WBCS Main Physics Questions and Answers (Previous Year Paper)

Answer: (A) Simplex

Explanation: In a TV transmission, the signal travels in only one direction, from the broadcasting station to the television receiver. The viewer’s TV cannot send anything back through the same channel. Communication that flows in a single fixed direction is called simplex transmission. Radio broadcasting, keyboards and loudspeakers work on the same principle.

Transmission Modes:

Answer: (B) Frequency

Explanation: When light passes from one medium to another, its frequency is fixed by the source and stays the same. The speed of light changes with the medium, and since velocity equals frequency times wavelength, the wavelength changes along with the velocity. The amplitude also changes, because part of the light is reflected at the boundary and part is absorbed, so the transmitted wave carries less energy.

Refraction:

Answer: (B) increases

Explanation: When a negative charge is placed on a soap bubble, the charges spread over the surface and repel one another. This mutual repulsion produces an outward electrostatic pressure that acts along with the gas pressure inside, opposing the inward pull of surface tension. The bubble therefore expands until the forces balance again, so its radius increases. The same happens if the bubble is given a positive charge, because only the repulsion between like charges matters.

Charged Soap Bubble:

Answer: (B) weight

Explanation: A poundal is the force needed to give a mass of one pound an acceleration of one foot per second squared. It is a unit of force in the foot-pound-second (FPS) system. Since weight is the force with which the earth pulls a body, weight is measured in force units, and so the poundal is a unit of weight.

Units of Force and Weight:

Answer: (B) Bernoulli’s theorem

Explanation: In a sprayer, air is forced at high speed over the top of a narrow tube dipped in the liquid. By Bernoulli’s theorem, where the speed of a fluid is high, its pressure is low. The pressure above the tube therefore drops below the atmospheric pressure acting on the liquid surface, and the liquid is pushed up the tube. It meets the fast air stream at the top and is broken into a fine spray.

Bernoulli’s Theorem:

Answer: (C) Frequency

Explanation: A transformer works on mutual induction and changes the voltage and current between its primary and secondary coils. The alternating flux in the core is set by the supply, so it varies at the same rate in both coils. The output therefore has the same frequency as the input. An ideal transformer also transfers the same power, so a rise in voltage is matched by a fall in current, and the reverse.

Transformer:

Answer: (D) 1,00,000

Explanation: The question is worded poorly, but the intended idea is the extreme temperature of the Sun’s outer atmosphere. The visible surface of the Sun (photosphere) is only about 5,500 K, but the corona above it reaches about one million kelvin, and flares and other hot regions can go higher still. Among the given options, only 1,00,000 K is a value the Sun’s temperature exceeds by a wide margin, so it is the best fit.

Temperature of the Sun:

Answer: (B) Lenz’s Law

Explanation: Faraday’s law gives the magnitude of the induced e.m.f., while Lenz’s law gives its direction. According to Lenz’s law, the induced e.m.f. acts in such a direction that the current it drives opposes the change in magnetic flux that produced it. This is a consequence of the conservation of energy.

Electromagnetic Induction:

Answer: (A) Angular acceleration

Explanation: Torque is the rotational counterpart of force. A constant torque acting on a body produces a constant angular acceleration, as given by τ = Iα, where I is the moment of inertia and α is the angular acceleration. The angular velocity keeps changing with time, while the angular acceleration stays fixed. Linear acceleration would arise from a net force, not from a torque alone.

Rotational Motion:

Answer: (A) decreased

Explanation: In an adiabatic process no heat enters or leaves the gas. When the gas expands, it does work on its surroundings, and since no heat is supplied, this work is done at the cost of its own internal energy. The internal energy therefore falls, and the temperature of the gas drops. This is why a gas cools on sudden expansion, as seen when a compressed gas cylinder is opened.

Thermodynamic Processes:

Answer: (B) Electronic

Explanation: A UV spectrometer uses ultraviolet and visible light, which carries enough energy to lift electrons from a lower orbital to a higher one. These electronic transitions (such as π to π* and n to π*) absorb light at characteristic wavelengths, and the spectrometer records that absorption. Rotational and vibrational changes need much lower energy, and nuclear transitions need far higher energy.

Spectroscopy:

Answer: (C) Instantaneous speed

Explanation: A car’s speedometer shows how fast the car is moving at that very moment, without any information about direction. A quantity that gives the magnitude of motion at a particular instant is instantaneous speed. Average speed would need total distance divided by total time, and velocity would need direction, which a speedometer does not show.

