Electrical Circuit
Electricity and Magnetism

Electrical power

Physics Narrative for 11-14 Supporting Physics Teaching

Defining electrical power, and calculating the amount

This quantity, the amount of energy shifted by the bulb each second, is called the electrical power of the bulb.

The power of any device (whether electrical or mechanical) is defined as power = energy transferred per second. You can write this as:

power = energyduration

In other words, a device has a high power output if it fills a selected store of energy very quickly. This idea can be applied to electrical devices, mechanical machines and even to people. For example, a trained athlete is able to ride a mountain bike up a steep slope quicker than a person of average fitness (so filling a gravitational store rapidly). The same amount of energy is needed for either person (provided they're about the same mass) to ride the bike up the hill. It's the athlete's ability to perform the task in a shorter period of time that shows that their power output is greater. We don't know, from these measurements, who has emptied their chemical store most rapidly. That would be a measure of the input power.

The standard unit of power is the watt, and the symbol used to represent this is W.

power/watt = energy/jouleduration/second

So: 1 watt is 1 joule of energy transferred per second and 100 watt is 100 joule of energy transferred per second.

Limit Less Campaign

Support our manifesto for change

The IOP wants to support young people to fulfil their potential by doing physics. Please sign the manifesto today so that we can show our politicians there is widespread support for improving equity and inclusion across the education sector.

Sign today