Energy is transferred by one of the following four types of energy pathway:
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.
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.
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.
When there are energy transfers in a closed system, there is no net change in the total energy in the system.
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.
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:
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 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 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.