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Heat and thermal energy

Ten questions of mixed difficulty, covering Heat and energy transfer. Print it, or work through it on screen — the answer key starts on its own page.

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Heat and thermal energy

Science 10 · maddyhelps.com

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Circle the best answer for each question. Show your work in the space provided.

  1. The Sun's energy reaches Earth by

    1. a) convection
    2. b) radiation
    3. c) conduction
    4. d) all three equally

  2. Heat is

    1. a) only produced by fire
    2. b) a substance inside an object
    3. c) the transfer of thermal energy from hotter to colder
    4. d) the same as temperature

  3. Convection happens in

    1. a) fluids, because warmer parts rise and cooler parts sink
    2. b) solids only
    3. c) metals only
    4. d) a vacuum

  4. A ball bouncing lower each time has not destroyed energy, because

    1. a) the missing energy became heat and sound in the ball and floor
    2. b) energy was created by the floor
    3. c) kinetic energy is not conserved
    4. d) gravity removed it permanently

  5. The second law of thermodynamics implies that in any conversion

    1. a) heat flows from cold to hot on its own
    2. b) efficiency can reach 100%
    3. c) energy is lost entirely
    4. d) some energy always becomes less useful, usually as heat

  6. A vacuum flask keeps drinks hot mainly by

    1. a) sealing in the heat as a substance
    2. b) being silver coloured only
    3. c) removing the air so conduction and convection cannot happen
    4. d) using thick plastic

  7. Water has an unusually high specific heat capacity, which means it

    1. a) warms up very quickly
    2. b) cannot be heated
    3. c) boils at a low temperature
    4. d) takes a lot of energy to warm up and releases a lot as it cools

  8. The energy needed to heat 2.0 kg of water by 10 °C, with c = 4200 J/(kg·°C), is

    1. a) 420 J
    2. b) 8400 J
    3. c) 21 000 J
    4. d) 84 000 J

  9. Conduction transfers energy by

    1. a) electromagnetic waves
    2. b) the movement of a fluid
    3. c) direct contact between particles
    4. d) chemical change

  10. The first law of thermodynamics states that energy

    1. a) cannot be created or destroyed, only converted
    2. b) always decreases
    3. c) can be destroyed by friction
    4. d) always increases

Answer key · Heat and thermal energy

Science 10 · maddyhelps.com

  1. b) radiation — Space is nearly empty, so there are no particles to collide or to circulate. Only radiation — electromagnetic waves — crosses a vacuum.
  2. c) the transfer of thermal energy from hotter to colder — An object does not contain heat; it contains thermal energy. Heat is that energy in transit, and it always flows from higher temperature to lower.
  3. a) fluids, because warmer parts rise and cooler parts sink — Warming a fluid makes it less dense, so it rises and colder fluid moves in beneath. That circulation is what drives weather, ocean currents and the water in a kettle.
  4. a) the missing energy became heat and sound in the ball and floor — Tracking where the energy went, rather than saying it was lost, is the habit this unit is really teaching. The total is unchanged; the useful, organised part is smaller.
  5. d) some energy always becomes less useful, usually as heat — The first law says the total is conserved; the second says its usefulness is not. That is why perpetual motion machines fail even though they do not obviously break the first law.
  6. c) removing the air so conduction and convection cannot happen — The vacuum between the walls blocks conduction and convection, and the silvered surfaces reflect radiation back. All three transfer methods are being attacked separately, which is why the design works so well.
  7. d) takes a lot of energy to warm up and releases a lot as it cools — It is why coastal climates are milder than inland ones and why water is used as a coolant. Specific heat capacity is the energy needed to raise one kilogram by one degree.
  8. d) 84 000 J — Q = mcΔT = 2.0 × 4200 × 10 = 84 000 J. All three factors multiply, so dropping any one of them changes the answer by a large factor.
  9. c) direct contact between particles — Fast particles collide with slower neighbours and pass energy along. Metals do this well because their free electrons carry energy through the material quickly.
  10. a) cannot be created or destroyed, only converted — This is conservation of energy. Friction does not destroy energy — it converts organised motion into thermal energy, which is harder to use.