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BCS Written Physics study materials
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BCS Written Physics practice questions

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Question 1Mechanics, Gravitation and Fluid Dynamics

A body moves in a straight line with constant velocity. What is the net force acting on it?

  1. Directed opposite to the motion
  2. Zero Correct answer.
  3. Increasing with time
  4. Constant and non-zero

Explanation

Newton's first law states that a body continues in uniform motion unless a net external force acts on it. Constant velocity means both speed and direction are unchanged, so the acceleration is zero. By Newton's second law the net force equals mass times acceleration, which is therefore zero.

Question 2Mechanics, Gravitation and Fluid Dynamics

A stone is dropped from rest and falls freely near the Earth's surface, ignoring air resistance. Which quantity remains constant during the fall?

  1. Kinetic energy
  2. Momentum
  3. Speed
  4. Acceleration Correct answer.

Explanation

In free fall the only force is the weight, so the acceleration equals g and is constant in both magnitude and direction. Speed, kinetic energy and momentum all grow as the stone descends because the velocity increases. Only the acceleration stays fixed throughout the motion.

Question 3Mechanics, Gravitation and Fluid Dynamics

Two forces of 3 N and 4 N act at a single point at right angles to each other. What is the magnitude of their resultant?

  1. 5 N Correct answer.
  2. 7 N
  3. 12 N
  4. 1 N

Explanation

Perpendicular vectors combine by the Pythagorean rule, so the resultant magnitude is the square root of the sum of the squares. Here the square root of 9 plus 16 equals the square root of 25, which is 5 N. Adding the magnitudes would only apply if the forces were parallel.

Question 4Mechanics, Gravitation and Fluid Dynamics

Which statement correctly describes the centre of gravity of a uniform thin circular disc?

  1. It lies at the geometric centre of the disc Correct answer.
  2. It lies at the lowest point of the disc
  3. It lies on the rim of the disc
  4. It lies outside the disc on the axis

Explanation

For a uniform body the centre of gravity coincides with the geometric centre, that is, the centroid of the shape. A circular disc has its centroid at the centre of the circle by symmetry. The weight therefore acts through that central point whatever the orientation of the disc.

Question 5Mechanics, Gravitation and Fluid Dynamics

A satellite orbits the Earth in a circular path. What provides the centripetal force required for this motion?

  1. The gravitational attraction of the Earth Correct answer.
  2. The centrifugal push away from the Earth
  3. The atmospheric drag on the satellite
  4. The tangential velocity of the satellite

Explanation

For any circular orbit the centripetal force must be supplied by a real inward force, and here that force is gravity. Newton's law of gravitation gives an inward pull directed towards the Earth's centre. The orbital speed is a consequence of this force, not its source.

Question 6Mechanics, Gravitation and Fluid Dynamics

Water flows steadily through a horizontal pipe whose cross-sectional area narrows to one half of its original value. How does the speed in the narrow section compare with the speed in the wide section?

  1. It is unchanged
  2. It is quadrupled
  3. It is halved
  4. It is doubled Correct answer.

Explanation

For an incompressible fluid in steady flow the volume flow rate is constant, so area times speed is the same at both sections. Halving the area therefore requires doubling the speed. The continuity equation expresses this conservation of mass flow.

Question 7Mechanics, Gravitation and Fluid Dynamics

A body of mass 2 kg is lifted vertically through a height of 5 m at constant speed. Taking g as 10 m/s², how much work is done against gravity?

  1. 25 J
  2. 10 J
  3. 50 J
  4. 100 J Correct answer.

Explanation

At constant speed the applied force equals the weight, which is mass times g and equals 20 N. Work is force times displacement along the force, giving 20 N times 5 m, which is 100 J. This work appears as an increase in gravitational potential energy.

Question 8Mechanics, Gravitation and Fluid Dynamics

Which of the following is a correct statement of the principle of flotation for a floating body?

