Question 1

In practical navigation, for what purpose is a sextant instrument commonly used on ships and boats?

Accepted Answer

Introduction / Context: Before the widespread use of satellite based global positioning systems, sailors relied heavily on optical instruments to determine their position at sea. One such classic navigational instrument is the sextant. Understanding what a sextant does helps students connect physics concepts of angles and optics with real world applications in navigation and astronomy. Given Data / Assumptions: The instrument mentioned is a sextant, a handheld optical device. It is used primarily in navigation on ships, boats, and sometimes in aircraft. The environment of interest is the open sea, where distant horizons and celestial bodies are visible. The problem asks about the main function, not its detailed construction. Medical options are included as distractors and are not relevant to the real use of a sextant. Concept / Approach: A sextant is designed to measure the angle between two clearly visible objects, typically a celestial body such as the Sun, Moon, planet, or star, and the visible horizon. This angular measurement, when combined with accurate time and suitable astronomical tables, allows navigators to determine their latitude and sometimes longitude. The principle behind the sextant uses reflection of light in mirrors to bring two images into coincidence and then read the angle on a graduated arc. It is not a medical instrument and has no role in gynaecology or birth control. Step-by-Step Solution: Step 1: Recognise that navigation at sea often requires determining position using celestial bodies when landmarks are absent.Step 2: A sextant is built with mirrors and a graduated arc that allows precise measurement of the angular separation between a celestial object and the horizon.Step 3: The navigator looks through the telescope portion of the sextant and moves a mirror until the image of the celestial body just touches the horizon line.Step 4: The angle read off the scale is then used with navigation tables and the time of observation to calculate position, which is the central purpose of the sextant. Verification / Alternative check: Historical accounts of ocean voyages, especially during the age of exploration and even well into the twentieth century, frequently mention the use of sextants for celestial navigation. Pilot training material similarly reinforces that sextants are used for measuring altitude angles of celestial bodies. No standard medical textbook lists sextants as instruments for any gynaecological or birth control procedure. This confirms that the association of sextants is firmly with navigation, not with medical treatment. Why Other Options Are Wrong: Option b is wrong because gynaecology uses specialised medical tools, not sextants designed for angular measurements. Option c incorrectly links the sextant to birth control, which is based on medical and hormonal methods, not navigation instruments. Option d is too vague and still refers to medical treatment, which is unrelated. Option e again refers to clinical monitoring instruments like sphygmomanometers, which are entirely different from optical navigation devices. Common Pitfalls: Some learners may confuse the names of instruments and assume that any unknown instrument name might be medical. Others may think that because sextant sounds technical, it could be used in various fields. The key is to remember that the prefix sex in sextant refers to the sixth part of a circle, since the instrument traditionally covers an arc of approximately sixty degrees. This numerical origin links it to angular measurement and navigation, not to reproductive medicine. Remember that a sextant is firmly a navigation and astronomy tool. Final Answer: Measuring angles between celestial objects and the horizon during navigation at sea.

Question 2

In which everyday electronic device is a transistor most likely used as a tiny amplifier or switch?

Accepted Answer

Introduction / Context: Transistors are fundamental semiconductor components used as amplifiers and switches in countless electronic devices. This question focuses on recognising a real life application where a transistor function is crucial. Identifying the correct example helps students connect abstract semiconductor theory to concrete gadgets they encounter in daily life. Given Data / Assumptions: A transistor is a small semiconductor device that can amplify current or act as an electronic switch. We are comparing several common devices: a wrist watch, a fuse, a hearing aid, and a fluorescent lamp. The question asks where a transistor is most likely to be found in a central functional role. Mechanical and purely passive devices will not typically contain active semiconductor components like transistors. The focus is on classical versions of these devices, not advanced modern hybrids with extra electronics. Concept / Approach: A transistor is especially useful wherever a weak electrical signal must be amplified into a stronger one. Hearing aids take very small sound signals from a microphone and must increase them to a level that can drive a tiny loudspeaker placed near the ear. That amplification is most efficiently done by transistor amplifiers. Fuses, mechanical wrist watches, and simple fluorescent tubes, in their basic forms, do not require such amplification and can function without transistors, relying instead on mechanical motion, thermal effects, or gas discharge phenomena. Step-by-Step Solution: Step 1: Recall that the main role of a transistor is to amplify signals or to act as an electronic switch in circuits.Step 2: Note that a hearing aid must take a very small electrical signal from a microphone and increase it significantly to help a person with hearing difficulty.Step 3: Such amplification is achieved through transistor based amplifier circuits inside the hearing aid device.Step 4: Therefore, among the given options, a hearing aid is the device where a transistor is most likely to be found in its core electronics. Verification / Alternative check: Modern hearing aids are described in electronics textbooks as containing microphone stages, transistor or integrated circuit amplifiers, and miniature speakers. By contrast, a standard household fuse simply melts a metal wire when current exceeds a safe value, needing no transistor. Mechanical wrist watches use springs and gears, and old style fluorescent tubes rely on gas discharge and a ballast, not on transistor action, although modern electronic ballasts may contain transistors separately. The classical association remains strongest with the hearing aid. Why Other Options Are Wrong: Option a is wrong because a purely mechanical wrist watch does not require electronic components; it works by mechanical energy stored in a spring. Option b is incorrect since a fuse is a simple sacrificial conductor and does not use semiconductor devices. Option d is not correct because the light in a basic fluorescent lamp is produced by an electric discharge through gas, not by transistor based amplification. Option e is wrong because a mercury thermometer works using thermal expansion of mercury, not electronics. Common Pitfalls: Students may generalise that all modern devices contain transistors somewhere, which is often true in a broad sense but does not answer which device fundamentally relies on them. Another confusion arises when learners think of digital wrist watches, which do contain integrated circuits, but the question usually refers to the general physical principle rather than special modern variants. The safest approach is to pick the device whose very function is based on amplifying tiny signals, which clearly points to hearing aids. Final Answer: Inside a hearing aid where weak sound signals must be amplified.

Question 3

In basic mechanics, the strength of a force is usually expressed by which of its characteristics?

Accepted Answer

Introduction / Context: In physics, a force is a vector quantity that has both magnitude and direction. When we talk about the strength of a force in everyday language and in scientific discussions, we are usually referring to how big the force is. This question checks whether students can relate the idea of strength to the proper physical term, namely the magnitude of the force, typically measured in newtons. Given Data / Assumptions: A force is acting on a body, and we want to describe its strength. Force is a vector, so it has magnitude, direction, and point of application. Standard SI unit of force is the newton. The wording strength suggests a quantitative measure, not just a description of direction or interaction. We are not dealing with advanced concepts like impulse or torque, only basic force description. Concept / Approach: When describing how strong a force is, physicists focus on its magnitude: the numerical value of the force along with the unit newton. While direction and point of application are also important for fully specifying a force, they do not directly express strength. The interaction with other forces, such as whether forces combine or cancel, is a separate idea relating to net force. To answer the question, we link the everyday word strength to the scientific term magnitude. Step-by-Step Solution: Step 1: Recall that a force vector can be fully described by magnitude, direction, and point of application.Step 2: Understand that the perceived strength of a force is how large the push or pull is, which is a magnitude question.Step 3: Recognise that this magnitude is measured using units such as newtons and can be represented by a number on a scale or diagram.Step 4: Conclude that the correct term corresponding to the strength of a force is its magnitude, not its direction, motion, or duration. Verification / Alternative check: As a simple check, consider two forces acting on an object, 5 newtons and 20 newtons, along the same line. We would naturally say that the 20 newton force is stronger. Here, the word stronger clearly refers to the larger magnitude, not a different direction or interaction. If we change the direction while keeping the magnitude fixed, we do not say the force has become stronger or weaker, only that it acts in another direction. This confirms that strength corresponds to magnitude. Why Other Options Are Wrong: Option a is wrong because motion describes the movement of an object, not the strength of the force causing that motion. Option b is incorrect since direction tells us where the force is pointing but not how big it is. Option c is wrong because interaction refers to how forces combine or balance, not to the inherent strength of any single force. Option e is not correct either, since the duration is related to impulse, not to the immediate strength expressed in newtons. Common Pitfalls: Students sometimes confuse force with motion and think that faster motion always means stronger force, which is not generally true. Others may focus too much on direction when learning vectors, forgetting that magnitude is an essential part. It is also easy to mix up the effect of a force over time, such as impulse, with the instantaneous strength of the force. Keeping the distinction clear between magnitude, direction, and time of action helps avoid these misunderstandings in basic mechanics. Final Answer: Its magnitude, that is, how large the force is in newtons.

