platinum resistance thermometers

Question 20

Hot water is poured simultaneously into four metallic tumblers that are painted on the outside with rough white, rough black, shining white, and shining black paints. After some time, in which painted tumbler will the water be found to have cooled t…

Accepted Answer

Introduction / Context: This question tests basic ideas of heat transfer by radiation and the effect of colour and surface finish on cooling. When hot water is placed in metallic tumblers that are painted differently on the outside, the rate at which heat leaves the water through the tumbler surface depends strongly on whether the surface is dark or light, and rough or shiny. Understanding this concept is very important in everyday physics applications such as designing radiators, cooking utensils, and thermos flasks. Given Data / Assumptions: Four metallic tumblers are identical in size, shape, and material. The outer surfaces are painted rough white, rough black, shining white, and shining black. The same quantity of hot water at the same initial temperature is poured simultaneously into each tumbler. Surrounding conditions such as room temperature and air movement are assumed to be the same for all four. Concept / Approach: Heat from the hot water is lost mainly through conduction through the metal and then radiation and convection from the outer painted surface. Since all tumblers are metallic and identical, the main difference in cooling rate comes from how effectively the outer paint allows radiant heat to be emitted. Black and rough surfaces are very good absorbers and emitters of thermal radiation. Shining and white surfaces are poor emitters and better reflectors of radiation. Therefore, the tumbler that behaves as the best radiator will cool the water fastest. Step-by-Step Solution: Step 1: Recognise that all tumblers conduct heat from water to metal at nearly the same rate because the metal and thickness are assumed identical. Step 2: Realise that the controlling factor for the rate of cooling is how easily heat leaves from the outer painted surface by radiation and convection. Step 3: Recall the rule from heat transfer: a black and rough surface is the best emitter and absorber of radiant heat, while a white or shiny surface is a poor emitter. Step 4: Compare the four paints: rough black is best for emission, rough white is worse, and both shining surfaces are poorer radiators than rough surfaces. Step 5: Conclude that the tumbler painted rough black will lose heat to the surroundings most rapidly and therefore its water will cool the most in a given time. Verification / Alternative check: A useful check is to recall practical examples. Radiators and the backs of some refrigerators are often painted dull black to enhance heat emission. On the other hand, thermos flasks and some cooking vessels use shiny or light coloured exteriors to reduce heat loss. These real life designs support the conclusion that a rough black surface promotes maximum cooling. Why Other Options Are Wrong: Rough white: Roughness helps, but white colour is a poor emitter of radiation compared with black, so cooling is less than rough black. Shining white: Both white colour and shiny finish reduce emission, so this tumbler cools the water more slowly. Shining black: Black colour helps, but the shiny surface still reflects more radiation and emits less than a rough black surface. Common Pitfalls: Many learners think that shiny black surfaces must be the best emitter simply because they are black. However, shine reduces emissivity. Others focus only on colour and ignore the role of roughness versus smoothness. It is important to consider both factors together, which is why rough black is superior to shining black in terms of radiative heat loss. Final Answer: The water cools the most in the tumbler painted with a rough black surface.

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