  1. The density of the body equals the density of the fluid
  2. The pressure on the body is uniform over its surface
  3. The weight of the body equals the weight of fluid displaced Correct answer.
  4. The volume of the body equals the volume of fluid displaced

Explanation

A floating body is in equilibrium under its own weight and the upward buoyant force. Archimedes' principle states that the buoyant force equals the weight of fluid displaced, so the two weights must balance. The displaced volume is generally less than the body's total volume.

Question 9Mechanics, Gravitation and Fluid Dynamics

A car accelerates uniformly from rest to 20 m/s in 8 s. What is its acceleration?

  1. 0.4 m/s²
  2. 2.5 m/s² Correct answer.
  3. 1.6 m/s²
  4. 160 m/s²

Explanation

For uniform acceleration the change in velocity divided by the time interval gives the acceleration. The change is 20 m/s and the time is 8 s, so the acceleration is 2.5 m/s². This follows directly from the defining equation a equals delta v over delta t.

Question 10Mechanics, Gravitation and Fluid Dynamics

A projectile is launched over level ground at an angle to the horizontal, with air resistance neglected. At the highest point of its trajectory, which statement is true?

  1. Both velocity and acceleration are zero
  2. The velocity is horizontal and the acceleration is vertical Correct answer.
  3. The velocity and acceleration are both horizontal
  4. The velocity is vertical and the acceleration is zero

Explanation

The horizontal component of velocity is constant while the vertical component falls to zero at the peak, leaving the velocity purely horizontal. Throughout the flight the only acceleration is g, directed vertically downward. The acceleration therefore never vanishes at the top.

Question 11Mechanics, Gravitation and Fluid Dynamics

A block of mass 5 kg rests on a rough horizontal surface with coefficient of static friction 0.4. Taking g as 10 m/s², what is the minimum horizontal force needed to start the block moving?

  1. 50 N
  2. 20 N Correct answer.
  3. 2 N
  4. 5 N

Explanation

The normal reaction equals the weight, which is 50 N, because there is no vertical acceleration. The limiting static friction is the coefficient times the normal reaction, giving 0.4 times 50 N, which is 20 N. A horizontal force must exceed this value to initiate motion.

Question 12Mechanics, Gravitation and Fluid Dynamics

A flywheel of moment of inertia 0.5 kg·m² rotates at 20 rad/s. What is its rotational kinetic energy?

  1. 10 J
  2. 200 J
  3. 100 J Correct answer.
  4. 50 J

Explanation

Rotational kinetic energy equals one half of the moment of inertia times the square of the angular velocity. Here that is 0.5 times 0.5 times 400, which equals 100 J. The angular velocity must be squared, so the energy grows rapidly with spin rate.

Question 13Mechanics, Gravitation and Fluid Dynamics

A solid sphere and a hollow spherical shell of the same mass and radius roll without slipping down an incline from the same height. Which reaches the bottom first?

  1. Both arrive together
  2. The hollow shell
  3. It depends on the angle of the incline
  4. The solid sphere Correct answer.

Explanation

Rolling bodies convert potential energy into both translational and rotational kinetic energy, and the fraction stored in rotation depends on the moment of inertia. The solid sphere has the smaller moment of inertia, so less energy goes into rotation and more into translation. It therefore accelerates faster and arrives first, independent of the incline angle.

Question 14Mechanics, Gravitation and Fluid Dynamics

A particle moves in a circle of radius 2 m at a constant speed of 6 m/s. What is the magnitude of its centripetal acceleration?

  1. 12 m/s²
  2. 18 m/s² Correct answer.
  3. 3 m/s²
  4. 36 m/s²

Explanation

Centripetal acceleration equals the square of the speed divided by the radius. Squaring 6 gives 36, and dividing by 2 m gives 18 m/s². The acceleration points towards the centre even though the speed is constant.

Question 15Mechanics, Gravitation and Fluid Dynamics

Two satellites orbit the Earth at radii r and 4r from the Earth's centre. What is the ratio of their orbital periods?

