8.1.1 Energy stores and transfer diagrams

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.

8.1.2 Ways of changing a system's energy

Changing the energy of a system

1. The energy of a system changes only when joules cross its boundary into or out of one of its stores.
2. An energy transfer pathway is the route those joules take, so a pathway is how energy moves while a store is where it ends up.
3. Three pathways change the energy of a system in this topic: work done by forces, electrical equipment and heating.
4. Naming the pathway and naming the store that changes are separate marks, so a full answer needs both.
5. No pathway makes or destroys joules, so the number leaving one store equals the number arriving in the others.

Work done by forces

1. A force does work on an object when the object moves in the direction of that force, and the energy is then transferred mechanically.
2. Lifting a crate off the floor transfers energy mechanically into the gravitational potential energy store of the crate and Earth.
3. Pushing a supermarket trolley from rest transfers energy mechanically into the trolley's kinetic energy store.
4. Drawing a bow transfers energy mechanically into the elastic potential energy store of the bow.
5. Friction between brake pads and a wheel transfers energy mechanically out of the bicycle's kinetic energy store and into thermal energy stores.
6. Holding a heavy bag still at one height transfers no energy mechanically, because the bag does not move.
7. Carrying that bag horizontally at a steady speed adds nothing to its gravitational store either, because the upward force and the movement are at right angles.

Electrical equipment

1. Energy is transferred electrically when charge is pushed through a component by a power supply.
2. The store of the supply falls as this happens, so a torch battery's chemical energy store decreases while the torch is switched on.
3. An electric motor transfers energy electrically and increases the kinetic energy store of its turning parts.
4. A kettle element transfers energy electrically and increases the thermal energy store of the water.
5. A filament lamp transfers energy electrically, then passes it on by light and by heating the surroundings.
6. A loudspeaker transfers energy electrically and then carries it away by sound waves through the surrounding air.
7. Electrical equipment never creates joules, so every joule delivered to the components has come out of the store of the supply.

Heating

1. Heating transfers energy whenever two regions are at different temperatures, and it always runs from the hotter region to the cooler one.
2. The thermal energy store of the hotter region decreases while the thermal energy store of the cooler region increases.
3. The transfer keeps going until the two temperatures are equal, and it then stops.
4. A gas hob heats a pan, so the pan's thermal energy store increases and its temperature rises.
5. A mug of coffee left on a desk cools down because energy is transferred by heating to the cooler air of the room.
6. Energy transferred by heating can travel by conduction, by convection or by radiation.
7. Burning gas in a boiler empties the chemical energy store of the fuel and then transfers energy by heating into the water in the radiators.

8.1.3 Work done by forces

Work done and energy transferred

1. Work done is the energy a force transfers when it moves an object through a distance in the direction of that force.
2. Work done and energy transferred are the same quantity, so the work done by a force equals the energy it transfers.
3. Both are measured in joules, $J$, while force is measured in newtons, $N$, and distance in metres, $m$.
4. One joule is the work done when a force of $1N$ moves an object $1m$ in the direction of the force, so $1J=1Nm$.
5. A force that acts without producing any movement does no work, so it transfers no energy.
6. Only the part of the movement lying along the force counts, so a bag carried horizontally gains nothing from the upward force holding it.

The work done equation

1. Work done is calculated from $E=F\times d$.
2. In this equation $E$ is the work done in joules, $F$ is the force in newtons and $d$ is the distance moved in the direction of the force in metres.
3. Rearranging gives $F=\frac{E}{d}$ when the force is wanted and $d=\frac{E}{F}$ when the distance is wanted.
4. Convert every distance into metres before substituting, so $60cm=0.60m$ and $1.5km=1500m$.
5. For a lift straight upwards at a steady speed the force needed equals the weight, so the work done is the weight in newtons multiplied by the vertical distance in metres.
6. Doubling the force doubles the work done over the same distance, and doubling the distance doubles the work done for the same force.
7. A force at right angles to the movement contributes no distance in its own direction, so it does no work.

Energy changes when work is done

1. Work done by a force always changes a store, so every $E=F\times d$ calculation can also be described as an energy transfer.
2. Lifting an object at a steady speed puts the work done into the gravitational potential energy store of the object and Earth.
3. A resultant force that speeds an object up puts the work done into the object's kinetic energy store.
4. Stretching a spring puts the work done into the elastic potential energy store of the spring.
5. Work done against friction or air resistance puts the energy into the thermal energy stores of the surfaces and the surroundings, so their temperature rises.
6. A car braking from $30m/s$ has the work done by the braking force taken out of its kinetic energy store and put into the thermal energy stores of the brakes, tyres and road.
7. To describe any of these changes, name the force, state that the object moves in the direction of that force, state that work is done, then name the store that increases and the store that decreases.