Define power as the rate at which energy is transferred and use examples…

The idea

Power measures how quickly energy moves from one store to another, or from one object to another, by heating, by radiation, mechanically or electrically. A 2000 W kettle transfers 2000 J of energy to the water every second, while a 100 W lamp transfers 100 J each second, so the kettle has the greater power even if both transfer the same total energy over different times. The definition links three ideas: energy transferred, measured in joules (J); time taken, measured in seconds (s); and the rate, which is energy divided by time. Two machines doing identical work differ in power if one finishes sooner. Power is not the same as energy, and a high-power device is not automatically the one that transfers the most energy overall.

What earns the marks

• States that power is the rate of energy transfer, that is energy transferred per unit time.
• Identifies the watt as the unit equal to one joule per second.
• Compares two devices by the energy each transfers in the same time interval.
• Explains that a device transferring more energy in a shorter time has the greater power.
• Distinguishes power from total energy transferred, recognising that a low-power device running for a long time can transfer more energy overall.

Common mistakes and how to put them right

• Confuses power with energy and treats a joule as a unit of power.

  Put it right: Keep joules for energy and watts for power.

• Assumes the device with the largest total energy transfer must have the highest power.

  Put it right: Compare energy transferred per second instead.

• Ignores the time element and compares only the energy values.

  Put it right: State the time interval before comparing rates.

• Describes power as the amount of work stored in an object.

  Put it right: Describe it as the rate at which work is done or energy transferred.

Exam tips

• Read the stem for the time interval, because a rate question is unanswerable without it.
• Convert minutes to seconds before comparing rates, since the watt is defined per second.
• Eliminate options that give power the unit of joules or energy the unit of watts.
• Check whether the question asks for the greater power or the greater total energy, as these can differ.