ESAT Physics: Thermal physics and matter, set 1

• Answer all questions. No calculator.
• Each question has exactly one correct answer.

1. 11 mark

   Metals are generally much better conductors of thermal energy than non-metals such as plastic or wood. Which of the following best explains why?

   1. A Metals contain free (delocalised) electrons that transfer thermal energy quickly through the metal, in addition to the vibration of particles.
   2. B Metal atoms are heavier than non-metal atoms, so they vibrate faster and pass on thermal energy more quickly.
   3. C Metals have weaker interatomic bonds than non-metals, allowing thermal energy to pass between particles with less resistance.
   4. D Metals absorb infrared radiation more strongly than non-metals, which is the main mechanism of heat transfer through a solid.
2. 21 mark

   A metal rod conducts thermal energy from its hot end to its cold end. Which single change, made on its own, would increase the rate of conduction along the rod?

   1. A Increasing the cross-sectional area of the rod
   2. B Decreasing the temperature difference between the two ends
   3. C Wrapping the rod in an insulating material
   4. D Increasing the length of the rod
3. 31 mark

   A radiator is placed near the floor of a room and warms the whole room by convection. Which statement correctly describes how this convection current is set up?

   1. A The air heated by the radiator contracts, becomes denser, and sinks; cooler air rises to take its place near the radiator.
   2. B The air heated by the radiator expands, becomes less dense, and rises; cooler, denser air sinks to replace it, setting up a circulating current.
   3. C Warm air particles move faster and directly pass energy to cold air particles throughout the room, with no bulk movement of air.
   4. D The radiator emits infrared radiation that heats the air uniformly throughout the room, without any change in air density.
4. 41 mark

   A beaker of water is heated gently from below, before any bubbles of steam have formed. Which statement correctly describes what happens to the water immediately above the flame, and why?

   1. A It stays in place because water is a poor conductor and convection cannot occur in liquids.
   2. B It becomes less dense than the water above it and rises, while cooler, denser water sinks to take its place.
   3. C It expands and rises only because bubbles physically push it upward, not because of any density change.
   4. D It becomes more dense than the water above it and sinks further, while cooler water rises above the flame.
5. 51 mark

   Which statement correctly describes thermal radiation and the type of surface that emits and absorbs it best?

   1. A Thermal radiation is a mechanical wave that needs a medium such as air to travel through.
   2. B Thermal radiation is infrared electromagnetic radiation; light, shiny surfaces are the best emitters and absorbers of it.
   3. C Thermal radiation is infrared electromagnetic radiation; dark, matt surfaces are the best emitters and absorbers of it.
   4. D Dark, matt surfaces are the best absorbers of infrared radiation, but shiny surfaces are the best emitters of it.
6. 61 mark

   A copper block and a water sample, both of the same mass, are each given the same amount of thermal energy. Copper has a much lower specific heat capacity than water. Which statement follows from this?

   1. A The copper block will show a smaller rise in temperature than the water for the same energy input.
   2. B Both will show the same rise in temperature, because they were given the same amount of energy.
   3. C The copper block requires more energy than the water to produce the same temperature rise.
   4. D The copper block will show a bigger rise in temperature than the water for the same energy input.
7. 71 mark

   A 2 kg block of metal has a specific heat capacity of 500 J/kg/degC. How much thermal energy is needed to raise its temperature from 20 degC to 40 degC?

   1. A 10000 J
   2. B 40000 J
   3. C 20000 J
   4. D 5000 J
8. 81 mark

   Which statement correctly compares the arrangement and motion of particles in a liquid with those in a gas?

   1. A In a liquid, particles are arranged in a fixed, regular pattern and vibrate in place; in a gas, particles move freely and are far apart.
   2. B In a liquid, particles are far apart and move slowly; in a gas, particles are close together and move quickly.
   3. C In a liquid, particles vibrate about fixed positions; in a gas, particles also vibrate about fixed positions, but with larger vibrations.
   4. D In a liquid, particles are close together and move in random directions; in a gas, particles are far apart and move much faster in random directions.
9. 91 mark

   A fixed mass of gas is heated in a sealed, rigid container, so its volume cannot change. Which statement correctly explains why the gas pressure increases?