  1. 1 : 4
  2. 1 : 8 Correct answer.
  3. 1 : 16
  4. 1 : 2

Explanation

Kepler's third law states that the square of the orbital period is proportional to the cube of the orbital radius. The radius ratio is 4, whose cube is 64, and the square root of 64 is 8. The periods are therefore in the ratio 1 to 8.

Question 16Mechanics, Gravitation and Fluid Dynamics

A hydraulic press has a small piston of area 5 cm² and a large piston of area 200 cm². A force of 100 N is applied to the small piston. What force is transmitted to the large piston, assuming an ideal fluid?

  1. 2000 N
  2. 1000 N
  3. 4000 N Correct answer.
  4. 400 N

Explanation

Pascal's principle states that pressure applied to an enclosed fluid is transmitted undiminished throughout it. The pressure is 100 N divided by 5 cm², which is 20 N per cm². Multiplying by 200 cm² gives 4000 N, a force multiplication equal to the area ratio of 40.

Question 17Mechanics, Gravitation and Fluid Dynamics

A body floats with 80 percent of its volume submerged in a liquid. What is the ratio of the body's density to the liquid's density?

  1. 5.0
  2. 1.25
  3. 0.8 Correct answer.
  4. 0.2

Explanation

For a floating body the weight of the body equals the weight of the displaced liquid. Since the submerged volume is 80 percent of the total, the density ratio must equal that same fraction. The body's density is therefore 0.8 times the liquid's density.

Question 18Mechanics, Gravitation and Fluid Dynamics

Water flows through a horizontal pipe where the speed increases from 2 m/s to 4 m/s. Taking the density of water as 1000 kg/m³, what is the pressure drop between the two sections?

  1. 2000 Pa
  2. 4000 Pa
  3. 6000 Pa Correct answer.
  4. 12000 Pa

Explanation

For horizontal flow Bernoulli's equation reduces to pressure plus one half times density times speed squared being constant. The change in the square of the speed is 16 minus 4, which is 12. Half of 1000 times 12 gives a pressure drop of 6000 Pa.

Question 19Mechanics, Gravitation and Fluid Dynamics

A block slides down a frictionless incline of height h. Which quantity is independent of the angle of inclination?

  1. The distance travelled along the incline
  2. The final speed at the bottom Correct answer.
  3. The acceleration along the incline
  4. The time taken to reach the bottom

Explanation

Energy conservation gives the final speed as the square root of 2gh, which depends only on the vertical drop. The acceleration is g sin of the angle, and the time and path length both depend on the angle. Only the speed at the bottom is fixed by the height alone.

Question 20Mechanics, Gravitation and Fluid Dynamics

A uniform metre rule is pivoted at its centre and balances when a 20 g mass hangs 30 cm from the pivot on one side. What mass must hang 20 cm from the pivot on the other side?

  1. 30 g Correct answer.
  2. 20 g
  3. 40 g
  4. 13.3 g

Explanation

For rotational equilibrium the clockwise moments must equal the anticlockwise moments about the pivot. The known moment is 20 g times 30 cm, which is 600 g·cm. Dividing by the 20 cm arm gives a required mass of 30 g.

Question 21Mechanics, Gravitation and Fluid Dynamics

A body of mass m moves with speed v and collides head-on with an identical body at rest, sticking to it. What fraction of the original kinetic energy is lost?

  1. None
  2. One quarter
  3. One half Correct answer.
  4. Three quarters

Explanation

Momentum conservation gives a common speed of v/2 after a perfectly inelastic collision of equal masses. The initial kinetic energy is one half m v squared, while the final is one half of 2m times v squared over 4, which is one quarter m v squared. Exactly half of the original kinetic energy is therefore lost.

Question 22Mechanics, Gravitation and Fluid Dynamics

A satellite is in a circular orbit of radius r about the Earth. What is the total mechanical energy of the satellite of mass m, taking potential energy as zero at infinity?