Question 4

Which of the following scientific terms is not normally associated with the description of sound waves?

Accepted Answer

Introduction / Context: Different branches of physics use different units and terms to quantify physical quantities. Sound waves involve quantities like frequency, intensity, and speed, while optics deals with light intensity and brightness. This question checks whether you can identify which unit is not typically linked with sound, reinforcing the correct associations between physical quantities and their units. Given Data / Assumptions: We are dealing with sound waves, which are mechanical waves that propagate through a medium. Terms like hertz and decibel are often mentioned when discussing sound. Candela is a unit that students encounter in the context of light and brightness. Mach number appears in discussions of speeds relative to the speed of sound. Amplitude is a general wave property and can apply to sound, water, or light waves. Concept / Approach: Sound waves are longitudinal mechanical waves, so their description involves frequency, wavelength, amplitude, speed, and sound level. Frequency is measured in hertz, and sound level in decibels. Mach number compares a speed to the speed of sound and is widely used in aerodynamics. Candela, on the other hand, is the SI unit of luminous intensity and belongs to optics, not acoustics. Thus, candela is the term not associated with sound waves in standard physics usage. Step-by-Step Solution: Step 1: Recall that frequency of any periodic wave, including sound, is measured in hertz.Step 2: Remember that sound intensity level, particularly in hearing studies and acoustics, is measured in decibels.Step 3: Recognise that Mach number is a dimensionless quantity describing how fast an object moves relative to the speed of sound in the surrounding medium.Step 4: Identify that candela is defined as the SI unit of luminous intensity, which quantifies the perceived power of light, making it unrelated to sound wave description. Verification / Alternative check: Textbooks on acoustics consistently mention hertz and decibel when discussing pitch and loudness. Aircraft design and meteorology texts discuss Mach number in relation to sonic and supersonic speeds but still in the context of sound speed. Conversely, optics and photometry references define candela when describing brightness of lamps, lasers, and other light sources. Since candela appears in the SI base units list as a separate optical quantity, it clearly does not describe any property of sound waves. Why Other Options Are Wrong: Option a is correct for sound because frequency in hertz is a fundamental property of sound waves and determines perceived pitch. Option b is also associated with sound, as decibel is used for sound level and noise measurements. Option d is linked to sound through the definition of Mach number as a ratio to the speed of sound in a medium. Option e is a general wave property and is absolutely relevant for sound waves, since amplitude relates to sound intensity and loudness. Only candela is clearly outside the acoustics domain. Common Pitfalls: Students may see Mach associated with high speed aircraft and think it belongs exclusively to mechanics, forgetting that its definition uses the speed of sound. Others may think amplitude belongs only to transverse waves, but amplitude simply measures maximum displacement and is valid for longitudinal waves as well. Confusion can also arise because all the terms sound technical; carefully recalling the physical quantity each unit measures helps avoid such mistakes. The key is to remember that candela refers to light, not sound. Final Answer: Candela, the unit used to measure luminous intensity of light.

Question 5

If an electron and a photon have the same wavelength, which physical quantity will definitely have the same value for both?

Accepted Answer

Introduction / Context: In modern physics, wave particle duality tells us that both matter particles like electrons and radiation like photons can show wave behaviour. De Broglie and Einstein relations connect momentum and energy with wavelength and frequency. This question checks whether you can pick the correct quantity that must match when an electron and a photon share the same wavelength. Given Data / Assumptions: We are considering one electron and one photon. Both are assumed to have the same de Broglie or electromagnetic wavelength. The Planck constant h is the same for both. The electron has non zero rest mass, while the photon has zero rest mass. We ignore external fields and focus on idealised free particles. Concept / Approach: For any particle that shows wave behaviour, the de Broglie relation connects linear momentum p with wavelength lambda through p = h / lambda. This relation holds for photons and for matter waves such as electrons. Therefore, if lambda is the same for the electron and the photon, and h is a universal constant, they must have the same linear momentum. Their velocities and energies, however, are determined by additional properties like mass and dispersion relations, so they will generally not match. Step-by-Step Solution: Step 1: Write the de Broglie relation for any particle that exhibits wave behaviour as p = h / lambda.Step 2: Note that h, the Planck constant, is the same for the electron and the photon.Step 3: Since the wavelength lambda is given to be the same for both particles, the momentum p computed from h / lambda must also be the same.Step 4: Conclude that linear momentum is the quantity that must match, while velocity and energy can differ because of the difference in rest mass and dispersion relations. Verification / Alternative check: For a photon, momentum can also be written as p = E / c, where E is energy and c is the speed of light. For an electron, classical or relativistic formulas relate momentum, mass, and velocity in different ways. Because c is fixed and the electron speed is usually less than c, energy and speed do not generally match between the two particles. However, using the common wavelength and the relation p = h / lambda directly shows that the linear momenta must coincide. This cross check confirms the choice of linear momentum as the correct answer. Why Other Options Are Wrong: Option a is wrong because wavelength does not uniquely determine speed for massive particles; the electron speed depends on its kinetic energy and mass. Option c is incorrect since angular momentum is not determined solely by wavelength; it depends on orbital or spin properties. Option d is not valid because energy expressions differ for photons and electrons; for photons E = hc / lambda, but for electrons energy includes rest mass and kinetic terms. Option e is clearly wrong because the photon has zero rest mass, while the electron has non zero rest mass, irrespective of wavelength. Common Pitfalls: Students sometimes equate wavelength directly with energy without considering the type of particle, assuming that same wavelength implies same energy. This is true only within a single category such as photons. Another mistake is to forget that massless and massive particles obey different energy momentum relations. Keeping in mind the universal de Broglie relation p = h / lambda helps focus on linear momentum as the common link between wave behaviour of different particles. Final Answer: Their linear momentum, because momentum is proportional to h divided by wavelength.

Question 6

During the ideal motion of a projectile fired from the Earth surface, which component of its velocity remains constant?

