Conservation of energy

In science, a system is a set of things and the processes that happen in them and between them. Energy can be stored or transferred, but it cannot be created or destroyed. This means that the total energy of a system stays the same. The idea that the total energy has the same value before and after a change is called conservation of energy.

For example, an electric lamp might transfer radiant energy to the surroundings. Some of this will be visible light and some will be infrared red light, which increases the temperature of the surroundings. The amount of energy transferred from the lamp will be the same as the amount of energy transferred into the lamp because energy can never be created or destroyed.

Sankey diagrams

You can show energy transfers in a Sankey diagram. In these diagrams, the thicker the line or arrow, the greater the amount of energy involved.

Electrical energy = 100J. 10J is used as light energy and 90J is used as heat energyA Sankey diagram for an electric lamp

Notice that 100 J of electrical energy is transferred to the lamp. Of this, 10 J is transferred to the surroundings as light energy. The remainder, 90 J (100 J – 10 J) is transferred to the surroundings as radiant energy that increases the temperature of the surroundings. Energy can never be created or destroyed so the energy at the beginning (here 100 J) must equal the total of the energy at the end (here also 100 J).

The energy transfer as light to the surroundings is the useful transfer. The rest is ‘wasted’ - it just makes the surroundings warmer rather than helping you see where you are going. This ‘wasted’ energy eventually becomes so spread out that it becomes useless.