Topic 1 of 4
Energy stores and transfers
Explain an energy change by naming the objects, the stores that change and the pathway that transfers energy.
A battery-powered motor raises a load. The battery's chemical store decreases, and the gravitational store of the load and Earth increases. Energy reaches the motor electrically, then reaches the rising load mechanically through the lifting force.
Energy is measured in joules, J. A store describes energy associated with the state of an object or system. A transfer pathway describes how energy is transferred. An energy account can include more than one object: the load and Earth form a useful system when considering gravitational energy.
A battery supplies a motor that lifts a load
This simplified steady lift ignores transfers to internal stores. The arrows label energy-transfer pathways.
Recognise the store from the physical situation
- Kinetic
- Associated with motion. A moving trolley has kinetic energy. If it slows down, this store decreases.
- Gravitational potential
- Associated with the positions of masses in a gravitational field. Lifting a load near Earth increases the gravitational store of the load-Earth system. Choose a reference height when assigning a value.
- Chemical
- Associated with the chemical state of substances. A battery, food or fuel can supply energy when chemical reactions change the substances.
- Elastic potential
- Associated with an object being stretched or compressed. Stretching a spring increases its elastic store. It can transfer energy as it returns towards its original shape.
- Nuclear
- Associated with the state of atomic nuclei. Changes in nuclei can release energy from nuclear stores, as in nuclear fuel undergoing fission.
- Internal
- Associated with the motion and interactions of the particles within a substance. A warmer metal block generally has a greater internal energy than the same block at a lower temperature in the same state.
Gravitational, chemical and elastic stores are examples of potential energy stores. One object may have several stores: a moving, warm trolley has kinetic energy as well as internal energy. Choose the stores whose changes matter to the situation.
Four ways energy is transferred
- Mechanically, by a force acting over a distance. A string pulls a load upwards as it moves upwards. The force transfers energy to the load-Earth gravitational store. A force without the relevant movement does not automatically transfer energy mechanically.
- Electrically, by an electric current. A battery supplies energy to a motor through a circuit. In an electric heater, an electrical transfer increases internal energy. Describe the current as the pathway, rather than inventing an extra store called "electricity".
- By heating, because of a temperature difference. A hot metal block in contact with a cooler block transfers energy to it. The hotter block's internal store decreases and the cooler block's increases. Heat describes a transfer, not a substance stored inside an object.
- By waves, both electromagnetic and mechanical. Sunlight is an electromagnetic wave: an absorbing surface can gain internal energy from it. Sound is a mechanical wave travelling through a medium such as air: it can make a receiving membrane vibrate. The energy travels with the disturbance; the medium does not need to travel bodily from source to receiver.
Some descriptions overlap: energy transferred by infrared waves from a hotter object to a cooler one is also a transfer by heating. Count that amount once in the energy account.
Include the surroundings
When a sliding trolley slows because of friction, its kinetic store decreases while internal stores of the trolley, track and surroundings increase. Saying that the energy is "lost" without identifying its destination leaves the account incomplete.
Energy cannot be created or destroyed. It can be transferred between stores. If no energy enters or leaves the system being considered, its total energy stays constant. If energy crosses the boundary, include that transfer when comparing the starting and final stores.
Keep stores and pathways distinct. Chemical and internal describe stores. Mechanically, electrically, heating and waves describe transfers. A complete explanation names what changes as well as how the transfer happens.