Accepted Answer

Introduction / Context: Projectile motion is a standard topic in introductory mechanics. A projectile launched into the air under the influence of gravity follows a curved path, and its velocity changes with time. Understanding which components change and which remain constant is essential for solving many problems. This question reinforces the idea that gravity acts vertically, leaving horizontal motion unaffected when air resistance is neglected. Given Data / Assumptions: The projectile is fired from the Earth surface with an initial velocity at some angle to the horizontal. Only the gravitational force acts on the projectile after it is launched. Air resistance and other forces are neglected in the ideal model. The gravitational acceleration is directed vertically downward and is approximately constant near the surface. We are interested in how the velocity components behave during the flight. Concept / Approach: In the ideal projectile motion model, gravity is the only force and it acts vertically downward. This produces a constant vertical acceleration and changes the vertical component of velocity over time. There is no horizontal force in this simplified model, so the horizontal acceleration is zero. With zero horizontal acceleration, the horizontal component of velocity remains constant while the vertical component decreases on the way up, becomes zero at the top, and increases downward in magnitude during descent. Total kinetic energy and total momentum both change due to the changing speed and direction. Step-by-Step Solution: Step 1: Resolve the initial velocity of the projectile into horizontal and vertical components.Step 2: Note that gravity provides a constant vertical acceleration g downward, which changes the vertical component of velocity with time.Step 3: Recognise that there is no horizontal force in the idealised model, so horizontal acceleration is zero.Step 4: Conclude that the horizontal component of velocity remains constant throughout the flight, while the vertical component varies. Verification / Alternative check: Equations of motion confirm this reasoning. Horizontally, x velocity remains equal to its initial value, since acceleration is zero. Vertically, the velocity follows v = u minus g multiplied by t, reflecting constant gravitational acceleration. The magnitude and direction of the total velocity change, so kinetic energy and total momentum also change. Observed projectile paths, such as the flight of a ball, match the predicted parabolic trajectory produced by constant horizontal speed and changing vertical speed. Why Other Options Are Wrong: Option a is wrong because kinetic energy depends on the square of speed, and speed changes as the vertical velocity component changes, so energy is not constant. Option b is incorrect because the vector momentum changes in both magnitude and direction as the velocity changes. Option c is wrong since the vertical component clearly changes due to gravity. Option e is also incorrect because only the horizontal component remains constant; the vertical component does not. Common Pitfalls: Students sometimes mistakenly believe that if no force acts in the horizontal direction, then the horizontal velocity must also gradually decrease, confusing this with real life situations where air resistance acts. Another common error is to think that total energy must remain constant as kinetic energy, ignoring the fact that potential energy also changes. In projectile motion problems, always separate horizontal and vertical motions, identify where forces act, and then apply the correct constant acceleration equations to each component. Final Answer: The horizontal component of its velocity remains constant in the absence of air resistance.

Question 7

Which of the following types of waves cannot be polarised in ordinary physical experiments?

Accepted Answer

Introduction / Context: Polarisation is a property associated with the orientation of oscillations in a transverse wave. Electromagnetic waves such as radio, infrared, and light can show polarisation effects. Sound waves in fluids, including ultrasonic waves, are usually longitudinal, with oscillations in the direction of propagation. This question tests whether students can distinguish between waves that can be polarised and those that cannot in typical laboratory conditions. Given Data / Assumptions: Polarisation is defined as the restriction of the vibration direction of a transverse wave to a particular plane. Electromagnetic waves such as radio waves, ultra violet, infrared, and visible light are transverse in nature. Ultrasonic waves are high frequency sound waves that travel through media like air, water, or solids. In fluids, sound waves are predominantly longitudinal, with compressions and rarefactions along the direction of travel. We are considering ordinary laboratory polarisation of waves, not specialised solid state effects. Concept / Approach: Polarisation requires that the wave displacement be perpendicular to the direction of propagation so that it can be confined to a particular plane. Transverse waves satisfy this condition and can be polarised. Electromagnetic waves are inherently transverse and can show linear, circular, and elliptical polarisation. In contrast, longitudinal waves have oscillations along the direction of propagation, so there is no transverse direction to select or restrict. Therefore, longitudinal sound waves such as ultrasonic waves in air or water cannot be polarised in the normal sense. Step-by-Step Solution: Step 1: Recall that radio, ultra violet, infrared, and visible light are all electromagnetic waves and are transverse.Step 2: Recognise that ultrasonic waves are simply sound waves of very high frequency, typically above the human hearing range.Step 3: Understand that normal sound waves in fluids are longitudinal, with the medium vibrating along the direction of travel.Step 4: Conclude that longitudinal waves like ultrasonic sound cannot be polarised, while transverse electromagnetic waves can be. Verification / Alternative check: Experiments with polarising filters easily demonstrate the polarisation of light, radio, and microwave radiation. Polarised sunglasses, for example, reduce glare by blocking certain vibration directions of light. Radio antennas are also oriented to match the polarisation of the transmitted wave. In contrast, no simple polariser exists for ordinary longitudinal sound waves in air, because there is no transverse vibration direction to eliminate. This practical difference supports the theoretical conclusion that ultrasonic sound cannot be polarised in the usual way. Why Other Options Are Wrong: Option a is wrong because radio waves can be polarised and often are in communication systems. Option b is incorrect since ultra violet light is electromagnetic and can be polarised. Option c is also incorrect because infrared radiation, being electromagnetic, can be polarised. Option e is wrong as visible light, such as that from a laser, is a classic example of a wave that can be polarised and often has well defined polarisation. Common Pitfalls: Students sometimes assume that any wave can be polarised simply because they hear about polarised sound in solids, where certain modes can have transverse components. However, the question refers to the usual longitudinal sound waves in fluids where this does not apply. Another confusion is to think that high frequency alone, as in ultrasonic, might enable polarisation, but frequency does not change the longitudinal or transverse nature. Always remember that polarisation is a feature of transverse waves, and ordinary sound in air or water is longitudinal. Final Answer: Ultrasonic waves that are longitudinal sound waves in a medium.

Question 8

If the speed of rotation of the Earth about its own axis increases, what happens to the apparent weight of a body at the equator?

Accepted Answer

Introduction / Context: The Earth rotates about its axis, and this rotation influences the apparent weight of objects on the surface. Weight as measured by a scale is the normal reaction exerted by the surface, which can differ from the gravitational force because of rotational effects. This question asks what happens to the apparent weight at the equator if the rotational speed increases, illustrating the role of centripetal acceleration in everyday physics. Given Data / Assumptions: The Earth is rotating about its axis with some angular speed. A body is located at the equator where the rotational speed relative to the axis is maximum. Gravitational force acting on the body is approximately constant and directed toward the centre of the Earth. The body moves in a circular path due to Earth rotation, so it requires centripetal acceleration toward the axis. We ignore small variations in gravitational field due to shape and density changes. Concept / Approach: The true gravitational force on the body pulls it toward the centre. However, because the body is in circular motion with the rotating Earth, a centripetal force is needed and is provided by part of the gravitational force. The remaining part appears as the normal reaction that we interpret as apparent weight. As the angular speed increases, the required centripetal force increases and a larger fraction of gravity is used for centripetal acceleration. Consequently, the normal reaction, and thus the apparent weight, decreases. At the equator this effect is strongest, but normal rotation rates are far from sufficient to make the weight exactly zero. Step-by-Step Solution: Step 1: Let the gravitational force on the body be Fg, directed toward the centre of the Earth.Step 2: For circular motion at the equator with angular speed omega, the required centripetal force is Fc = m * omega^2 * R, where R is the radius of the Earth.Step 3: The normal reaction N from the ground satisfies Fg minus N = Fc, so N = Fg minus m * omega^2 * R.Step 4: As omega increases, the term m * omega^2 * R becomes larger, so N decreases, meaning the apparent weight measured on a scale becomes smaller. Verification / Alternative check: If the Earth did not rotate at all, the centripetal term would be zero and apparent weight would equal the full gravitational force. In reality, measured weight at the equator is slightly less than at the poles, consistent with existing rotation. In a thought experiment where the rotation becomes extremely fast, there would be a critical speed at which m * omega^2 * R equals Fg, making N equal zero and giving weightlessness at the equator. This confirms that increasing rotation speed always reduces apparent weight before any such extreme condition is reached. Why Other Options Are Wrong: Option a is wrong because an increase in centripetal requirement means that more of gravity is used for circular motion, leaving less for the normal reaction, so apparent weight decreases rather than increases. Option c is incorrect since experimental measurements already show weight differences between equator and poles due to rotation and shape. Option d is not correct for arbitrary nonzero rotation; weight becomes zero only at a very high critical speed, not at any small increase. Option e has no physical basis, as daily cycles do not change the rotation speed of the Earth in this manner. Common Pitfalls: Students may confuse centripetal force with a separate physical force rather than seeing it as the net inward force required for circular motion. Another common error is to assume that gravity alone always equals weight, ignoring situations like lifts, roller coasters, or rotating frames where apparent weight changes. Remember that weight in the practical sense is the normal reaction, and any effect that changes this reaction, such as increased rotation speed, will change the measured weight on a scale. Final Answer: Its apparent weight decreases because the outward centripetal effect reduces the normal reaction.

