Explain, with reference to examples, the definition of power as the rate…

The idea

Power measures how quickly energy is transferred, not how much energy is transferred in total. The definition is that power is the rate at which energy is transferred, so a device with a larger power transfers the same energy in less time, or more energy in the same time. Use examples to make this concrete. A 2000 W kettle transfers energy to water faster than a 1000 W kettle, so it boils the same amount of water in less time. A lift motor of higher power raises a load in a shorter time. Two lamps may transfer the same total energy, but the brighter, higher-power lamp does it in less time. When explaining, name the device, state the energy transfer involved, and compare the time taken or the energy per second, linking this back to the definition of power as a rate.

What earns the marks

• States that power is the rate at which energy is transferred, linking the quantity to energy transferred per unit time.
• Uses a named example, such as two kettles of different power, to show that a higher power transfers the same energy in less time.
• Compares two devices transferring the same total energy and explains that the higher-power device takes less time.
• Explains that a higher-power device transfers more energy in the same time than a lower-power device.
• Links the example back to the definition by referring to energy transferred per second rather than total energy alone.

Common mistakes and how to put them right

• Describes power as the total amount of energy transferred.

  Put it right: Explain instead that power is the rate, that is energy transferred per unit time.

• Assumes a higher-power device always transfers more total energy.

  Put it right: Recognise that it transfers energy faster, so total energy depends on time as well.

• Gives an example without linking it to the definition.

  Put it right: State the energy transfer and the time comparison, then connect them to the rate idea.

• Confuses power with force or with work done.

  Put it right: Keep power as the rate of energy transfer and work done as the energy transferred by a force.

Exam tips

• Begin your explanation with the definition, then support it with a specific example.
• Use a comparison of two devices so the rate idea is visible, for example same energy in different times.
• Refer to energy transferred per second to make the rate explicit.
• Name the device and the energy transfer in each example rather than speaking in general terms.