Conservation of energy

This lesson covers: 

  1. The principle of conservation of energy
  2. Energy transfers and usefulness
  3. Drawing energy transfer diagrams

The principle of conservation of energy

One of the most important principles in physics is the principle of conservation of energy.

This states:

  1. Energy cannot be created nor destroyed.
  2. Energy is only ever transferred from one store to another store.

What this means is that energy never disappears, it simply moves to a different energy store, where it takes on a different form.

Energy is only useful when transferred

Diagram showing no transfer of energy as not useful and transfer of energy as useful with a battery and light bulb.

An important consequence of the principle of conservation of energy is that energy is only useful when it is transferred between stores:

  • Useful machines rely on transferring energy from one store (e.g. chemical energy in fuel) to power the machine.
  • Simply having energy stored in one place does not allow it to do any useful work.
  • So in devices like lamps, engines, etc. there must be an energy transfer taking place to make them work.

Most energy transfers are not perfect

When energy gets transferred:

  1. Some energy always gets wasted, usually as heat.
  2. The total energy output will equal the total energy input.

This means the total energy stays the same. But typically not all of it ends up being put to useful work. The rest warms up the surroundings.

Drawing energy transfer diagrams

Diagram showing energy transfer with 100 J input energy, 20 J useful energy, and 80 J waste energy.

We can use diagrams to represent energy transfers between different stores:

  • The boxes show the energy stores.
  • Arrows show the direction of energy transfer.
  • Energy values can be included to show amounts transferring.

These diagrams help visualise how energy moves between different places and forms.