Energy transfers

Transferring energy

Energy is transferred by one of the following four types of energy pathway:

  • 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 transferred as a wave eg light, infrared, sound - the Sun emits light radiation and infrared radiation
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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 from one store to another.

Example

Energy transfer

A toy car rolls to a stop

Kinetic energy store decreases as energy is transferred into the thermal energy store of the surroundings by the work done by the force of friction.

A toy car rolls to a stop on a table with no incline which demonstrates kinetic energy.

An electric motor lifting a weight

The chemical energy store decreases as energy is transferred via the electrical pathway and the gravitational potential energy store increases.

An electric motor rests on a table lifting a weight. The motor is connected to a Joulemeter which in turn is connected to a low voltage supply.

Bringing water to a boil on a gas hob

Energy from the chemical store in the gas increases the internal (thermal) energy of the water in the pan.

A pan of water on a gas hob, bringing water to a boil.

When there are energy transfers in a closed system, there is no net change in the total energy in the system.

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 the gravitational potential energy store to the kinetic energy store and the internal energy store (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 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.