The energy of a system can be changed in three ways: by work done by forces, by work done in electrical equipment, and by heating.
- Work done by forces. When a force moves an object, energy is transferred. Lifting a box transfers energy to its gravitational potential store. Pushing a box along a rough floor transfers energy to the thermal store of the box and floor, because friction acts against the movement.
- Electrical equipment. An electric current flowing through a device does work, and transfers energy. A motor lifts a load, and a kettle raises the thermal store of the water.
- Heating. Energy is transferred by heating when there is a temperature difference. Energy moves from the hot object to the cooler one. A hot drink warms the cup and the cooler air around it.