Speed and Velocity:

Answer: (A) Amplitude modulation

Explanation: In India, All India Radio uses amplitude modulation (AM) for its medium wave and short wave broadcasts, which cover long distances and large areas. In AM, the amplitude of the carrier wave is varied in step with the audio signal while its frequency stays fixed. FM is also used in India, mainly for local city stations, but the standard answer for radio transmission is AM.

Modulation Types:

Answer: (D) 10 ⁻ ¹³ m

Explanation: An X-unit (also known as a Siegbahn unit or X-ray unit, abbreviated as xu) is a non-SI unit of length historically used in spectroscopy to express the wavelengths of X-rays and gamma rays.

• Exact Value: 1 xu ≈ 1.0021 × 10 ⁻ ¹³ meters
• Therefore, it is approximately equal to 10 ⁻ ¹³ m

Comparison with other options:

• 10 ⁻ ¹⁵ m: Equal to 1 femtometer (or 1 fermi), typically used for measuring atomic nuclei sizes.
• 10 ⁻ ¹² m Equal to 1 picometer, used for molecular or atomic-scale distances.
• 10 ⁻ ¹⁰ m: Equal to 1 angstrom (Å), commonly used to denote atomic radii and chemical bond lengths. 

Answer: (C) 1 to 100

Explanation: Nanoparticles are particles whose size lies between 1 and 100 nanometres in at least one dimension. At this scale, materials show properties different from their bulk form, such as changes in colour, melting point, strength and chemical reactivity, mainly because of the very large surface area relative to volume.

Nanoparticles: Scale, Types and Facts

Answer: (B) atomic energy reactor

Explanation: In a nuclear reactor the chain reaction is kept under control by absorbing the extra neutrons released in fission, so that on average only one neutron from each fission goes on to cause another fission. The energy is then released slowly and steadily and can be used to produce electricity. In an atom bomb the chain reaction is uncontrolled, and the energy is released in a fraction of a second.

Nuclear Reactions:

Answer: (D) peak voltage for A.C. is much larger than 220 volt.

Explanation:

Answer: (C) both (A) and (B)

Explanation: A magnetic field is produced by a moving charge, which is an electric current, as shown by Oersted’s discovery. It is also produced by a changing electric field, as Maxwell showed through the displacement current term added to Ampere’s law. Both sources create a magnetic field, so option (C) is correct.

Sources of Magnetic Field:

Answer: (D) All of the above

Explanation: Industrial robots are used in factories to take over repetitive, hazardous and precise tasks. They reduce the number of workers needed on the line, raise productivity through continuous and fast operation, and improve the quality of output. They also handle loads and operations gently and consistently, which reduces wear and misuse of production machines and so extends their working life.

Industrial Robots:

Answer: (C) Copper

Explanation: The temperature coefficient of resistance indicates how the electrical resistance of a material changes when its temperature changes.

• Negative Temperature Coefficient (NTC): This means that as temperature increases, the electrical resistance decreases. Semiconductors, insulators, and electrolytes possess a negative coefficient.
• Positive Temperature Coefficient (PTC): This means that as temperature increases, the electrical resistance of the material increases. Metals/conductors possess a positive coefficient. When a metal like Copper is heated, the thermal vibration of its metal ions increases. This creates more frequent collisions and obstructions for the flowing electrons, thereby raising the resistance.

Comparison of the Options:

Material	Type	Temperature Coefficient (𝛼)	Behavior with Heat
Carbon	Non-metal / Semiconductor behavior	Negative	Resistance decreases
Germanium	Semiconductor	Negative	Resistance decreases
Copper	Metallic Conductor	Positive	Resistance increases
An electrolyte	Ionic solution	Negative	Resistance decreases (viscosity drops, ions move faster)

Answer: (C) neutral

Explanation: An n-type semiconductor is made by doping pure silicon or germanium with a pentavalent impurity such as phosphorus or arsenic. The extra electrons supplied by the impurity atoms are free to move, but each impurity atom also has an extra proton in its nucleus. The total number of positive and negative charges is therefore equal, and the crystal as a whole remains electrically neutral.

Extrinsic Semiconductors:

Answer: (B) 4 GHz

Explanation: In the C-band satellite communication system, the uplink (earth station to satellite) frequency is about 6 GHz and the downlink (satellite to earth station) frequency is about 4 GHz. The downlink uses the lower frequency because it suffers less attenuation, and the satellite has limited power to transmit. This is why the C-band is commonly called the 6/4 GHz band.

