Systems and energy stores

1. A system is the object or group of objects you decide to study, and everything outside it is called the surroundings.
2. Drawing the boundary of a system decides which energy changes you count and which ones are treated as happening outside it.
3. Energy is measured in joules, $J$, whichever store is holding it.
4. Every store is named after the reason the energy is there, so naming a store also means naming the object it belongs to.
5. Eight energy stores are used at this level.
   1. A kinetic energy store holds energy because an object is moving.
   2. A gravitational potential energy store holds energy because two objects are separated in a gravitational field.
   3. An elastic potential energy store holds energy because an object has been stretched, squashed or twisted.
   4. A thermal energy store holds energy because of the random motion of the particles inside an object, and it grows as the temperature rises.
   5. A chemical energy store holds energy in the bonds inside fuels, food and the chemicals in a battery.
   6. An electrostatic energy store holds energy because electric charges have been pulled apart.
   7. A magnetic energy store holds energy because magnets or magnetic materials are held apart or pushed together.
   8. A nuclear energy store holds energy inside the nuclei of atoms.
6. The gravitational potential energy store belongs to the object and the Earth together, because a separation needs two objects.
7. A store is where energy sits, so words such as light, sound and electricity name ways of moving energy rather than stores.

Changes in the way energy is stored

1. When a system changes, one store decreases while one or more other stores increase.
2. Describing a change means naming the store that falls, naming the store that rises, and naming the object each store belongs to.
3. As a ball falls, the gravitational potential energy store of the ball and Earth decreases while the ball's kinetic energy store increases, so the ball speeds up.
4. When a stretched catapult is released, the elastic potential energy store of the rubber decreases and the kinetic energy store of the stone increases.
5. When a battery drives a motor, the chemical energy store of the battery decreases and the kinetic energy store of the turning parts increases.
6. When a mug of tea cools on a kitchen worktop, the thermal energy store of the tea decreases and the thermal energy store of the room increases.
7. When a crane raises a steel beam, the gravitational potential energy store of the beam and Earth increases.
8. When a firework burns, the chemical energy store of the powder decreases while the thermal and kinetic energy stores of the hot gases increase.

Drawing energy transfer diagrams

1. An energy transfer diagram shows the store the energy starts in, the store or stores it ends in, and an arrow for each transfer.
2. Write both the store and the object it belongs to inside each box, such as chemical store of the battery.
3. Draw every arrow from the store that decreases towards the store that increases, and put an arrowhead on it.
4. One arrow means all of the energy ends in a single store, while several arrows mean the energy is shared between several stores.
5. Label an arrow with the way the energy travels when the question asks for it, for example an arrow marked electrically for a battery driving a motor.
6. For that battery and motor the diagram reads: chemical store of the battery $⟶$ kinetic store of the motor.
7. To interpret a diagram, follow each arrow from its tail to its head, then state that the tail store decreases and the head store increases.
8. A Sankey diagram is an energy transfer diagram drawn with arrow widths in proportion to the number of joules, so the widest branch carries the most energy.
9. The single arrow entering a Sankey diagram on the left stands for the total energy supplied, and the branches leaving it show how those joules are shared out.
10. The widths of all the branches leaving a point add up to the width of the arrow entering it, which is a quick check that the diagram is drawn correctly.

Energy in a closed system

1. Conservation of energy means the total number of joules in a closed system is the same before and after any change, so $E_{total\; before}=E_{total\; after}$.
2. Energy is never created and never destroyed, so a change only moves joules from one store to another.
3. A closed system therefore shows no net change in its total energy, and any decrease in one store is matched by an equal total increase in the others.
4. If one store of a closed system falls by $250J$, the remaining stores must gain $250J$ between them.
5. Whether a system counts as closed depends on where the boundary is drawn, so a swinging pendulum on its own is not closed while the pendulum together with the air and its support is much closer to closed.
6. Widening the boundary to include the surroundings makes the total easier to account for, because joules that leave the object are still inside the system.