Describe power as the energy transferred per second and recall that it is…

The idea

Power tells you how quickly energy is transferred, not how much energy is moved in total. One watt means one joule of energy transferred each second, so a 60 W lamp transfers 60 J every second while it is on. A 2000 W kettle transfers 2000 J each second, so it heats water far faster than the lamp even if both run for the same time. To compare devices, divide the energy transferred by the time taken: a heater transferring 9000 J in 30 s has a power of 9000 J ÷ 30 s = 300 W. The unit is the watt (W), and the symbol W always means watt, never work done. Power is a rate, so doubling the time for the same energy halves the power.

What earns the marks

• States that power is the energy transferred per second.
• Recalls that power is measured in the watt (W).
• Explains that one watt equals one joule transferred each second.
• Uses the relationship between energy transferred, time taken and power to compare two devices.

Common mistakes and how to put them right

• Confuses power with total energy transferred.

  Put it right: Keep power as energy per second and energy as the total amount.

• Treats the symbol W as work done rather than the watt.

  Put it right: Read W in a power context as the unit of power.

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

  Put it right: Compare only over the same time, since energy depends on both power and time.

Exam tips

• Read the unit in the question: a value in W is a power, a value in J is an energy.
• When comparing devices, check the time interval is the same before saying which transfers more energy.
• Write the unit as W or watt, and remember 1 W = 1 J/s.