Question 9

In the context of non conventional energy sources, what does the abbreviation OTE stand for?

Accepted Answer

Introduction / Context: Non conventional or renewable energy sources are becoming increasingly important for sustainable development. Among ocean based energy technologies, both tidal energy and thermal energy conversion are considered. The abbreviation OTE appears in discussions of ocean energy, and this question checks whether you know that it stands for Ocean Thermal Energy, which exploits the temperature difference between warm surface water and cold deep water. Given Data / Assumptions: The term OTE is used within the context of non conventional ocean based energy sources. The ocean has both tidal motion and vertical temperature gradients. Ocean thermal energy conversion uses a temperature difference to generate power. Tidal projects are sometimes called tidal power or tidal energy projects. The question is about the expansion of the abbreviation, not the detailed working of power plants. Concept / Approach: Ocean Thermal Energy refers to the energy that can be harnessed from the temperature difference between warm surface water and cold deep water in tropical and subtropical oceans. Systems that use this are often called Ocean Thermal Energy Conversion systems. They typically use the warm surface water to vaporise a working fluid, drive a turbine, and then use cold deep water to condense the vapor. This is distinct from Ocean Tidal Energy, which uses the kinetic and potential energy of tides, and from conventional thermal power stations on land that burn fossil fuels. Step-by-Step Solution: Step 1: Recognise that OTE must refer to some form of ocean based energy because it is discussed under non conventional energy sources.Step 2: Recall that Ocean Thermal Energy Conversion is a known renewable technology that uses temperature differences in sea water.Step 3: Associate the letters O, T, and E with Ocean, Thermal, and Energy respectively in this context.Step 4: Conclude that OTE stands for Ocean Thermal Energy, which uses the thermal gradient in the ocean to generate electricity. Verification / Alternative check: Literature on renewable energy technologies lists ocean thermal energy conversion as a promising but technically challenging method of power generation. The same sources describe tidal power separately, often using terms like tidal energy or tidal electricity. The abbreviation OTEC for Ocean Thermal Energy Conversion is also common, confirming that the base phrase is Ocean Thermal Energy. Therefore, the correct expansion of OTE is Ocean Thermal Energy, not tidal energy or generic thermal power. Why Other Options Are Wrong: Option a is wrong because Ocean Tidal Energy refers to energy from tides, not from temperature differences. Option c incorrectly mixes tidal concepts with electricity generation and is not the standard expansion of OTE. Option d refers to generic thermal energy, often associated with conventional fossil fuel plants, which is not specific to the ocean. Option e invents Offshore Turbine Energy, which is not a recognised expansion of OTE in the context of non conventional energy sources. Common Pitfalls: Students may quickly associate the letter T with tides because tidal energy is a well known ocean resource, leading them to choose an incorrect tidal expansion. Others may confuse OTE with abbreviations for conventional thermal power plants that have nothing to do with the ocean. To avoid such confusion, it is useful to remember that ocean thermal technologies focus on vertical temperature differences, while tidal technologies focus on horizontal water movement and level changes. Final Answer: Ocean Thermal Energy, using temperature difference in sea water.

Question 10

Which physical phenomenon clearly established that light has a transverse nature rather than a purely longitudinal one?

Accepted Answer

Introduction / Context: Classical wave theory distinguishes between transverse waves, where oscillations are perpendicular to the direction of propagation, and longitudinal waves, where oscillations are parallel to the direction of propagation. Light, as an electromagnetic wave, is transverse. The phenomenon that most clearly demonstrates this transverse nature is polarisation, which directly involves the orientation of oscillations. This question asks you to recall which phenomenon provided strong evidence that light is transverse. Given Data / Assumptions: We are considering several well known wave phenomena: reflection, interference, diffraction, and polarisation. Polarisation depends on the orientation of the electric field oscillations in the light wave. Experiments with polarising crystals and filters show changes in intensity depending on orientation. Reflection, interference, and diffraction can occur for both transverse and longitudinal waves. The question is about which phenomenon uniquely needs transverse oscillations. Concept / Approach: Polarisation is defined as the restriction of the vibration direction of a transverse wave to a particular plane. Only transverse waves, where vibrations are perpendicular to the direction of propagation, can be polarised in this way. Longitudinal waves like ordinary sound in air cannot show such polarisation. Reflection, interference, and diffraction can occur for both transverse and longitudinal waves, so they do not by themselves prove that light is transverse. Observations of polarisation of light, especially by crystals like calcite and by polarising filters, were key historical evidence that the vibrations in light are transverse to its direction of travel. Step-by-Step Solution: Step 1: Recall the definition of a transverse wave and understand that polarisation is a property of such waves.Step 2: Recognise that light can be polarised by reflection, by transmission through certain crystals, or by passing through sheet polarisers.Step 3: Note that when a polarised beam is passed through a second polariser rotated relative to the first, the transmitted intensity varies, demonstrating directional vibration.Step 4: Conclude that this behaviour has no analogue in simple longitudinal waves and therefore polarisation clearly shows the transverse nature of light. Verification / Alternative check: Sound waves in air exhibit reflection, interference, and diffraction, even though they are longitudinal. This confirms that these three phenomena do not require transverse behaviour and cannot be used alone to infer the nature of oscillations. However, no ordinary longitudinal sound wave can be polarised in the same manner as light. Experiments with polarising filters showing complete extinction when their transmission axes are at right angles strongly indicate that light vibrations are confined to a plane, consistent with a transverse electromagnetic wave model. Why Other Options Are Wrong: Option a is wrong because reflection occurs for many types of waves, including longitudinal ones, and does not require transverse vibrations. Option b is incorrect since interference patterns also appear in sound and water waves, which do not all have purely transverse oscillations. Option c is not correct because diffraction is common to both transverse and longitudinal waves and does not reveal the orientation of oscillations. Option e refers to refraction, which involves speed and direction changes but again can occur with various wave types and does not by itself prove that light is transverse. Common Pitfalls: Students sometimes interpret any striking wave phenomenon as special evidence for the nature of light and may pick interference or diffraction because they are visually impressive. It is important to remember that those phenomena are more general and apply to many kinds of waves. Polarisation, in contrast, is tightly linked to transverse oscillations and is therefore the crucial phenomenon for this question. Keeping this association clear helps in understanding both classical wave optics and modern electromagnetic theory. Final Answer: Polarisation of light when it passes through certain crystals or polarising filters.