Satellite Frequency Bands:

Answer: (C) Polarisation

Explanation: Sound and light both show reflection, refraction, interference and diffraction, because these are general properties of all waves. Polarisation, however, is possible only for transverse waves, in which the vibrations are perpendicular to the direction of travel. Light is a transverse wave, but sound in air is a longitudinal wave, with vibrations along the direction of travel, so it cannot be polarised.

Transverse and Longitudinal Waves:

Answer: (D) Steel

Explanation: Elasticity is the ability of a body to regain its original shape after the deforming force is removed, and it is measured by Young’s modulus. Steel has a much higher Young’s modulus than copper, rubber or sponge, which means it needs a much larger force to produce the same strain and returns fully to its shape. Rubber stretches a lot, but that is high extensibility, not high elasticity in the physical sense.

Elastic Properties of Materials:

Answer: (B) SYNCOM-3

Explanation: SYNCOM-3, launched by NASA in August 1964, was the first geostationary satellite, placed in an orbit about 35,786 km above the equator so that it stayed fixed over one point on Earth. It was used to relay television pictures of the 1964 Tokyo Olympics across the Pacific. The earlier SYNCOM-2 (1963) was geosynchronous but not exactly geostationary, because its orbit was inclined.

Early Communication Satellites:

Answer: (A) Hydrogen gas discharge lamp

Explanation: A hydrogen (or deuterium) gas discharge lamp produces a continuous spectrum in the ultraviolet region when an electric discharge is passed through the gas at low pressure. This is why it is the standard UV source in UV-visible spectrometers. An RF oscillator and a klystron give radio and microwave frequencies, and a Nernst filament gives infrared radiation.

Radiation Sources:

Answer: (D) accelerating protons

Explanation: A cyclotron is a particle accelerator in which charged particles move in a circular path inside a magnetic field and gain energy each time they cross the gap between two D-shaped electrodes (dees) connected to an alternating voltage. It was designed by E. O. Lawrence to accelerate heavy positively charged particles such as protons, deuterons and alpha particles. It cannot accelerate electrons, because their very small mass makes them gain speed so quickly that they go out of step with the oscillating voltage.

Particle Accelerators:

Answer: (C) 50 Hz

Explanation: The alternating current supplied in India has a frequency of 50 Hz, which means the current changes its direction 100 times every second (twice in each cycle). The standard household supply is 220 to 240 V. This frequency is fixed and maintained by the national power grid.

Power Supply Frequency and Voltage:

Answer: (A) Steel

Explanation: A permanent magnet must keep its magnetism for a long time after the magnetising field is removed. This needs a material with high retentivity and high coercivity, so that it is not easily demagnetised. Steel has both properties, which is why it is the standard choice for permanent magnets. Soft iron has low retentivity and low coercivity, so it loses its magnetism easily and is used for temporary magnets and electromagnets.

Magnetic Materials:

Answer: (C) the field between the plates

Explanation: In a charged capacitor, energy is stored in the electric field set up in the space between the plates, not in the charges themselves. The energy is stored while the capacitor is charged, and it is released when the field collapses on discharging. The energy density of the field is ½ε₀E² per unit volume, so the total energy depends on the volume of the field region.

Energy Stored in a Capacitor:

Answer: (B) increase

Explanation: When a non-volatile solid such as salt or sugar is dissolved in water, it lowers the vapour pressure of the solution. The solution must then be heated to a higher temperature before its vapour pressure equals the atmospheric pressure, so its boiling point rises. This effect is called elevation of boiling point, and it is a colligative property, which depends on the number of solute particles and not on their nature.

Colligative Properties:

Answer: (B) Oersted

Explanation: The magnetic effect of electric current was discovered by the Danish physicist Hans Christian Oersted in 1820. He noticed that a compass needle placed near a wire carrying current got deflected, which showed that a current produces a magnetic field around it. This was the first link found between electricity and magnetism.

Electricity and Magnetism:

Answer: (C) 2f

Explanation: A simple pendulum oscillates with frequency f, but its energy (kinetic and potential) is not tied to the direction of the swing. The kinetic energy is maximum at the mean position and zero at both extreme positions, and the potential energy behaves oppositely. In one complete oscillation, the mean position is crossed twice, and each extreme is reached once on each side, so the energy cycles twice per oscillation. The frequency of change of energy is therefore 2f.

Simple Pendulum and SHM:

Answer: (D) Black

Explanation: Space has no atmosphere to scatter sunlight, so there is nothing to spread the light and colour the sky. An astronaut therefore sees the sky as black, even though the Sun and stars are visible against it. On Earth, the sky looks blue because air molecules scatter the shorter blue wavelengths of sunlight in all directions.

