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Science 10 · Worksheets

Mixed review · Energy and heat

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

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Mixed review · Energy and heat

Science 10 · maddyhelps.com

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

  1. A machine does 600 J of work in 20 s. Its power is

    1. a) 30 W
    2. b) 620 W
    3. c) 12 000 W
    4. d) 0.03 W

  2. A 2 kg ball is dropped from 5 m. Just before it lands, its kinetic energy is about

    1. a) 98 J
    2. b) 10 J
    3. c) 20 J
    4. d) 49 J

  3. An efficiency above 100% is impossible because it would mean

    1. a) the machine is too fast
    2. b) more energy came out than went in, breaking conservation of energy
    3. c) heat was produced
    4. d) friction was removed

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

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

  5. Heat is

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

  6. A force of 20 N moves a box 4 m in the direction of the force. The work done is

    1. a) 5 J
    2. b) 24 J
    3. c) 80 J
    4. d) 80 W

  7. The first law of thermodynamics states that energy

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

  8. In every real energy conversion, some energy is

    1. a) stored permanently
    2. b) created
    3. c) converted into forms that are not useful, usually heat
    4. d) destroyed

  9. Power is

    1. a) the same as work
    2. b) a kind of force
    3. c) the rate at which energy is transferred
    4. d) the total energy transferred

  10. Work is done when

    1. a) a force moves an object through a distance
    2. b) a force is applied with no movement
    3. c) energy is stored
    4. d) an object is heavy

Answer key · Mixed review · Energy and heat

Science 10 · maddyhelps.com

  1. a) 30 W — P = W/t = 600/20 = 30 W. Multiplying instead of dividing gives 12 000, which would be a very large power for a very small job — checking whether the number is plausible catches it.
  2. a) 98 J — All the potential energy has become kinetic: Ep = mgh = 2 × 9.8 × 5 = 98 J. Conservation of energy lets you skip the motion entirely, which is usually the fastest route.
  3. b) more energy came out than went in, breaking conservation of energy — Conservation of energy is not a design limit that better engineering could beat — it is an accounting identity. A device claiming over 100% is either measuring something wrong or omitting an input.
  4. b) 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.
  5. b) 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.
  6. c) 80 J — W = Fd = 20 × 4 = 80 J. The unit must be joules — answering in watts would be answering a power question.
  7. b) 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.
  8. c) converted into forms that are not useful, usually heat — Energy is always conserved; what falls is the <i>useful</i> fraction. A light bulb converts almost all its electrical energy to heat and only a little to light, which is why efficiency is worth measuring.
  9. c) the rate at which energy is transferred — P = W/t, measured in watts. Two motors can do the same job while using very different power — one just takes longer.
  10. a) a force moves an object through a distance — W = Fd. Holding a heavy box still is exhausting and is no work at all in the physics sense, because nothing moved. Work is a transfer of energy, not an effort.