Energy Stores & Transfers
Energy Stores
Everything in the universe involves energy. Energy is not a substance you can hold; it is a quantity that describes the capacity of a system to do work. In GCSE physics, we describe energy using the idea of energy stores. There are eight energy stores you need to know:
Kinetic — energy of a moving object. Gravitational potential — energy stored when an object is raised above the ground. Elastic potential — energy stored when an object is stretched or compressed. Thermal — energy related to the temperature of an object. Chemical — energy stored in bonds between atoms (fuels, food, batteries). Magnetic — energy of magnets attracting or repelling. Electrostatic — energy of charged particles attracting or repelling. Nuclear — energy stored in the nucleus of an atom.
The eight energy stores are: kinetic, gravitational potential, elastic potential, thermal, chemical, magnetic, electrostatic, and nuclear.
Energy Transfers
Energy can be transferred between stores by four main pathways:
Mechanically — a force acting on an object does work and transfers energy (e.g. pushing a trolley transfers energy from chemical stores in your muscles to kinetic stores in the trolley). Electrically — charges flowing through a circuit transfer energy (e.g. from the chemical store of a battery to the thermal store of a filament lamp). By heating — energy transfers from a hotter object to a cooler one by conduction, convection, or evaporation. By radiation — energy is transferred by electromagnetic waves, such as light or infrared radiation from the Sun.
The four energy transfer pathways are: mechanically (by forces doing work), electrically (by charges in a circuit), by heating (conduction, convection, evaporation), and by radiation (electromagnetic waves).
Energy Transfers in Systems
A system is an object or group of objects. When a system changes, energy is transferred between stores. Here are some important examples from the specification:
When a ball is thrown upwards, energy is transferred from the kinetic store to the gravitational potential energy store. As the ball rises, it slows down: kinetic energy decreases while gravitational potential energy increases. At the highest point, all the kinetic energy has been transferred to gravitational potential energy (ignoring air resistance). On the way down, the reverse happens.
When a car crashes into a barrier, energy is transferred from the kinetic store to the thermal store (the car and barrier heat up) and to the elastic potential store (the car body deforms). Some energy is also transferred by sound waves to the thermal store of the surroundings.
When a constant force accelerates a car along a road, energy is transferred from the chemical store of the fuel to the kinetic store of the car. Some energy is dissipated to the thermal store of the surroundings through friction and air resistance.
Drawing Energy Transfer Diagrams
Energy transfer diagrams use boxes to represent energy stores and arrows to show the transfer pathway. The width of each arrow can show the relative amount of energy transferred along that pathway. For example, boiling water in a kettle: the chemical store of the electricity supply transfers energy electrically to the thermal store of the water (useful) and to the thermal store of the surroundings (wasted, through heating of the kettle body and sound).
When drawing these diagrams, always label each store clearly and show the transfer pathway on or beside the arrow. The total energy in must equal the total energy out — this links directly to the principle of conservation of energy.
Energy transfer diagrams show energy stores as labelled boxes and transfer pathways as arrows. The total energy in always equals the total energy out.
Exam Tip: When describing energy changes, always name the store energy is transferred from, the store it goes to, and the pathway (mechanically, electrically, by heating, or by radiation). Vague answers like “energy is lost” lose marks — say “energy is dissipated to the thermal store of the surroundings by heating.”
Energy is stored in eight different stores and transferred between them by four pathways. Being able to describe these changes precisely — naming stores and pathways — is essential for almost every energy question on the exam.