   1. A The gas particles move faster on average and collide with the container walls more frequently and with greater force.
   2. B The gas particles move closer together, increasing the number of collisions with the walls.
   3. C The gas particles increase in size as they gain thermal energy, taking up more of the container's volume.
   4. D The gas particles move faster but collide with the walls less often, transferring more energy per collision.
10. 101 mark

    A fixed mass of gas at constant temperature has a volume of 6 m^3 at a pressure of 100 kPa. The gas is compressed at constant temperature until its volume is 2 m^3. What is the new pressure?

    1. A 150 kPa
    2. B 33.3 kPa
    3. C 300 kPa
    4. D 600 kPa
11. 111 mark

    A pure substance is heated steadily until it melts. While it is melting, its temperature stays constant even though heating continues. Which statement best explains why?

    1. A The thermal energy supplied is being reflected away from the substance while it changes state, so none is actually absorbed.
    2. B The thermal energy supplied is being used to break the bonds between particles as the substance changes state, rather than to increase the particles' kinetic energy.
    3. C The particles' kinetic energy is increasing during melting, which is why more energy is required, but the thermometer cannot detect this increase.
    4. D The rate of heating has been reduced automatically by the heat source while the substance melts.
12. 121 mark

    The specific latent heat of fusion of ice is 340000 J/kg. How much thermal energy is needed to melt 0.5 kg of ice at 0 degC, with no change in temperature?

    1. A 680000 J
    2. B 340000 J
    3. C 170000 J
    4. D 85000 J
13. 131 mark

    A solid block has a mass of 750 g and a volume of 250 cm^3. What is its density?

    1. A 0.33 g/cm^3
    2. B 3 g/cm^3
    3. C 187500 g/cm^3
    4. D 500 g/cm^3
14. 141 mark

    A box exerts a force of 240 N on the ground through its base, which has an area of 0.4 m^2. What pressure does the box exert on the ground?

    1. A 240.4 Pa
    2. B 96 Pa
    3. C 600 Pa
    4. D 1500 Pa
15. 151 mark

    What is the hydrostatic pressure at a depth of 3 m below the surface of a liquid of density 1000 kg/m^3? Take the gravitational field strength as g = 10 N/kg, and ignore atmospheric pressure.

    1. A 3000 Pa
    2. B 30000 Pa
    3. C 10000 Pa
    4. D 30 Pa

Worked solutions

Every question below carries the reasoning, not just the answer. The official material for this test publishes a correct option letter and nothing else.

1. Question 1Answer: A

   1. Conduction transfers thermal energy through a material without the material as a whole moving.
   2. In any solid, particles vibrate more vigorously when they have more thermal energy, and pass some of that energy to neighbouring particles.
   3. Metals have an additional mechanism: their structure contains free, delocalised electrons that can move through the whole metal, carrying thermal energy much faster than vibration alone.
   4. Non-metals such as plastic or wood have no such free electrons, so they rely only on the slower particle vibration mechanism and conduct far less well.
   5. Therefore the answer is A.
   • Why not B: This assumes heavier particles automatically vibrate faster and conduct better, but atomic mass is not what the specification gives as the reason metals conduct well; it is the presence of free electrons.
   • Why not C: This has the bonding the wrong way round: metals are held together by strong metallic bonding, and it is the delocalised electrons, not weak bonds, that make conduction fast.
   • Why not D: This confuses conduction with a different transfer mechanism. Absorbing infrared radiation well is a separate property from conducting thermal energy through a solid by particle and electron movement.