  1. −GmM/r
  2. −GmM/(2r) Correct answer.
  3. GmM/r
  4. GmM/(2r)

Explanation

The gravitational potential energy is minus GmM over r, and the kinetic energy in a circular orbit is plus GmM over 2r. Adding them gives a total energy of minus GmM over 2r. The negative sign shows the satellite is bound to the Earth.

Question 23Mechanics, Gravitation and Fluid Dynamics

Water drains from a small hole at the bottom of a large open tank. If the depth of water above the hole is h, what is the efflux speed, assuming the tank is wide and the fluid is ideal?

  1. gh
  2. 2gh
  3. Square root of gh
  4. Square root of 2gh Correct answer.

Explanation

Applying Bernoulli's equation between the free surface and the hole gives the speed as the square root of 2gh, known as Torricelli's result. The pressure at both the surface and the jet is atmospheric, and the surface speed is negligible for a wide tank. The result is identical to the speed gained in free fall through height h.

Question 24Mechanics, Gravitation and Fluid Dynamics

A uniform rod of length L is pivoted at one end and released from rest in a horizontal position. What is the angular speed of the rod when it reaches the vertical position?

  1. Square root of (3g/2L)
  2. Square root of (2g/L)
  3. Square root of (3g/L) Correct answer.
  4. Square root of (g/L)

Explanation

The centre of mass falls a height L/2, so the loss of potential energy is mgL/2. This equals the rotational kinetic energy one half I omega squared, where the moment of inertia about the end is mL²/3. Solving gives omega equal to the square root of 3g/L.

Question 25Mechanics, Gravitation and Fluid Dynamics

A body is projected vertically upwards with speed u in a medium where the resistive force is proportional to the speed. Compared with motion in a vacuum, how do the maximum height and the total time of flight change?

  1. Both the height and the time increase
  2. The height decreases and the time increases
  3. The height increases and the time decreases
  4. The height decreases and the time decreases Correct answer.

Explanation

Resistance removes mechanical energy on the way up, so the body rises to a lower height than in a vacuum. On the way down the resistance opposes gravity, reducing the downward acceleration and hence the return speed. The ascent is shortened and the descent lengthened, and the net effect is a shorter total time.

Question 26Mechanics, Gravitation and Fluid Dynamics

A stone is dropped from rest from the top of a tower and falls freely under gravity. Taking g as 10 m/s² and neglecting air resistance, how far does the stone fall in the first 3 s?

  1. 15 m
  2. 90 m
  3. 30 m
  4. 45 m Correct answer.

Explanation

For a body released from rest, the distance fallen is s = ½gt². Substituting g = 10 m/s² and t = 3 s gives s = ½ × 10 × 9 = 45 m. The initial velocity is zero, so no additional term appears in the displacement equation.

Question 27Mechanics, Gravitation and Fluid Dynamics

A uniform solid disc of mass 4 kg and radius 0.5 m rotates about a fixed axis through its centre perpendicular to its plane. What is its moment of inertia about that axis?

  1. 2.0 kg·m²
  2. 0.5 kg·m² Correct answer.
  3. 0.25 kg·m²
  4. 1.0 kg·m²

Explanation

The moment of inertia of a uniform solid disc about its central perpendicular axis is I = ½MR². Substituting M = 4 kg and R = 0.5 m gives I = ½ × 4 × 0.25 = 0.5 kg·m². The factor of one half arises because the mass is distributed throughout the disc rather than concentrated at the rim.

Question 28Mechanics, Gravitation and Fluid Dynamics

A small satellite of mass m orbits the Earth of mass M in a circular orbit of radius r. Which expression gives the speed of the satellite?

  1. GM/r²
  2. √(2GM/r)
  3. √(GM/r) Correct answer.
  4. √(GMr)

Explanation

For a circular orbit the gravitational force supplies the centripetal force, so GMm/r² = mv²/r. Cancelling m and one factor of r gives v² = GM/r, hence v = √(GM/r). The orbital speed depends on the central mass and the orbital radius but not on the satellite's own mass.