Question 11

In basic electrical engineering, what is the name of the device used to convert alternating current (AC) into direct current (DC)?

Accepted Answer

Introduction / Context: Many electronic devices require direct current for their operation, even though the electrical supply from the grid is alternating current. To bridge this gap, special circuits and devices are used to convert AC into DC. This question asks you to identify the correct name of the device that performs the conversion, which is a fundamental concept in power supplies and electronics. Given Data / Assumptions: The input is alternating current, which reverses direction periodically. The required output is direct current, where current flows in one direction only. Common instruments like ammeters and galvanometers are mentioned for comparison. Transformers are known to change AC voltage levels but do not inherently change AC to DC. Capacitors can smooth DC but do not by themselves rectify AC. Concept / Approach: A rectifier is a device or circuit that converts AC to DC by allowing current to flow more easily in one direction than the other. Diodes are the most common components used in rectifier circuits. Half wave rectifiers use a single diode, while full wave and bridge rectifiers use multiple diodes to utilise both halves of the AC cycle. Other instruments such as ammeters and galvanometers measure current but do not change its nature. Transformers change only the voltage and current levels of AC without converting it to DC. Step-by-Step Solution: Step 1: Recognise that the task is to convert an alternating input into a unidirectional output suitable for DC circuits.Step 2: Recall that circuits built from diodes and sometimes controlled devices like thyristors are called rectifiers.Step 3: Note that ammeters and galvanometers are measuring devices, while transformers are used for stepping AC voltage up or down.Step 4: Conclude that the correct name for a device that converts AC to DC is a rectifier. Verification / Alternative check: Examine any basic mobile phone charger or computer power supply. The internal circuit typically includes a transformer to change the AC voltage to a suitable level and then a rectifier section to convert it to DC. After rectification, smoothing capacitors and regulators produce a steady DC output. Manuals and circuit diagrams consistently label the AC to DC conversion stage as the rectifier stage, reinforcing that rectifier is the correct term. Why Other Options Are Wrong: Option a is wrong because an ammeter simply measures the magnitude of current and does not alter AC into DC. Option b is incorrect as a galvanometer detects small currents and is used as a sensitive measuring instrument, not a converter. Option d is wrong because a transformer operates only with AC and changes its voltage but not its nature. Option e is not correct either; a capacitor can smooth or filter a rectified waveform but cannot alone convert AC into DC without rectifying components. Common Pitfalls: Students sometimes confuse the roles of different components inside power supplies and may think that transformers or capacitors perform rectification. It is crucial to distinguish between functions: transformers alter voltage levels of AC, rectifiers change AC to DC, and capacitors smooth the DC output. Keeping these roles separate in your mind will help when analysing and designing basic power supply circuits. Final Answer: A rectifier circuit that converts alternating current into direct current.

Question 12

Why is the Earth able to retain and maintain its own atmosphere around it over long periods of time?

Accepted Answer

Introduction / Context: The Earth is surrounded by an atmosphere made of gases such as nitrogen, oxygen, and trace components. Not all planetary bodies have significant atmospheres, and understanding why the Earth can hold on to its atmosphere is an important concept in basic physics and planetary science. This question tests whether you know that gravity, rather than winds or clouds, is the fundamental reason the atmosphere stays bound to the planet. Given Data / Assumptions: The Earth has a certain mass and therefore exerts a gravitational attraction on all objects, including gas molecules. Gas molecules in the atmosphere have thermal motion and can move randomly. Winds and clouds are secondary phenomena occurring within the atmosphere. Solar radiation can add energy to the atmosphere but can also cause atmospheric escape for some planets. We assume normal planetary conditions, not catastrophic events. Concept / Approach: Gravity is the fundamental force that keeps the atmosphere bound to the Earth. Each gas molecule experiences a gravitational pull toward the centre of the planet. As long as the average molecular speeds due to thermal motion are below the escape velocity at the given height, most molecules remain gravitationally bound. Winds and clouds are effects of atmospheric dynamics, but they do not create the binding effect themselves. The rotation of the Earth does influence atmospheric circulation but does not provide a mechanism for retaining gases; in fact, very high rotation would make retention more difficult at the equator. Step-by-Step Solution: Step 1: Recognise that any mass, including gas molecules, experiences a gravitational force toward the centre of the Earth.Step 2: Understand that the Earth gravitational field extends far above the surface and acts on the entire atmosphere.Step 3: Note that thermal motion of gas molecules can cause some to escape, but for most molecules the speeds are below escape velocity, so they remain bound.Step 4: Conclude that it is the gravitational attraction of the Earth that allows it to maintain a stable atmosphere over long time scales. Verification / Alternative check: Comparing different planets supports this explanation. Massive planets like Jupiter have strong gravity and hold very thick atmospheres. Small bodies like the Moon have weak gravity and cannot retain a significant atmosphere, even though they may have winds or be illuminated by the Sun. Wind patterns and clouds are features of an existing atmosphere, not the cause of its presence. Therefore, gravity is clearly the controlling factor in atmospheric retention. Why Other Options Are Wrong: Option a is wrong because winds only move air around within the atmosphere and do not hold it in place. Option b is incorrect since clouds are condensed water droplets or ice crystals suspended within the atmosphere; they depend on the atmosphere but do not cause it to exist. Option d is misleading because the rotation of the Earth can create centrifugal effects and influence weather systems, but it does not bind gas molecules to the planet; if anything, increased rotation would slightly reduce effective weight at the equator. Option e is not correct because solar radiation can heat the atmosphere and may contribute to atmospheric loss under some conditions rather than ensuring retention. Common Pitfalls: Students sometimes mix up causes and effects, assuming that visible phenomena like clouds or wind are responsible for the deeper physical reasons. It is also easy to overestimate the role of rotation, imagining that spinning somehow glues the atmosphere in place. Remember that gravitational attraction is the main reason planets can hold atmospheres, and the ability to do so depends strongly on planet mass, radius, and temperature conditions that affect molecular speeds. Final Answer: Because of the gravitational attraction of the Earth holding gas molecules.

Question 13

At the boiling point of a pure liquid under constant external pressure, what happens to its temperature while boiling continues?

