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Energy, work and efficiency

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

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Energy, work and efficiency

Science 10 · maddyhelps.com

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

  1. A motor takes in 500 J and produces 150 J of useful output. Its efficiency is

    1. a) 70%
    2. b) 30%
    3. c) 350%
    4. d) 3.3%

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

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

  3. Gravitational potential energy is calculated as

    1. a) Ep = mgh
    2. b) Ep = ½mv²
    3. c) Ep = mv
    4. d) Ep = Fd/t

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

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

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

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

  6. Kinetic energy is the energy of

    1. a) sound only
    2. b) position
    3. c) chemical bonds
    4. d) motion

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

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

  8. Power is

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

  9. The unit of energy and work is the

    1. a) watt
    2. b) pascal
    3. c) newton
    4. d) joule

  10. In every real energy conversion, some energy is

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

Answer key · Energy, work and efficiency

Science 10 · maddyhelps.com

  1. b) 30% — Efficiency = useful out ÷ total in × 100 = 150/500 × 100 = 30%. The other 350 J went to heat and sound, so 70% is what was <i>wasted</i>, not the efficiency.
  2. 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.
  3. a) Ep = mgh — Mass times gravitational field strength times height. Only the change in height matters, so the reference level can be chosen wherever it is convenient.
  4. 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.
  5. c) 80 J — W = Fd = 20 × 4 = 80 J. The unit must be joules — answering in watts would be answering a power question.
  6. d) motion — Ek = ½mv². Potential energy is the energy of position or stored arrangement. Note the v is squared, so doubling the speed quadruples the kinetic energy.
  7. c) 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.
  8. a) 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.
  9. d) joule — One joule is one newton-metre. The watt is a joule per second — a rate, not an amount — and the newton is a force.
  10. a) 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.