Colour of the Sky:

Answer: (B) 380 – 750 nm

Explanation: The visible spectrum is the small part of the electromagnetic spectrum that the human eye can detect. It is commonly taken as 380 to 750 nm, running from violet at the short-wavelength end to red at the long-wavelength end. Radiation just below this range is ultraviolet and just above it is infrared, both invisible to the eye.

Visible Spectrum:

Answer: (B) Current density

Explanation: Current density is the current flowing per unit area of cross-section, and it has a definite direction, the direction of flow of the current at that point. It is therefore a vector quantity, written as J. Electric current itself is treated as a scalar, because it does not obey the vector law of addition, even though it has a direction along the wire. Power is a scalar, and wattless current is just the component of current that does no work, also a scalar.

Scalar and Vector Quantities:

Answer: (A) high voltage and low current

Explanation: Power loss in transmission lines is given by I²R, so it depends on the square of the current. For the same power transmitted (P = VI), raising the voltage lowers the current, which sharply reduces the heat lost in the wires. Alternating current is therefore stepped up to a high voltage with transformers at the power station, sent over long distances at low current, and stepped down near the consumer.

Power Transmission:

Answer: (D) Parabolic

Explanation: When a liquid flows steadily through a capillary tube in streamline (laminar) flow, the layer touching the wall is at rest because of viscosity, and the speed increases towards the axis, where it is maximum. The velocity at a distance r from the axis is v = (P/4ηl)(R² − r²), which is a parabolic function of r. A plot of velocity against the radius therefore forms a parabola.

Poiseuille’s Flow: Laminar Flow in a Tube

Answer: (B) Pascal’s law

Explanation: A hydraulic press works on Pascal’s law, which says that pressure applied to an enclosed liquid is transmitted equally and undiminished in all directions. A small force on a small piston creates a pressure that acts on a much larger piston, producing a large force there. The force is multiplied in the ratio of the piston areas, though the larger piston moves a correspondingly smaller distance.

Pascal’s Law:

Answer: (B) π/2

Explanation: The average power in an A.C. circuit is P = V I cosφ, where φ is the phase difference between voltage and current. When φ = π/2 (90°), cosφ is zero, so the average power consumed is zero even though current flows. The current in this case is called wattless (or idle) current. It occurs in a circuit containing a pure inductor or a pure capacitor.

Phase Difference and Power in A.C. Circuits:

Answer: (D) Voltage regulator

Explanation: A Zener diode is a specially doped p-n junction designed to work in the reverse breakdown region. In this region the voltage across it stays almost constant (the Zener voltage) even when the current through it changes over a wide range. Connected in reverse bias across a load, it holds the output voltage steady, so it is used as a voltage regulator.

Diodes and Their Uses:

Answer: (B) Angular momentum

Explanation: A moving bicycle has angular momentum because its wheels spin. Angular momentum is conserved unless an external torque acts, so the spinning wheels resist any change in the direction of their axis. This gyroscopic effect, together with the rider’s steering corrections, keeps the bicycle upright. Linear momentum and kinetic energy are not conserved here, since friction and air resistance act on the bicycle.

Angular Momentum and Stability:

Answer: (D) Reflection noise

Explanation: Lining the walls and ceiling of a room with sound absorbing material such as acoustic tiles, felt or porous boards reduces the sound waves that bounce back from these surfaces. This lowers the reverberation and echo inside the room, which is the noise caused by repeated reflection. Noise that travels directly through the air from the source, or through the building structure, is not cut by wall lining in the same way.

Noise and Its Control:

Answer: (D) transverse nature of light

Explanation: Polarisation means restricting the vibrations of a wave to a single plane perpendicular to its direction of travel. Only transverse waves, whose vibrations are at right angles to the direction of propagation, can be polarised. Since light shows polarisation, it proves that light waves are transverse in nature.

Polarisation of Light:

Answer: (D) >90°

Explanation: The contact angle is the angle between the liquid surface and the solid wall, measured inside the liquid. Mercury does not wet glass, because the cohesive force between mercury molecules is stronger than the adhesive force between mercury and glass. The mercury surface therefore curves downward at the wall, and the contact angle is obtuse, about 140°. This is also why mercury is depressed in a glass capillary tube.

Contact Angle and Capillarity:

Answer: (A) Radio Detection and Ranging

Explanation: RADAR stands for Radio Detection and Ranging. It is a system that sends out short pulses of radio waves or microwaves and receives the echoes reflected from a distant object. The time taken by the echo to return gives the distance of the object, and the direction of the returning signal and the Doppler shift give its position and speed.