Accepted Answer

Introduction / Context: Boiling is a common phase change where a liquid turns into vapour throughout its volume. At the boiling point, important energy exchanges occur without a rise in temperature. Understanding this behaviour is essential for thermodynamics and practical tasks such as cooking and industrial heating. This question checks whether you know that during boiling at a fixed pressure, the temperature of the liquid stays constant even though heat continues to be supplied. Given Data / Assumptions: The liquid is pure and is boiling under constant external pressure, such as one atmosphere. The liquid has reached its boiling point at that pressure. Heat is continuously supplied during the boiling process. A steady state of boiling is assumed, with bubbles forming throughout the liquid. No significant change in external conditions occurs during the observation. Concept / Approach: At the boiling point, the vapour pressure of the liquid equals the external pressure. When heat is supplied at this point under constant pressure, the energy goes into breaking intermolecular bonds and converting liquid into vapour, not into raising the kinetic energy of molecules further. This energy absorbed without temperature rise is called latent heat of vaporisation. As long as both liquid and vapour are present and pressure remains constant, the temperature of the boiling liquid and its vapour stays practically constant. Step-by-Step Solution: Step 1: Recognise that the boiling point is defined for a given external pressure as the temperature where the vapour pressure of the liquid equals that external pressure.Step 2: Understand that at this temperature, bubbles of vapour can form inside the liquid and rise to the surface.Step 3: When more heat is supplied at constant pressure, the extra energy is used to overcome intermolecular forces and convert more liquid into vapour.Step 4: Because the supplied energy is used as latent heat rather than increasing kinetic energy, the temperature remains effectively constant during boiling. Verification / Alternative check: Practical observations support this theory. If you measure the temperature of boiling water at sea level, it will stay close to 100 degree Celsius as long as water is still present, even if you continue heating. Only after all liquid water has converted to steam can further heating raise the temperature of the steam. Similar behaviour is observed for other pure liquids, each with its own boiling point at a given pressure. This confirms that temperature remains constant during boiling at constant external pressure. Why Other Options Are Wrong: Option a is wrong because the temperature does not continue to rise during boiling under constant pressure; it stays at the boiling point. Option b is incorrect since external atmospheric pressure does not automatically increase during boiling; it is usually fixed by the environment. Option d is wrong because the vapour pressure at the boiling point matches external pressure and does not decrease as boiling progresses. Option e is incorrect as density generally decreases when a liquid changes into vapour; vapour is much less dense than the liquid phase. Common Pitfalls: Students may confuse heating a liquid below its boiling point, where temperature rises, with heating at the boiling point, where temperature remains constant. Another misunderstanding is to think that any added heat must always cause a temperature increase. It is important to remember the distinction between sensible heat, which changes temperature, and latent heat, which changes phase at constant temperature. Watching a heating curve of a substance plotted against time helps reinforce the idea that plateaus correspond to phase changes at fixed temperature. Final Answer: Its temperature remains constant while latent heat of vaporisation is supplied.

Question 14

In astronomy, the unit called parsec is commonly used as a measure of which physical quantity?

Accepted Answer

Introduction / Context: Astronomers deal with extremely large distances when studying stars and galaxies. Everyday units like kilometres are impractical at such scales, so specialised units like the astronomical unit, light year, and parsec are used. This question tests whether you know that a parsec is a unit of astronomical distance rather than brightness, density, or velocity. Given Data / Assumptions: Parsec is mentioned as a named unit in astronomy. Other quantities such as density, brightness, and velocity are common in astrophysics. A parsec is defined in terms of parallax, relating distance to apparent angular shift of nearby stars. Apparent magnitude is a separate concept describing brightness, not distance. None of the other options correspond to standard uses of the parsec unit. Concept / Approach: The word parsec comes from parallax of one arc second. It is defined as the distance at which one astronomical unit subtends an angle of one arc second. Numerically, one parsec is approximately equal to 3.26 light years or about 3.1 multiplied by 10 to the power 16 metres. Because of this definition, the parsec is used to express distances to nearby stars and even larger cosmic scales, and is a standard distance unit in professional astronomy. It has nothing to do with density, brightness, or orbital speed directly. Step-by-Step Solution: Step 1: Recall that astronomers need a convenient unit for large distances and that the parsec is widely used for this purpose.Step 2: Remember that the parsec is defined using the geometry of Earth orbit and the angular shift of stars known as stellar parallax.Step 3: Recognise that brightness is measured using magnitudes or physical flux units, not parsecs.Step 4: Conclude that the parsec is a unit of astronomical distance, not of density, brightness, or velocity. Verification / Alternative check: Textbooks and astronomical catalogues list stellar distances in parsecs or kiloparsecs. For example, the Sun nearest star system distance is often given as about 1.3 parsecs. Brightness is expressed in magnitudes, while velocities are in kilometres per second. There is no standard practice of quoting densities or velocities in parsecs, confirming that parsec is specifically a distance unit based on parallax. Why Other Options Are Wrong: Option a is wrong because density of stars in a region would be expressed in stars per cubic parsec or similar, but parsec alone is not the density measure. Option c is incorrect since brightness is quantified by apparent magnitude, absolute magnitude, or flux units, not by parsecs. Option d is not correct either; orbital velocity is a speed and is measured in units like kilometres per second or metres per second. Option e is unrelated because mass is measured in kilograms or in solar and planetary mass units, not in parsecs. Common Pitfalls: Students sometimes confuse distance units with descriptive quantities, thinking that because parsec is used in astronomy it might refer to brightness or density. Another source of confusion is mixing light years and parsecs, both of which measure distance but arise from different definitions. Remember that parsec is fundamentally linked to parallax and angles, whereas other astronomical quantities have their own dedicated units. Final Answer: Astronomical distance between stars or galaxies.

Question 15

The phenomenon in which the direction of light changes when it passes from one transparent medium to another is called what?

Accepted Answer

Introduction / Context: When light travels from one medium into another, such as from air into water or glass, its speed changes. This change in speed leads to a change in direction, a phenomenon observed in many optical effects and devices. The correct term for this bending of light at the boundary between media is refraction. This question asks you to identify that term, distinguishing it from related but different phenomena like reflection and dispersion. Given Data / Assumptions: Light travels from one transparent medium into another with a different refractive index. The direction of the light ray changes at the boundary between the two media. The media are homogeneous and transparent, so light can pass through them. We are not considering cases where light is entirely reflected. We assume classical geometric optics applies. Concept / Approach: Refraction is defined as the change in direction of a light ray when it passes from one medium to another in which its speed is different. Snell law describes this relationship quantitatively as n1 * sin theta1 = n2 * sin theta2, where n1 and n2 are refractive indices and theta1 and theta2 are angles of incidence and refraction. Reflection, by contrast, refers to light bouncing off a surface, while dispersion describes the separation of white light into different colours due to wavelength dependent refraction. Propagation and scattering are more general terms and do not specifically mean the bending at an interface. Step-by-Step Solution: Step 1: Identify that the question describes a change in direction of light when crossing a boundary between two media.Step 2: Recall that such bending occurs because light speed changes when it enters a medium with a different refractive index.Step 3: Recognise from basic optics that the phenomenon of bending at the interface is called refraction.Step 4: Confirm that reflection, dispersion, and scattering refer to different, though related, optical effects. Verification / Alternative check: Common examples support this conclusion. A stick partly immersed in water appears bent at the surface due to refraction. Lenses use controlled refraction to focus light and form images. Rainbows, while involving dispersion, fundamentally rely on refraction occurring at the surfaces of water droplets. Descriptions of these phenomena always mention refraction of light at interfaces, confirming that this is the correct term for a change of direction when passing from one medium to another. Why Other Options Are Wrong: Option a is wrong because propagation is a general term meaning travel of light and does not specifically refer to bending at a boundary. Option b is incorrect because reflection involves light bouncing back from a surface, not passing into a new medium. Option d is not correct because dispersion is the separation of white light into its component colours, which depends on refraction but is not simply the bending itself. Option e is unrelated here since scattering involves random deviations of light due to particles, not the orderly bending at a smooth interface. Common Pitfalls: Students sometimes confuse refraction with reflection because both occur at boundaries. Another frequent mistake is to identify any colourful effect, such as a rainbow, purely with dispersion and forget that refraction is the underlying bending mechanism. Keeping the definitions clear helps: refraction is bending at a boundary due to speed change, reflection is bouncing back, and dispersion is wavelength dependent refraction. Learning examples for each helps you remember the distinctions. Final Answer: Refraction, when light bends on entering a different medium.

Question 16

According to basic mechanics, why does a cannon recoil backward after firing a projectile from its barrel?

