Energy transfers

Systems and stores

Energy can remain in the same store for millions of years or sometimes just for a fraction of a second. There are energy transfers going on all the time - whenever a system changes there is a change in the way some or all of the energy is stored.

Examples of energy transfers include:

Boat suspended from column, swings in circular arc. Highest point: no kinetic energy,  max gravitational potential energy. Lowest point: max kinetic energy, minimum gravitational potential energy.

A swinging pirate ship ride at a theme park

Kinetic energy is transferred into gravitational potential energy

Transferring energy

In each of these examples, energy is transferred by one of the following four types of energy transfer:

  • mechanical work - a force moving an object through a distance
  • electrical work - charges moving due to a potential difference
  • heating - due to temperature difference caused electrically or by chemical reaction
  • radiation - energy tranferred as a wave, eg light and infrared - light radiation and infrared radiation are emitted from the sun

Doing 'work' is the scientific way of saying that energy has been transferred. For example, a grazing cow, a firing catapult and a boiling kettle are all doing 'work', as energy is being transferred.

Energy flow diagrams

Diagrams can be used to show how energy is transferred from one store to another. Two examples are the transfer diagram and the Sankey diagram.

Transfer diagrams

In transfer diagrams the boxes show the energy stores and the arrows show the energy transfers.

For example, a transfer diagram for a child at the top of a slide may be:

Energy transfer diagram of a child on a slide

Gravitational energy stored in the child at the top of the slide is transferred as mechanical work done to speed up and to do work against friction. The result of this is a shift of energy from gravitational potential energy to kinetic energy and internal energy (raising the temperature of the child and the slide).

Sankey diagrams

Sankey diagrams start off as one arrow that splits into two or more points. This shows how all of the energy in a system is transferred into different stores.

Sankey diagram of a child on a slide

Sankey diagrams are really useful when the amount of energy in each of the energy sources is known. The width of the arrow is drawn to scale to show the amount of energy.