Energy is measured in joules (J)Power is measured is watts (W)
Equations to know Efficiency (%) = (useful energy out ÷ total energy in) x 100. Gravitational Potential Energy = mass x gravity x height. GPE = mgh. Kinetic Energy = 0·5 x mass x velocity2. KE = ½mv2. Work done = Force x Distance. W = F x d. Work done = Energy transferred. W = E. Power = Energy ÷ Time. P = E ÷ t.
Amount of energy does not increase or decrease during energy transfer, it’s only changed from one form into others. Energy cannot be created or destroyed. This is called the ‘conservation of energy’. Not all the energy is changed into the form we want (the useful form).
Examples of the conversion of energy If we want to light a room, we can switch on a light bulb. This transfers electrical energy into light energy. If we want to travel from one place to another, we can walk. We transfer chemical energy into kinetic energy. A cell (or battery) transfers chemical energy into electrical energy.