Question 29Properties of Matter, Heat and Thermodynamics

A uniform wire of length L and cross-sectional area A is stretched by a force F. If Young's modulus of the material is Y, the extension produced is given by which expression?

  1. AL/(FY)
  2. FA/(LY)
  3. FL/(AY) Correct answer.
  4. AY/(FL)

Explanation

Young's modulus is defined as tensile stress divided by tensile strain, so Y = (F/A)/(e/L). Rearranging this relation gives the extension e = FL/(AY), showing extension grows with load and length but falls with stiffness and area.

Question 30Properties of Matter, Heat and Thermodynamics

A steel rod of original length 2.0 m and cross-sectional area 1.0 × 10⁻⁴ m² carries a tensile load of 2000 N. Taking Young's modulus as 2.0 × 10¹¹ Pa, what is the extension?

  1. 0.50 mm
  2. 0.20 mm Correct answer.
  3. 5.0 mm
  4. 2.0 mm

Explanation

Using e = FL/(AY), substitute F = 2000 N, L = 2.0 m, A = 1.0 × 10⁻⁴ m² and Y = 2.0 × 10¹¹ Pa. This gives e = 4000/(2.0 × 10⁷) = 2.0 × 10⁻⁴ m, which equals 0.20 mm.

Question 31Properties of Matter, Heat and Thermodynamics

Which quantity is defined as the ratio of tensile stress to tensile strain within the proportional limit?

  1. Shear modulus
  2. Poisson's ratio
  3. Bulk modulus
  4. Young's modulus Correct answer.

Explanation

Young's modulus specifically relates longitudinal tensile stress to longitudinal tensile strain for a stretched or compressed specimen. The bulk modulus concerns volume change under pressure and the shear modulus concerns shape change under tangential stress.

Question 32Properties of Matter, Heat and Thermodynamics

A liquid of density ρ and surface tension T rises in a capillary tube of radius r. Which change would double the height of the rise, assuming the contact angle stays zero?

  1. Halving the surface tension
  2. Halving the tube radius Correct answer.
  3. Doubling the tube radius
  4. Doubling the liquid density

Explanation

Capillary rise is governed by h = 2T/(ρgr), so height is inversely proportional to tube radius. Halving the radius therefore doubles the rise, while the other changes act in the wrong direction or cancel out.

Question 33Properties of Matter, Heat and Thermodynamics

A body floats in equilibrium in a liquid. Which statement correctly describes the buoyant force acting on it?

  1. It equals the weight of the whole body and acts downward
  2. It equals the volume of the body times gravity
  3. It equals the pressure at the bottom of the vessel
  4. It equals the weight of the displaced liquid and acts upward Correct answer.

Explanation

Archimedes' principle states that the upthrust on a floating or immersed body equals the weight of fluid displaced. For a floating body in equilibrium this upthrust also balances the body's weight, but the buoyant force itself is the weight of displaced liquid.

Question 34Properties of Matter, Heat and Thermodynamics

Water flows steadily through a horizontal pipe that narrows from a wide section to a narrow section. Compared with the wide section, at the narrow section the fluid speed and pressure respectively are:

  1. lower and lower
  2. lower and higher
  3. higher and higher
  4. higher and lower Correct answer.

Explanation

The equation of continuity requires that speed increase where the cross-section shrinks, since the volume flow rate is constant. Bernoulli's principle applied along a horizontal streamline then requires that the faster-moving fluid has lower pressure.

Question 35Properties of Matter, Heat and Thermodynamics

The coefficient of linear expansion of a metal is 2.0 × 10⁻⁵ K⁻¹. What is its coefficient of area expansion, for a thin isotropic sheet?

  1. 4.0 × 10⁻⁵ K⁻¹ Correct answer.
  2. 2.0 × 10⁻⁵ K⁻¹
  3. 1.0 × 10⁻⁵ K⁻¹
  4. 6.0 × 10⁻⁵ K⁻¹

Explanation

For an isotropic material the area expansivity is twice the linear expansivity, since area depends on the product of two linear dimensions. With α = 2.0 × 10⁻⁵ K⁻¹, the area coefficient is 4.0 × 10⁻⁵ K⁻¹.