RADAR:

Answer: (B) 500 – 5000 Hz

Explanation: The human ear can hear sounds from 20 Hz to 20,000 Hz, but it is not equally sensitive across this range. Sensitivity is greatest in the middle band of about 500 to 5000 Hz, with the peak near 3000 to 4000 Hz, because of the resonance of the ear canal. Speech sounds lie mostly in this band, which is why we hear them so clearly.

Human Hearing:

Answer: (A) the scattering of sunlight by air molecules.

Explanation: Sunlight is made of many colours. As it passes through the atmosphere, air molecules scatter the short wavelengths (blue) far more strongly than the long wavelengths (red), because scattering intensity is proportional to 1/λ⁴. The scattered blue light reaches our eyes from every direction, so the sky looks blue. Water vapour, absorption or emission by the atmosphere are not the cause.

Colour of the Sky and Scattering:

Answer: (B) Bourdon pressure gauge

Explanation: A Bourdon gauge has a curved, flattened metal tube closed at one end. When fluid under pressure enters the tube, it tends to straighten, and this small movement is passed through a linkage to a pointer on a dial. Because the tube is made of strong metal, the gauge can measure very high pressures, up to about 10⁵ psi in special designs. U-tube manometers suit only low pressures, while McLeod and Pirani gauges are meant for very low pressures (high vacuum).

Pressure Gauges:

Answer: (C) Phosphorous

Explanation: An n-type semiconductor is made by doping ultrapure silicon with a pentavalent impurity, an element with five valence electrons. Phosphorus is pentavalent, so four of its electrons bond with neighbouring silicon atoms and the fifth is left free to conduct. Boron, aluminium and indium are trivalent and produce p-type semiconductors.

Doping Elements: Donors and Acceptors

Answer: (A) Forward biased

Explanation: A light emitting diode is a p-n junction made of a direct band gap semiconductor. When it is forward biased, electrons from the n-side and holes from the p-side cross the junction and recombine. The energy released in each recombination is given out as a photon, so the diode glows. Under reverse bias almost no current flows, so no light is produced.

LED: Working, Materials and Colours

Answer: (D) GaAs

Explanation: LED and LASER diodes are made from compound semiconductors with a direct band gap, in which electron-hole recombination releases energy as light. Gallium arsenide (GaAs) is the standard example. The individual elements Al, Ga and As alone are not the semiconductor material; it is the compound GaAs (and its alloys such as GaAsP and AlGaAs) that is used.

Semiconductor Materials for LEDs and Lasers:

Answer: (A) increases with temperature

Explanation: In a conductor (metal), the free electrons collide with the vibrating positive ions of the lattice. As temperature rises, the ions vibrate more strongly, so electrons are scattered more often and their relaxation time falls. The resistivity therefore increases with temperature, following ρₜ = ρ₀(1 + αΔT).

Resistivity and Temperature:

Answer: (A) Air pressure

Explanation: It is the standard SI unit of pressure (including air pressure and stress). One pascal is equal to a force of one newton per square metre (1 N/m2). 

Pressure and Related Units:

Answer: (D) converts electrical energy into mechanical energy.

Explanation: An electric motor works on the principle that a current-carrying conductor placed in a magnetic field experiences a force. This force turns the coil (armature) and produces rotation, so electrical energy supplied to the motor is changed into mechanical energy. A motor does not generate energy by itself, and the reverse conversion of mechanical energy into electrical energy is done by a generator (dynamo).

Electric Motor and Generator:

Answer: (C) Anemometer

Explanation: An anemometer is the instrument used to measure the speed of wind. The common cup type has three or four cups mounted on a rotating arm, and the faster the wind blows, the faster the cups spin. The rotation rate is converted into wind speed. A speedometer measures vehicle speed, a tachometer measures rotational speed of an engine, and a hygrometer measures humidity.

Meteorological Instruments:

Answer: (C) γ-rays

Explanation: Gamma rays have the highest energy and the greatest penetrating power among the given radiations. They pass through the body and damage cells deep inside, breaking DNA and causing mutations, burns and cancer. Alpha rays are stopped by skin or paper and beta rays by thin metal, so they are less dangerous from outside the body, while X-rays are similar in nature but have lower energy than gamma rays.

Radiations:

Answer: (D) a heat engine working in reverse order.

Explanation: A refrigerator takes heat from a cold space (the freezer or cabinet) and rejects it to the warmer surroundings, with the help of work done by a compressor. A heat engine does the opposite: it takes heat from a hot source, converts part of it into work, and rejects the rest to a cold sink. Since the refrigerator reverses this process, it is a heat engine working in reverse order.