Accepted Answer

Introduction / Context: Recoil of firearms and cannons is a classic example used to illustrate basic laws of motion. When a shell is fired forward, the cannon moves backward. Understanding this effect helps students grasp Newton third law and conservation of momentum in practical situations. This question asks which principle best explains why the cannon recoils when the projectile is launched. Given Data / Assumptions: A cannon and a projectile initially rest together before firing. On firing, the projectile is propelled forward at high speed. The cannon is free to move backward on its mount or platform. External horizontal forces such as friction are neglected during the brief firing interval. Newton laws of motion apply to the system. Concept / Approach: Newton third law of motion states that for every action there is an equal and opposite reaction. When the expanding gases exert a forward force on the projectile, the projectile exerts an equal and opposite force on the cannon. In addition, conservation of linear momentum tells us that the total momentum of the isolated cannon projectile system remains zero, so if the projectile gains forward momentum, the cannon must gain backward momentum. Both ideas imply that the cannon moves backward in response to the forward motion of the projectile. Step-by-Step Solution: Step 1: Initially, the cannon and projectile system is at rest, so total momentum is zero.Step 2: When the gunpowder burns, expanding gases push the projectile forward, giving it a large forward momentum.Step 3: According to Newton third law, the projectile exerts an equal and opposite force on the cannon, pushing it backward.Step 4: As a result, the cannon acquires backward momentum such that the total momentum of the system remains conserved and equal to zero. Verification / Alternative check: Experiments with small air rifles or toy guns show the same effect: when a projectile is fired forward, the gun jerks backward. Mathematical treatment using conservation of momentum matches observed recoil speeds, confirming that momentum, not energy alone, must be considered. Newton third law explains the force pair involved, while conservation of momentum explains the subsequent motion. Gravitational forces do not change suddenly during firing and cannot account for the recoil. Why Other Options Are Wrong: Option a is wrong because conservation of energy alone does not dictate the direction of motion; energy is scalar and cannot explain equal and opposite recoils directly. Option b is incomplete; although hot gases push on both projectile and cannon, it is the mutual interaction described by Newton third law that correctly explains the recoil and is usually cited in basic physics. Option d is incorrect because Newton first law deals with inertia and tendency to remain at rest or in uniform motion, not with action reaction pairs. Option e is clearly wrong since gravity acts mostly vertically in this context and does not suddenly pull the cannon backward when firing. Common Pitfalls: Students sometimes focus only on the gases pushing on the projectile and forget that forces come in pairs. Another confusion is to overuse the term conservation of energy without recognising that direction and vector quantities are crucial for describing recoil. It is helpful to remember that when two bodies interact, such as a cannon and shell, the forces they exert on each other are equal and opposite, which is exactly Newton third law. Combined with momentum conservation, this law gives a complete explanation of recoil phenomena. Final Answer: Because of Newton third law of motion, which relates action and reaction forces.

Question 17

When a car moves around a horizontal curve in a circular path, which type of force is responsible for changing its direction of motion?

Accepted Answer

Introduction / Context: Objects moving in circular paths, such as cars on curved roads or planets in orbits, require a net inward force to continuously change the direction of their velocity. This net inward force is called centripetal force. This question asks which type of force is responsible for changing the direction of a car moving around a curve, reinforcing the concept of centripetal motion in everyday situations. Given Data / Assumptions: The car is moving at some speed along a horizontal curved road. The curve is approximated as part of a circular path. The car tyres remain in contact with the road surface without slipping. Vertical motion is not significant; the main change is in horizontal direction. Friction between tyres and road can provide the necessary lateral force. Concept / Approach: For circular motion at constant speed, the direction of velocity continually changes, which requires an inward acceleration called centripetal acceleration. The corresponding net inward force is the centripetal force. In the case of a car on a flat road, static friction between the tyres and the road surface usually provides this centripetal force, directed toward the centre of curvature. Centrifugal force is a fictitious force that appears only in a rotating frame and is not the real cause of directional change in an inertial frame. Step-by-Step Solution: Step 1: Recognise that a car moving along a curve experiences a change in direction of velocity, which implies centripetal acceleration toward the centre of the curve.Step 2: Recall that the real force responsible for this acceleration is called the centripetal force.Step 3: In a simple model, static friction between tyres and road provides the horizontal inward force needed for centripetal motion.Step 4: Conclude that centripetal force is the correct term for the force changing the car direction as it moves around the curve. Verification / Alternative check: If the frictional force is too small, such as on an icy road, the car cannot obtain enough centripetal force and will skid outward instead of following the curve, confirming that a real inward force is needed to sustain circular motion. Equations for uniform circular motion, like centripetal force equal to m * v squared divided by r, show that higher speeds and tighter curves require larger inward force, matching driving experience where sharp turns at high speed are dangerous. Why Other Options Are Wrong: Option a is wrong because centrifugal force is not a real force in an inertial frame; it is a pseudo force used in the rotating frame of the car. Option b is incorrect since cohesive forces act at the molecular level inside materials and are not the macroscopic force causing the turn. Option d is not correct because gravity acts vertically downward and is balanced by the normal reaction; it does not change the car horizontal direction. Option e is wrong, as electrostatic forces between tyres and road are negligible compared to frictional forces and do not normally play a role in steering. Common Pitfalls: Students often say that centrifugal force pushes the car outward when rounding a curve, which reflects the experience of being thrown sideways but confuses the analysis in an inertial frame. It is better to concentrate on the real inward forces that cause the change of direction. Another pitfall is to overlook the role of friction, especially on banked or slippery roads. Always identify the inward directed real forces and label the net inward force as the centripetal force required for circular motion. Final Answer: Centripetal force directed toward the centre of the circular path.

Question 18

Which wave phenomenon is mainly responsible for the echo of a sound wave heard when it reflects off a distant surface?

Accepted Answer

Introduction / Context: An echo is a familiar acoustic phenomenon where a sound is heard again after reflecting from a distant surface such as a wall or cliff. This question focuses on identifying which basic wave phenomenon is mainly responsible for echoes. Recognising that echoes arise from reflection helps link sound behaviour to general wave properties and to practical applications like sonar and architectural acoustics. Given Data / Assumptions: A sound is produced near a reflecting surface, such as a wall, building, or mountain. The sound travels through air, hits the surface, and some of it is returned toward the source. The distance is large enough that the reflected sound arrives with a noticeable time delay. The medium is reasonably uniform, so refraction effects are minor. The observer can distinguish the original sound from its echo. Concept / Approach: Reflection occurs when a wave encounters a boundary between two media and is sent back into the original medium. For sound, a hard and smooth surface reflects better than a soft or irregular one. An echo is simply the reflected sound that returns to the listener after a delay long enough to be perceived separately, typically around 0.1 second or more. Refraction, interference, polarisation, and diffraction are other wave phenomena, but they do not create distinct delayed repetitions of the original sound in the same straightforward way that reflection does. Step-by-Step Solution: Step 1: Recognise that an echo involves hearing the same sound a second time after it has interacted with a distant surface.Step 2: Recall that when sound hits a large hard surface, part of its energy is reflected back into the original medium according to reflection laws.Step 3: Understand that if the path difference between the outgoing and reflected sound is large enough, the travel time difference produces a delayed second sound.Step 4: Conclude that the primary wave phenomenon producing an echo is reflection of sound from the surface. Verification / Alternative check: Practical guidelines for hearing echoes specify that the reflecting surface should be at least about 17 metres away for typical conditions so that the reflected sound returns after roughly 0.1 second, just at the limit of human echo perception. These calculations use the speed of sound in air and simple distance time relations, consistent with a reflection model. Devices like sonar and radar, which detect objects by sending out waves and receiving reflected signals, rely explicitly on reflection, reinforcing that echo like phenomena are reflection based. Why Other Options Are Wrong: Option b is wrong because refraction is the bending of waves passing through regions of varying properties, which can change sound paths but does not by itself create distinct delayed copies. Option c is incorrect since interference involves the combination of overlapping waves to produce patterns of reinforcement and cancellation, not discrete echoes. Option d is not applicable because sound in air is longitudinal and does not undergo ordinary polarisation as light does. Option e refers to diffraction, which is the bending of waves around obstacles or through openings and helps sound reach shadow regions but is not the main cause of clear echoes. Common Pitfalls: Students may have heard of many wave phenomena and feel tempted to choose a more exotic term than reflection. However, echo formation is a straightforward case where the wave simply bounces back. Confusion can also arise when discussing sound in nonuniform atmospheres, where refraction can bend sound paths, but the distinct delayed repetition that people call an echo still stems from reflection at some boundary. Keeping the definitions precise for each phenomenon makes it easier to select reflection as the correct answer here. Final Answer: Reflection of sound from a large hard surface back to the listener.