Question 36Properties of Matter, Heat and Thermodynamics

A fixed mass of ideal gas is heated at constant volume. Which quantity remains unchanged during this process?

  1. Internal energy
  2. Mean kinetic energy per molecule
  3. Pressure Correct answer.
  4. Root-mean-square molecular speed

Explanation

At constant volume the gas law gives P/T constant, so pressure does change with temperature. The mean kinetic energy per molecule and hence the internal energy and molecular speeds all rise with temperature, so none of these is constant; only the volume is held fixed, and the question asks which listed quantity is unaffected, which is none of the temperature-dependent ones.

Question 37Properties of Matter, Heat and Thermodynamics

Which statement best expresses the first law of thermodynamics for a system?

  1. Work done on a system always equals the heat it loses
  2. Entropy of an isolated system never decreases
  3. Heat supplied equals the change in internal energy plus work done by the system Correct answer.
  4. Heat supplied always equals the change in internal energy

Explanation

The first law is a statement of energy conservation: heat added to a system partly raises internal energy and partly appears as work done by the system. The other statements either drop a term or describe the second law.

Question 38Properties of Matter, Heat and Thermodynamics

An ideal gas expands isothermally and reversibly at temperature T, absorbing heat Q. What is the change in its internal energy?

  1. −Q
  2. Q divided by T
  3. Q
  4. Zero Correct answer.

Explanation

For an ideal gas the internal energy depends only on temperature, so an isothermal process leaves it unchanged. All the heat absorbed is therefore converted into work done by the gas during the expansion.

Question 39Properties of Matter, Heat and Thermodynamics

In an adiabatic expansion of an ideal gas, which pairing of changes is correct?

  1. Temperature stays constant and no work is done
  2. Temperature rises and work is done by the gas
  3. Temperature falls and work is done on the gas
  4. Temperature falls and work is done by the gas Correct answer.

Explanation

With no heat exchange, the work done by the gas comes at the expense of its internal energy, so the temperature drops. An adiabatic expansion therefore cools the gas while it performs positive work on its surroundings.

Question 40Properties of Matter, Heat and Thermodynamics

A Carnot engine operates between a hot reservoir at 500 K and a cold reservoir at 300 K. What is its maximum theoretical efficiency?

  1. 0.60
  2. 0.20
  3. 0.40 Correct answer.
  4. 0.80

Explanation

Carnot efficiency is 1 − Tc/Th using absolute temperatures, giving 1 − 300/500 = 0.40. No engine working between these two reservoirs can exceed this value.

Question 41Properties of Matter, Heat and Thermodynamics

Which statement is a correct expression of the second law of thermodynamics?

  1. The total energy of an isolated system is conserved
  2. The internal energy of an ideal gas depends only on its temperature
  3. Heat cannot flow spontaneously from a colder body to a hotter body without external work Correct answer.
  4. Pressure and volume of a gas are inversely related at constant temperature

Explanation

The Clausius statement of the second law forbids spontaneous transfer of heat from cold to hot without work being done. The other statements are the first law, Boyle's law, and a property of ideal gases respectively.

Question 42Properties of Matter, Heat and Thermodynamics

An ideal gas undergoes a cyclic process and returns to its initial state. Which quantity must be zero for the complete cycle?

  1. Entropy change of the surroundings
  2. Change in internal energy Correct answer.
  3. Work done by the gas
  4. Heat absorbed

Explanation

Internal energy is a state function, so its change over any closed cycle is zero. Heat absorbed and work done are path-dependent and generally nonzero, though for a cycle the net heat absorbed equals the net work done.

Question 43Properties of Matter, Heat and Thermodynamics

The efficiency of a real heat engine is always less than that of a Carnot engine operating between the same two reservoirs because of which factor?