Refrigerator and Heat Engine:

Answer: (D) Steel

Explanation: Sound travels fastest in solids, then liquids, then gases, and not at all in vacuum. The speed of sound depends on the elasticity and density of the medium, and solids like steel have very high elasticity compared to their density. Sound therefore moves at about 5000 m/s in steel, much faster than in water or air.

Speed of Sound in Different Media:

Answer: (C) interference

Explanation: A thin oil film on water shows colours because of interference of light. Light reflected from the top surface of the film and light reflected from the bottom surface (the oil-water boundary) meet and interfere. For a given film thickness, some wavelengths are strengthened and others are cancelled, so particular colours are seen. As the thickness varies across the film, the colours change from place to place.

Thin Film Interference:

Answer: (D) Joule

Explanation: The MKS system uses metre, kilogram and second as base units. Energy in this system is measured in joule, where one joule is the work done when a force of one newton moves a body through one metre in the direction of the force. Volt is a unit of potential difference, ohm is a unit of resistance, and erg is the unit of energy in the CGS system.

Units of Energy:

Answer: (A) Gravitational forces

Explanation: Of the four fundamental forces, gravitation is by far the weakest. It is about 10⁻³⁸ times the strength of the strong nuclear force, and about 10⁻³⁶ times that of the electromagnetic force. It appears strong in everyday life only because it is always attractive and acts between huge masses like the earth, which have no net electric charge to cancel it.

Fundamental Forces:

Answer: (D) Inertia

Explanation: A lubricant forms a thin layer between two surfaces in contact and reduces the friction between them, whether the surfaces are at rest, sliding or rolling. Inertia, however, is the property of a body to resist any change in its state of rest or motion, and it depends only on mass. It has nothing to do with the surfaces in contact, so a lubricant cannot reduce it.

Friction and Lubrication:

Answer: (D) decreases with temperature.

Explanation: Surface tension arises from the cohesive forces between molecules at the liquid surface. When temperature rises, the molecules move faster and the cohesive forces weaken, so the surface tension falls. It becomes zero at the critical temperature, where the liquid and vapour become indistinguishable. Surface tension does not depend on the area of the surface, because it is force per unit length (or energy per unit area), which is a property of the liquid.

Surface Tension:

Answer: (D) dispersion

Explanation: White light is a mixture of seven colours, and each colour has a different speed in a transparent medium such as glass, so each is bent by a different amount. When white light passes through a prism, violet bends the most and red bends the least, and the light spreads out into its component colours. This splitting of white light into its colours is called dispersion.

Dispersion of Light:

Answer: (D) Both Electrons and Holes

Explanation: In a semiconductor, current is carried by two types of charge carriers. Free electrons in the conduction band move under an electric field, and the vacancies left behind in the valence band, called holes, also drift as neighbouring electrons fill them. In an intrinsic semiconductor the two are present in equal numbers, and in doped semiconductors one type dominates but the other is still present.

Conduction in Semiconductors:

Answer: (C) with a speed greater than the speed of sound.

Explanation: Supersonic aircraft fly at speeds greater than the speed of sound in air, that is, above Mach 1 (about 331 m/s at 0°C, or roughly 1200 km/h). When an aircraft crosses this speed, it outruns its own sound waves, which pile up into a shock wave that is heard on the ground as a loud sonic boom. Speeds below the speed of sound are called subsonic, and speeds above Mach 5 are called hypersonic.

Speed Regimes of Aircraft:

Answer: (D) 9

Explanation: The question asks for the ratio R/R′, where R is the original resistance and R′ is the equivalent resistance after the wire is cut into three equal parts and joined in parallel. Each part has resistance R/3, because resistance is proportional to length. Three equal resistors of R/3 in parallel give R′ = (R/3)/3 = R/9. The ratio R/R′ is therefore 9.

Resistance and Combinations:

Answer: (A) 5 Hz

Explanation: The audible range for human beings is about 20 Hz to 20,000 Hz. A frequency of 5 Hz lies below 20 Hz, in the infrasonic region, so it cannot be heard. The frequencies 50 Hz, 500 Hz and 5000 Hz all fall within the audible range.

Sound Frequency Ranges:

Answer: (D) Both (i) and (ii) are true

Explanation: An emf is induced in a circuit whenever the magnetic flux linked with it changes. Flux can change in two ways: a conductor can move through a steady magnetic field (motional emf, e = Blv), or the magnetic field itself can change with time while the conductor stays still (transformer emf). Both statements describe valid ways of inducing an emf, so both are true.

