Power: Power is defined as the rate at which energy is transferred or the rate at which work is done.

The idea

Power measures how quickly energy moves or how quickly work is done, so it always involves an amount divided by a time. Two devices may transfer the same total energy, but the more powerful one does it in less time. For example, a 2000 W kettle transfers 2000 J every second, while a 1000 W kettle transfers 1000 J every second; after 60 s the first has transferred 120 000 J and the second 60 000 J. Because energy transferred and work done are equivalent in this context, either wording may be used. The watt is the unit: 1 W = 1 J/s. A rate compares a quantity with time, so power is not the total energy transferred, and a large energy transfer over a long time can still mean low power.

What earns the marks

• States that power is the rate of energy transfer or the rate of doing work, linking an amount of energy or work to the time taken.
• Recognises that the watt is the unit of power and that 1 W equals 1 J transferred each second.
• Explains that two devices transferring equal energy can have different powers if the times taken differ.
• Uses the idea of rate to compare devices, for example a 2000 W heater transfers energy twice as fast as a 1000 W heater.
• Distinguishes power from energy: energy is the total transferred, while power describes how quickly that transfer happens.

Common mistakes and how to put them right

• Treating power as a total amount of energy rather than a rate.

  Put it right by dividing energy or work by time.

• Writing the unit of power as J or J s.

  Put it right by using W, where 1 W = 1 J/s.

• Assuming a more powerful device always transfers more energy.

  Put it right by noting that a low-power device running for longer can transfer more total energy.

Exam tips

• Define power using the word rate and name both energy transferred and work done as acceptable quantities.
• When comparing appliances, quote their power values and state the time interval, such as energy per second.
• Check that any calculation answer has the unit W and that the time used is in seconds.