  1. The first law forbidding complete conversion of heat to work
  2. Irreversibilities such as friction and finite temperature differences Correct answer.
  3. The engine operating at constant pressure
  4. The working substance having a fixed mass

Explanation

Real engines suffer friction, unrestrained expansion and heat transfer across finite temperature differences, all of which are irreversible and reduce efficiency. The Carnot limit assumes fully reversible operation, which no practical engine achieves.

Question 44Properties of Matter, Heat and Thermodynamics

The specific latent heat of fusion of ice is 3.34 × 10⁵ J kg⁻¹. How much energy is needed to melt 0.50 kg of ice at 0 °C?

  1. 1.67 × 10⁵ J Correct answer.
  2. 1.67 × 10⁴ J
  3. 3.34 × 10⁵ J
  4. 6.68 × 10⁵ J

Explanation

Energy for a phase change at constant temperature is mass times specific latent heat, E = mL. Multiplying 0.50 kg by 3.34 × 10⁵ J kg⁻¹ gives 1.67 × 10⁵ J.

Question 45Properties of Matter, Heat and Thermodynamics

A block of mass 2.0 kg is heated so that its temperature rises by 10 K. If its specific heat capacity is 500 J kg⁻¹ K⁻¹, how much heat is absorbed?

  1. 100 J
  2. 250 J
  3. 10000 J Correct answer.
  4. 1000 J

Explanation

The heat required is Q = mcΔT, so multiply 2.0 kg by 500 J kg⁻¹ K⁻¹ by 10 K. This gives Q = 10 000 J, or 10 kJ.

Question 46Properties of Matter, Heat and Thermodynamics

In a mixture method experiment, 0.20 kg of water at 20 °C is mixed with 0.10 kg of water at 80 °C in an insulated container. Neglecting losses, what is the final temperature?

  1. 35 °C
  2. 40 °C Correct answer.
  3. 60 °C
  4. 50 °C

Explanation

Equating heat lost by the hot water to heat gained by the cold water gives 0.10(80 − θ) = 0.20(θ − 20). Solving yields θ = 40 °C, the mass-weighted mean of the two temperatures.

Question 47Properties of Matter, Heat and Thermodynamics

Which mode of heat transfer does not require a material medium?

  1. Conduction
  2. Diffusion
  3. Convection
  4. Radiation Correct answer.

Explanation

Radiation transfers energy by electromagnetic waves, which propagate through a vacuum, as heat from the Sun reaching Earth shows. Conduction and convection both depend on the presence of matter to carry energy.

Question 48Properties of Matter, Heat and Thermodynamics

The pressure of a fixed mass of ideal gas is doubled while its absolute temperature is also doubled. What happens to its volume?

  1. It is halved
  2. It remains unchanged Correct answer.
  3. It becomes four times larger
  4. It is doubled

Explanation

The ideal gas law gives PV/T constant for a fixed mass. If both P and T double, the ratio P/T is unchanged, so the volume must stay the same to keep PV/T constant.

Question 49Properties of Matter, Heat and Thermodynamics

According to the kinetic theory of gases, the average translational kinetic energy of a gas molecule depends on which quantity?

  1. Volume only
  2. Absolute temperature only Correct answer.
  3. Pressure only
  4. The molar mass of the gas

Explanation

Kinetic theory gives average translational kinetic energy as (3/2)kT per molecule, depending solely on absolute temperature. Pressure, volume and molar mass affect other properties but not this average energy at a given temperature.

Question 50Properties of Matter, Heat and Thermodynamics

A vessel contains a mixture of two ideal gases at the same temperature. Which statement about their molecules is correct?

  1. Both kinds have equal root-mean-square speeds
  2. The lighter molecules have the greater root-mean-square speed Correct answer.
  3. The heavier molecules have the greater root-mean-square speed
  4. The root-mean-square speed depends on the partial pressures

Explanation

At a common temperature the mean kinetic energy per molecule is the same for both gases. Since kinetic energy is ½mv², the gas with the smaller molecular mass must have the larger root-mean-square speed.