Electromagnetic Induction:

Answer: (A) 1/6 of that on the surface of the earth

Explanation: The acceleration due to gravity on a planet is g = GM/R². The moon has about 1/81 of the earth’s mass and about 1/3.7 of its radius, which together give a surface gravity of about 1.62 m/s², roughly one sixth of the earth’s 9.8 m/s². A body therefore weighs about one sixth as much on the moon as on the earth, while its mass stays the same.

Gravity on Celestial Bodies:

Answer: (D) Joule and Calorie

Explanation: Joule and calorie are both units of energy (more precisely, of work and heat), with 1 calorie equal to 4.186 joule. Kelvin is a unit of temperature, and newton is a unit of force. Only the joule and calorie therefore measure the same physical quantity.

Units and Their Physical Quantities:

Answer: (B) convection currents

Explanation: During summer days, the land on the shore heats up faster than the water of the river, because water has a high specific heat. The air above the land becomes warm and light and rises, and cooler air from above the water moves in to take its place. This circulation of air is a convection current, and it is felt as a mild breeze from the river towards the shore.

Sea Breeze and Land Breeze:

Answer: (B) (-) 40°

Explanation: The conversion from Celsius to Fahrenheit is F = (9/5)C + 32. Putting C = -40 gives F = (9/5)(-40) + 32 = -72 + 32 = -40. The two scales therefore read the same value at -40 degrees, which is the only temperature at which this happens.

Temperature Scales:

Answer: (C) 225 Joule

Explanation: The work done in lifting a body against gravity equals the gain in its potential energy, W = mgh. Here m = 15 kg, g = 10 m/s² and h = 1.5 m, so W = 15 × 10 × 1.5 = 225 J. The porter’s walking or standing with the load does no work in the physical sense, because the force is vertical and the displacement is horizontal.

Work, Energy and Power:

Answer: (C) Silicon

Explanation: Silicon is a semiconductor, a material whose electrical conductivity lies between that of a conductor and an insulator. Its band gap is about 1.1 eV, so at room temperature a small number of electrons can jump to the conduction band, and conductivity rises when it is heated or doped. Aluminium, mercury and silver are metals and are good conductors.

Semiconductors and Other Materials:

Answer: (D) ML²T⁻²

Explanation: Energy has the same dimensions as work, which is force multiplied by distance. Force has dimensions MLT⁻² and distance has dimension L, so energy has dimensions ML²T⁻². This can also be checked from kinetic energy, ½mv², which gives M × (LT⁻¹)² = ML²T⁻².

Dimensions of Physical Quantities:

Answer: (D) photo electric effect

Explanation: Albert Einstein received the Nobel Prize in Physics for 1921 “for his services to theoretical physics, and especially for his discovery of the law of the photoelectric effect.” His 1905 explanation used the idea that light is made of quanta of energy E = hν, and that an electron is emitted only if a quantum has enough energy to overcome the work function. The theory of relativity, though more famous, was not what the prize was awarded for.

Einstein and Nobel Prizes in Physics:

Answer: (B) photon

Explanation: A quantum of light is called a photon. Each photon carries energy E = hν, where h is Planck’s constant and ν is the frequency of the light. Photons have zero rest mass and travel at the speed of light in vacuum. The term was coined by G. N. Lewis in 1926, though the quantum idea came from Planck and Einstein.

Particles and Their Quanta:

Answer: (A) Frequency

Explanation: When light passes from one medium to another, its frequency is fixed by the source and does not change. The speed of light changes with the medium, and since velocity equals frequency times wavelength, the wavelength changes along with the velocity. The amplitude also changes, because part of the light is reflected at the boundary and part may be absorbed.

Refraction:

Answer: (C) 1905

Explanation: Einstein proposed the special theory of relativity in 1905, in his paper “On the Electrodynamics of Moving Bodies.” It is based on two postulates: the laws of physics are the same in all inertial frames, and the speed of light in vacuum is the same for all observers. It led to time dilation, length contraction and the mass-energy relation E = mc². The general theory followed in 1915.

Einstein’s Annus Mirabilis and Relativity:

Answer: (C) X-ray

Explanation: The first Nobel Prize in Physics, awarded in 1901, went to Wilhelm Conrad Röntgen for the discovery of X-rays, which he made in 1895 in Würzburg, Germany. He found that an unknown penetrating radiation from a discharge tube could pass through soft tissue and produce shadow images of bones on a fluorescent screen. He called them X-rays because their nature was unknown.