2. Question 2Answer: A

   1. The rate of conduction along a rod depends on its cross-sectional area, its length, and the temperature difference between its ends.
   2. A bigger temperature difference and a larger cross-sectional area both increase the rate of conduction, while a greater length decreases it.
   3. Increasing the length, decreasing the temperature difference, or adding insulation all act to slow conduction down, not speed it up.
   4. Increasing the cross-sectional area gives more material for thermal energy to pass through at once, which increases the rate of conduction.
   5. Therefore the answer is A.
   • Why not B: This is the wrong direction: a smaller temperature difference between the ends reduces the rate of conduction, since a bigger difference is what drives faster heat flow.
   • Why not C: This is the wrong direction: wrapping the rod in an insulator reduces the rate at which thermal energy escapes, rather than increasing the rate of conduction along it.
   • Why not D: This is the wrong direction: a longer rod gives thermal energy further to travel, which decreases the rate of conduction, rather than increasing it.

3. Question 3Answer: B

   1. Convection in a fluid is driven by changes in density caused by changes in temperature.
   2. Air heated by the radiator gains thermal energy, expands, and becomes less dense than the surrounding cooler air.
   3. This less dense, warmed air rises, and cooler, denser air sinks to take its place near the radiator.
   4. This continual rising and sinking sets up a circulating convection current that spreads warmth around the room.
   5. Therefore the answer is B.
   • Why not A: This reverses the density change: heating a fluid makes it expand and become less dense, so it rises, rather than contracting and sinking as this option claims.
   • Why not C: This describes particle-to-particle energy transfer without bulk movement, which is conduction, not convection; a convection current requires the heated fluid itself to move.
   • Why not D: This names radiation, not convection, as the mechanism, and radiation does not require or produce the density changes that actually drive this circulating current.

4. Question 4Answer: B

   1. Heating the water directly above the flame gives those particles more thermal energy, so they spread further apart.
   2. This makes the heated water less dense than the cooler water above it, even before any bubbles have formed.
   3. The less dense, warmed water rises, and cooler, denser water sinks down to replace it near the flame.
   4. This density-driven movement is convection, and it occurs in liquids just as it does in gases.
   5. Therefore the answer is B.
   • Why not A: This wrongly claims convection cannot occur in liquids; convection happens in any fluid, liquid or gas, whenever heating causes a density difference.
   • Why not C: This attributes the movement to bubbles, but no bubbles have formed yet at this stage; the water moves because heating has already made it less dense, which is the actual cause of convection.
   • Why not D: This reverses the density change: the heated water becomes less dense, not more dense, which is why it rises rather than sinking further.

5. Question 5Answer: C

   1. Thermal radiation is infrared electromagnetic radiation, so unlike sound it needs no medium and can travel through a vacuum.
   2. The rate at which a surface absorbs and emits infrared radiation depends on its colour and texture.
   3. Dark, matt surfaces absorb and emit infrared radiation well, while light, shiny surfaces are poor absorbers and emitters and instead reflect radiation well.
   4. A surface's absorbing and emitting ability go together, so a good absorber such as a dark, matt surface is also a good emitter.
   5. Therefore the answer is C.
   • Why not A: This wrongly classifies thermal radiation as a mechanical wave needing a medium, confusing it with sound; electromagnetic waves, including infrared, travel through a vacuum with no medium needed.
   • Why not B: This reverses which surfaces perform best: light, shiny surfaces are poor emitters and absorbers of infrared radiation, and are instead good reflectors of it.
   • Why not D: This wrongly separates emitting and absorbing ability between two different surface types; a surface that is a good absorber of infrared radiation, such as a dark, matt one, is also a good emitter of it, not the reverse.

6. Question 6Answer: D

   1. Specific heat capacity c is defined by thermal energy = mass x specific heat capacity x temperature change, so temperature change = thermal energy / (mass x c).
   2. For a fixed mass and a fixed amount of thermal energy, a smaller value of c gives a bigger temperature change.
   3. Copper has a lower specific heat capacity than water, so for the same mass and the same energy input, copper's temperature rise is bigger.
   4. Therefore the answer is D.
   • Why not A: This is the wrong direction: a lower specific heat capacity means a given mass needs less energy for each degree of temperature rise, so for the same energy input its temperature rise is bigger, not smaller.
   • Why not B: This ignores specific heat capacity altogether; specific heat capacity = thermal energy / (mass x temperature change) means that, for the same mass and energy, a lower specific heat capacity gives a bigger temperature change.
   • Why not C: This is the wrong direction: because copper has a lower specific heat capacity, it needs less energy than water for the same temperature rise, not more.

