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Energy – Forces Doing Work

Power as the rate of energy transfer

  • 3 things to know
  • Paper 2

Overview

Power as energy transferred each second, comparing motors, cranes and heaters, the watt, the power equation, and calculations from work done and time.

What you need to know

Answer each one before you reveal it.

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Key term

Define power.

Key term

Two cranes lift identical loads to the same height, but one takes half the time of the other. Compare the powers of the two cranes.

Key term

Give the unit of power and state what that unit measures.

Questions from this subtopic

Each one with the model answer underneath.

  1. 1

    Define power.

    Model answer

    Power is the rate at which energy is transferred, that is the work done each second.

  2. 2

    Explain what it means to say that one motor is more powerful than another.

    Model answer

    It transfers the same energy in a shorter time, or more energy in the same time.

  3. 3

    Two cranes lift identical loads to the same height, but one takes half the time of the other. Compare the powers of the two cranes.

    Model answer

    The faster crane has twice the power, because it does the same work in half the time.

  4. 4

    Explain why a 2000 W heater warms a room faster than a 1000 W heater.

    Model answer

    It transfers twice as much energy each second.

  5. 5

    Give the unit of power and state what that unit measures.

    Model answer

    The watt, W, which measures the energy transferred each second.

  6. 6

    Explain why the power of a person running up a flight of stairs is greater than that of the same person walking up them.

    Model answer

    The same work is done against gravity in a shorter time.

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ALength is measured in metres, with the symbol m.
BMass is measured in kilograms, symbol kg.
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Watt means the unit of power, symbol W.
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