Early Nobel Prizes in Physics:

Answer: (C) Water

Explanation: The time taken to cool depends on the heat the body must lose, which is Q = mcΔT. For the same mass and the same fall in temperature (80°C to 20°C), the substance with the largest specific heat capacity holds the most heat and therefore takes the longest to cool. Water has a specific heat of about 4186 J/kg·K, which is higher than that of glycerine, iron and copper, so water takes the longest.

Specific Heat Capacities:

Answer: (B) 346 meter

Explanation: An echo is the reflection of sound from a distant surface. The 2 s taken is the time for the sound to travel to the cliff and back, so the sound covers twice the distance of the cliff. Total distance = speed × time = 346 × 2 = 692 m, and the distance of the cliff is 692 / 2 = 346 m.

Echo and Reflection of Sound:

Answer: (B) 628 meter

Explanation: One complete round of a circular track covers a distance equal to its circumference. With diameter 200 m, the circumference is C = πd = 3.14 × 200 = 628 m. The distance covered is therefore 628 m, while the displacement after one full round is zero, since the athlete returns to the starting point.

Circle, Distance and Displacement:

Answer: (A) gets larger

Explanation: When a metal sheet is heated, every part of it expands, including the material around the hole. The hole behaves as if it were made of the same metal, so its dimensions increase in the same ratio as any other part of the sheet. The hole therefore becomes larger, not smaller. A common mistake is to think the metal expands into the hole, but the whole sheet scales up uniformly.

Thermal Expansion:

Answer: (C) Heisenberg

Explanation: The uncertainty principle was proposed by the German physicist Werner Heisenberg in 1927. It states that the position and momentum of a particle cannot both be measured exactly at the same time, and the product of their uncertainties is at least h/4π. This limit comes from the wave nature of matter and not from faulty instruments.

Quantum Mechanics:

Answer: (B) refraction

Explanation: Starlight passes through layers of the earth’s atmosphere whose density and refractive index keep changing because of air movement and temperature differences. The light bends by slightly different amounts from moment to moment, so the apparent position and the amount of light reaching the eye fluctuate. This makes the star seem to flicker, which is called twinkling, and it is caused by atmospheric refraction.

Twinkling of Stars:

Answer: (C) the surface of the earth

Explanation: Above the surface, g falls with height as g′ = g R²/(R + h)², so it decreases as we go up. Below the surface, g falls linearly with depth as g′ = g (1 − d/R), and it becomes zero at the centre. Taking the earth as a uniform sphere, the value is therefore greatest on the surface itself.

Variation of g:

Answer: (C) Both will reach simultaneously

Explanation: The vertical motion of a horizontally fired bullet is independent of its horizontal motion. Both bullets start with zero vertical velocity from the same height, and both fall under the same acceleration due to gravity, so the time of fall is t = √(2h/g). This time does not depend on the mass or on the horizontal velocity of the bullet, so both bullets hit the ground at the same instant. The faster bullet only lands farther away.

Projectile Motion:

Answer: (C) increases heavily

Explanation: A short circuit occurs when the live and neutral wires touch directly, so the resistance of the circuit drops to almost zero. By Ohm’s law, I = V/R, so the current rises to a very large value. This large current produces intense heating (H = I²Rt), which can melt wires and start fires, and it is the reason fuses and circuit breakers are fitted.

Short Circuit and Electrical Safety:

Answer: (D) 1 : 2

Explanation: In a series circuit the same current flows through both resistors. The heat produced is H = I²Rt, so for equal current and equal time, the heat developed is directly proportional to the resistance. The ratio of heat in R and 2R is therefore R : 2R = 1 : 2.

Heating Effect of Current:

Answer: (B) gm/cm³

Explanation: Density is mass per unit volume. In the CGS system mass is measured in grams and length in centimetres, so volume is in cm³ and density is in gm/cm³. The SI unit is kg/m³, and 1 gm/cm³ equals 1000 kg/m³.

Density:

Answer: (D) 16 km

Explanation: One mile is equal to about 1.609 km. Therefore 10 miles = 10 × 1.609 = 16.09 km, which is closest to 16 km. A quick method is to multiply the miles by 1.6 to get kilometres.

Length Units:

Answer: (C) 50°F

Explanation: The conversion from Celsius to Fahrenheit is F = (9/5)C + 32. Putting C = 10 gives F = (9/5)(10) + 32 = 18 + 32 = 50. So 10°C is equal to 50°F.

Temperature Conversions: