Power: how rapidly can you shift energy?

Energy, power and time

The amount of energy that is shifted to or from a store during one second is called the power. The unit of power is the watt (W).

Suppose a light bulb has a power of 100 watt. As a consequence, when the bulb is operating under normal conditions, it shifts 100 joule of energy each second from the chemical store of the supply to the internal store of the surroundings (along heating and lighting pathways).

If you know the power output for a device such as an electric bulb, you can calculate the energy shifted in a given time as follows:

energy = power × duration

energyjoule = powerwatt × durationsecond

And, expressed using symbols:

E = P × t

Shifting at different rates

Here is an example where energy is shifted to a kinetic store by accelerating a pellet.

There is a trade-off. You can have a big power for a short time, or a smaller power for a longer time. In both cases the same mass achieves the same top speed, and the same amount of energy is shifted into the kinetic store.