1.2b – Language of power
It is easy to dismiss power as “simply” the rate at which energy is transferred. However, it is worthy of discussion in its own right. In many situations it is easier to give an intuitive sense of power than use equations – students can readily recognise machines and devices that are powerful; and have a sense of the brightness and loudness of 100 watt devices.
For example, when discussing changes that are continuous over time, such as lighting a room, power calculations for electrical or lighting applications are much more meaningful than descriptions in energy terms.
Energy allows us to analyse difference between two snapshots – or two fixed situations.
We use energy values (in joules) to analyse:
- resources: food, combustible fuels, nuclear fuels and a high tide;
- tasks: stretching a spring, making tea, and launching a ball in the air.
Power tells us how rapidly energy stores are emptying or filling.
We use power values (in watts) to analyse:
- devices: light bulbs, loudspeakers, kettles, circuits and power stations;
- processes: lighting a room, generating electricity, moving at constant speed.
Sankey diagrams are a good way of representing power and efficiencies. The Sankey diagrams of different light bulbs are shown below.
Point to note: it is more helpful to discuss the power radiated in the infra-red part of the spectrum rather than the “heat given out by the light bulb”.
1.2a – Force, energy, and power examples