7. Question 7Answer: C

   1. Specific heat capacity gives thermal energy = mass x specific heat capacity x temperature change.
   2. The temperature change here is 40 - 20 = 20 degC, not the final temperature of 40 degC on its own.
   3. Substituting mass = 2 kg, specific heat capacity = 500 J/kg/degC and temperature change = 20 degC gives thermal energy = 2 x 500 x 20.
   4. 2 x 500 x 20 = 20000 J.
   5. Therefore the answer is C.
   • Why not A: This leaves the mass out of the calculation, giving just specific heat capacity x temperature change (500 x 20 = 10000), as if the mass were 1 kg rather than 2 kg.
   • Why not B: This uses the final temperature of 40 degC as the temperature change itself, instead of the actual rise of 20 degC (40 - 20), giving 2 x 500 x 40 = 40000.
   • Why not D: This divides by the mass instead of multiplying by it: (500 x 20) / 2 = 5000, when the mass should multiply the other two quantities, not divide them.

8. Question 8Answer: D

   1. In a liquid, particles are close together, held by weaker forces than in a solid, and move around each other in random directions.
   2. In a gas, particles are far apart, with negligible forces between them, and move much faster in random directions than liquid particles do.
   3. A fixed, regular arrangement with vibration in place describes a solid, not a liquid or a gas.
   4. Only one option keeps a liquid's particles close together and randomly moving, and a gas's particles far apart and moving faster.
   5. Therefore the answer is D.
   • Why not A: This gives the liquid a fixed, regular arrangement with particles vibrating in place, which is a description of a solid, not a liquid.
   • Why not B: This swaps the two states around: it is liquid particles that are close together, and gas particles that are far apart, not the reverse given here.
   • Why not C: This gives both states particles fixed about vibrating positions, which describes a solid; neither a liquid's nor a gas's particles are held in fixed positions in this way.

9. Question 9Answer: A

   1. Gas pressure is caused by particles colliding with the walls of their container.
   2. Heating the gas gives its particles more kinetic energy, so they move faster on average.
   3. Faster particles hit the container walls more often and with greater force each time.
   4. Both the increased frequency and increased force of collisions increase the pressure the gas exerts.
   5. Therefore the answer is A.
   • Why not B: This is not possible in a rigid, sealed container: the volume is fixed, so the particles cannot move closer together; the pressure rise instead comes from faster particle motion.
   • Why not C: This wrongly claims the particles themselves grow in size with temperature; heating changes how fast particles move, not their physical size.
   • Why not D: This gets the collision frequency backwards: heating a gas makes its particles collide with the walls more often, not less, in addition to hitting them harder.

10. Question 10Answer: C

    1. At constant temperature, pressure and volume of a fixed mass of gas obey pressure x volume = constant.
    2. The constant is the initial pressure multiplied by the initial volume: 100 kPa x 6 m^3 = 600.
    3. The new pressure is this constant divided by the new volume: 600 / 2 m^3.
    4. 600 / 2 = 300 kPa.
    5. Therefore the answer is C.
    • Why not A: This misreads the new volume as the decrease in volume (6 - 2 = 4 m^3) rather than the final volume of 2 m^3 actually stated, giving (100 x 6) / 4 = 150.
    • Why not B: This treats pressure as directly proportional to volume rather than inversely proportional, calculating 100 x (2/6) = 33.3 instead of dividing the constant pressure-volume product by the new volume.
    • Why not D: This gives the constant product of the initial pressure and volume (100 x 6 = 600) as if that product were itself the new pressure, without dividing by the new volume at all.

