### KS3 Physics

Energy
Motion and Forces
Waves
Electricity and Electromagnetism
Matter
Space Physics

# Conservation of Energy

Energy cannot be created or destroyed, therefore the total energy before and after a change or transfer remains the same. This is known as the conservation of energy.

So Total energy before = Total energy after

However, energy can also be wasted.

## Wasted energy

When transfers between different energy stores take place, there can be wasted energy.

For example, when a bulb is lit, it may be small, but it releases heat energy to the surroundings. This heat energy is not required for the bulb to work, which is why it is wasted energy.

Most wasted energy ends up in the thermal energy stores of the surroundings, which causes the surroundings to heat up a little.

# Sankey diagrams

Sankey diagrams are also used to show energy transfers. In these diagrams, the size of the arrows is important, as it shows the amount of energy.

The Sankey diagram above is for an electric lamp. As you can see, the total amount of energy given out by the lamp is 100 J. Of the total 100 J, 10 J is transferred as light energy (which is the transfer that is useful to us) and 90 J is transferred as thermal energy (which is the transfer that is unuseful to us).

The arrow for the transfer to heat energy is larger because more energy is transferred here. However, it is wasted energy, as we use lamps solely to produce light, not to produce heat.

Quizzes