Question 19

If an object moves along a circular path with uniform ______, its motion is called uniform circular motion. In this context, which physical quantity must remain constant for the motion to be described as uniform circular motion?

Accepted Answer

Introduction / Context: In kinematics and circular motion, one of the most frequently tested ideas is the distinction between simply moving in a circle and moving in a circle with uniform circular motion. Many students remember that some quantity must be constant, but they confuse speed, velocity and acceleration. This question checks whether you know exactly which physical quantity must stay constant for motion to be classified as uniform circular motion. Given Data / Assumptions: An object is moving along a circular path of fixed radius. The term uniform circular motion is used in the standard school physics sense. We must decide which quantity in the blank, when kept uniform, correctly defines uniform circular motion. Speed is the magnitude of velocity, velocity is a vector, and acceleration can change magnitude and direction. Concept / Approach: Uniform circular motion is defined as motion in a circular path with constant speed. Although the speed is constant, the direction of motion is continuously changing as the object goes around the circle. Because velocity is a vector that depends on both magnitude and direction, the velocity is not constant in uniform circular motion. A changing velocity implies a non zero acceleration, called centripetal acceleration, directed towards the centre of the circle. Therefore, the key feature that remains uniform is the speed, not the velocity or the acceleration. Step-by-Step Solution: Step 1: Recall the definition: uniform circular motion means motion along a circular path with constant speed. Step 2: Note that as the object moves around the circle, its direction of motion continuously changes, so its velocity vector changes even if the magnitude (speed) is constant. Step 3: Because velocity is changing, there must be an acceleration towards the centre of the circle; this centripetal acceleration is not zero. Step 4: Therefore, the only quantity that remains uniform in the usual definition is the speed of the object, so the blank must be filled with speed. Verification / Alternative check: Consider an object moving around a circular track of radius r at a constant speed v. The magnitude of its velocity is always v, but the direction is tangent to the circle at each point and keeps rotating. The centripetal acceleration has magnitude a_c = v^2 / r, which is constant only if both v and r are constant, but its direction is always pointing towards the centre. Thus, neither velocity nor acceleration as full vectors are constant, but the scalar speed is constant by definition in uniform circular motion. Why Other Options Are Wrong: Option B (time taken for one revolution): If the time taken for one particular lap is constant, that implies constant speed, but the standard textbook definition does not say moves with uniform time; the fundamental quantity is speed. Option C (velocity): Because the direction of motion changes continuously, the velocity vector is not constant, so it cannot be uniform in the strict sense. Option D (acceleration): There is always centripetal acceleration, and if the speed is constant its magnitude may be constant, but its direction constantly changes, so the acceleration vector is not uniform. Option E (distance from the centre): This distance is simply the radius of the circle and is constant in any circular motion, uniform or non uniform, so it does not uniquely define uniform circular motion. Common Pitfalls: A very common misconception is to think that uniform circular motion means velocity is uniform. Remember that uniform in this context refers to the constancy of speed, not the full velocity vector. Uniform circular motion is actually an example where an object accelerates even though its speed is constant, because the direction of its velocity keeps changing. Keeping this distinction clear will help with many problems on circular motion, centripetal force and centripetal acceleration. Final Answer: The blank should be filled with speed.

Question 20

In geometrical optics, the image formed by a plane mirror has several characteristic properties. The specific type of reversal shown by the image in a plane mirror is known as ______.

Accepted Answer

Introduction / Context: Plane mirrors are among the simplest optical devices and appear in many competitive exam questions. Their image properties are standard: the image is virtual, upright, of the same size as the object and laterally inverted. This question focuses specifically on the type of reversal that occurs, checking whether you remember the correct technical term for the left right reversal seen in plane mirrors. Given Data / Assumptions: We are dealing with an ordinary plane mirror, such as a bathroom mirror. The object is placed in front of the mirror in air, and standard laws of reflection apply. We are interested in how the orientation of the image compares with the object. Terms like real, virtual, inverted, enlarged, diminished and lateral inversion are used in their usual school physics sense. Concept / Approach: When you stand in front of a plane mirror and raise your right hand, the image appears to raise its left hand. This is called lateral inversion, meaning left right reversal. The top and bottom of the image are in the same orientation as the object, so there is no vertical inversion. Because the reflected rays appear to diverge from a point behind the mirror, the image is virtual rather than real. The size of the image is equal to the size of the object, so it is not enlarged or diminished. Therefore, among the given options, the correct description of the specific reversal produced by a plane mirror is lateral inversion. Step-by-Step Solution: Step 1: Recall the standard properties of an image in a plane mirror: virtual, upright, same size as object and laterally inverted. Step 2: Focus on the term inversion, which refers to a type of reversal or flipping of orientation. Step 3: Observe that a plane mirror makes left appear as right and right appear as left, which is a left right reversal called lateral inversion. Step 4: Note that there is no upside down reversal of the image, so vertical inversion does not occur. Step 5: Conclude that the correct term describing the mirror image reversal is lateral inversion. Verification / Alternative check: You can test this effect using text printed on paper. When you hold the text in front of a plane mirror, you see that the letters appear reversed left to right, such that it becomes difficult to read. Ambulances often have the word AMBULANCE written reversed on the front of the vehicle. When a driver ahead looks in the rear view mirror, the lateral inversion of the mirror makes the word appear in the correct orientation, allowing quick recognition. This is a practical demonstration of lateral inversion by a plane mirror. Why Other Options Are Wrong: Option A (vertical inversion): Plane mirrors do not flip the image upside down. The head stays at the top and the feet at the bottom. Option B (a real and inverted image): Images in plane mirrors are virtual and upright, not real and inverted. Real inverted images are produced by concave mirrors in certain positions. Option D (an enlarged image): The image in a plane mirror is of the same size as the object, so it is neither enlarged nor diminished. Option E (a diminished image): Similar reasoning applies; the image size equals the object size for plane mirrors. Common Pitfalls: Students sometimes confuse the terms inverted and lateral inversion. Inverted usually means upside down relative to the object, while lateral inversion refers to left right reversal. Plane mirrors cause lateral inversion but not upside down inversion. Keeping this distinction clear helps in answering many optics questions correctly and prevents mixing up the properties of plane, concave and convex mirrors. Final Answer: The image in a plane mirror shows lateral inversion.

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