11. Question 11Answer: B

    1. Temperature is a measure of the average kinetic energy of a substance's particles.
    2. While a substance is melting, its temperature stays constant, so the particles' average kinetic energy is not increasing during that time.
    3. The thermal energy supplied during melting is instead used to overcome the forces (break the bonds) holding particles in the solid's fixed arrangement.
    4. Once melting is complete, further heating can again increase the particles' kinetic energy, and the temperature starts rising once more.
    5. Therefore the answer is B.
    • Why not A: This wrongly claims the energy is not absorbed at all; the energy IS absorbed by the substance, it is simply used to change state rather than to raise the temperature.
    • Why not C: This contradicts the question itself: temperature is a measure of average particle kinetic energy, so if temperature is genuinely constant during melting, kinetic energy is not increasing.
    • Why not D: This wrongly assumes the heat source changes its own output; nothing about the heating rate needs to change for the temperature to stay constant during melting.

12. Question 12Answer: C

    1. Melting at constant temperature needs thermal energy = mass x specific latent heat of fusion.
    2. Substituting mass = 0.5 kg and specific latent heat of fusion = 340000 J/kg gives thermal energy = 0.5 x 340000.
    3. 0.5 x 340000 = 170000 J.
    4. Therefore the answer is C.
    • Why not A: This divides the specific latent heat by the mass instead of multiplying by it: 340000 / 0.5 = 680000, when the mass should multiply the specific latent heat, not divide it.
    • Why not B: This leaves the mass out of the calculation altogether, giving the specific latent heat value itself as the answer, as if the mass were 1 kg rather than 0.5 kg.
    • Why not D: This squares the mass instead of using it directly: 340000 x 0.5 x 0.5 = 85000, applying an extra, unwarranted factor of 0.5 on top of the correct calculation.

13. Question 13Answer: B

    1. Density is defined as density = mass / volume.
    2. Substituting mass = 750 g and volume = 250 cm^3 gives density = 750 / 250.
    3. 750 / 250 = 3 g/cm^3.
    4. Therefore the answer is B.
    • Why not A: This inverts the density formula, calculating volume / mass (250 / 750 = 0.33) instead of mass / volume.
    • Why not C: This multiplies the mass and volume together (750 x 250 = 187500) instead of dividing the mass by the volume.
    • Why not D: This subtracts the volume from the mass (750 - 250 = 500) instead of dividing one by the other.

14. Question 14Answer: C

    1. Pressure is defined as pressure = force / area.
    2. Substituting force = 240 N and area = 0.4 m^2 gives pressure = 240 / 0.4.
    3. 240 / 0.4 = 600 Pa.
    4. Therefore the answer is C.
    • Why not A: This adds the force and the area together (240 + 0.4 = 240.4) instead of dividing one by the other.
    • Why not B: This multiplies the force by the area (240 x 0.4 = 96) instead of dividing the force by the area, as the pressure formula requires.
    • Why not D: This squares the area before dividing (240 / (0.4 x 0.4) = 1500), as if the formula required the area squared, instead of dividing by the area as given.

15. Question 15Answer: B

    1. Hydrostatic pressure is given by pressure = h x rho x g, where h is the depth, rho is the liquid's density, and g is the gravitational field strength.
    2. Substituting h = 3 m, rho = 1000 kg/m^3 and g = 10 N/kg gives pressure = 3 x 1000 x 10.
    3. 3 x 1000 x 10 = 30000 Pa.
    4. Therefore the answer is B.
    • Why not A: This leaves the gravitational field strength g out of the calculation, computing only depth x density (3 x 1000 = 3000) rather than depth x density x g.
    • Why not C: This leaves the depth h out of the calculation, computing only density x gravitational field strength (1000 x 10 = 10000) rather than including the depth as well.
    • Why not D: This leaves the density rho out of the calculation, computing only depth x gravitational field strength (3 x 10 = 30) rather than including density as well.