AQA GCSE Physics

## Systems and stores A **system** is an object or a group of objects that you choose to look at. Energy is not a substance that flows around; it is a quantity that is stored in different ways, and when a system changes the way its energy is stored changes too. The main stores are kinetic (moving objects), gravitational potential (objects raised above the ground), elastic potential (stretched or compressed objects), thermal (the random motion of particles), chemical (fuels, food and batteries), nuclear and magnetic. Energy moves from one store to another in four ways: by a force doing work (mechanical work), by an electric current doing work, by heating, and by waves such as light and sound. The total amount of energy is the same before and after a change; only its distribution differs. Describing a change means naming the stores that lose energy and the stores that gain it: - An object thrown upwards starts with energy in its kinetic store. As it rises, the kinetic store shrinks and the gravitational potential store grows, and at the top of its flight almost all the energy is in the gravitational store. - A moving object that hits an obstacle loses kinetic energy. Some goes to the thermal store of the object and obstacle, and some is carried away as sound. - An object accelerated by a constant force gains kinetic energy, and the force does work on it. The energy comes from whatever is driving the force, for example the chemical store of a fuel. - A vehicle slowing down…