A system: decide what you are looking at first

1. Before you can say where energy has gone, you first have to decide what you are looking at, and that choice of object or objects is the system.
2. A system can be a single object, such as a falling ball, or several objects together, such as a kettle, its water and the mains supply.
3. Once you have set that boundary, you list the stores inside it and say which ones are empty and which ones are full.

Energy stores: the eight ways energy is held

1. Energy is measured in joules, $J$.
2. Energy is not a substance and not a "type": it is a quantity you account for by naming the store it sits in.
3. There are eight energy stores:
   1. kinetic: anything moving
   2. gravitational potential: anything raised in a gravitational field
   3. elastic potential: anything stretched, squashed or twisted, such as a spring
   4. thermal (internal): every object, and larger when the object is hotter
   5. chemical: fuels, food and batteries
   6. nuclear: the nuclei of atoms
   7. magnetic: magnets interacting
   8. electrostatic: separated electric charges

Transfers: stores empty and fill along pathways

1. When a system changes, energy is transferred from one store to another.
2. The route it takes is a pathway, and there are four:
   1. mechanically: a force acts on an object and moves it through a distance
   2. electrically: a charge is moved through a potential difference, that is, a current flows
   3. by heating: energy passes from a hotter object to a cooler one
   4. by radiation: energy is carried away by waves, such as light or sound

Describing the changes: one frame fits every situation

1. Every answer below fits one frame: energy is transferred [pathway] from the [store] of the [object] to the [store] of the [object].
2. Describe all the changes when the system changes, including the wasted transfers, not just the main one.

An object projected upwards

1. As the object rises, energy is transferred mechanically from its kinetic store to its gravitational potential store.
2. Thrown straight up, its kinetic store is empty at the highest point, and the transfer reverses as it falls.
3. Air resistance does work on it throughout, so a little energy also goes to the thermal store of the surroundings.

A moving object hitting an obstacle

1. Energy leaves the kinetic store of the moving object.
2. Where it goes depends on the collision: to the kinetic store of the obstacle if that is pushed along, to the elastic potential stores of whatever bends or squashes, to the thermal stores of both objects as they warm up, and away by radiation as sound waves.

An object accelerated by a constant force

1. The force does work on the object, so energy is transferred mechanically to its kinetic store.
2. Say where it came from: the chemical store of the fuel in an engine, the chemical store of the athlete pushing, or the gravitational potential store of a falling mass.

A vehicle slowing down

1. The brakes press on the discs and friction does work, so energy is transferred mechanically from the kinetic store of the vehicle to the thermal store of the brakes, which is why brakes get hot.
2. It then transfers by heating to the thermal store of the surrounding air.

Bringing water to the boil in an electric kettle

1. Energy is transferred electrically from the mains supply to the thermal store of the heating element, then by heating to the thermal store of the water, whose temperature rises until it boils.
2. Some energy also goes to the thermal store of the kettle body and the surrounding air.

1.1.1b Calculating energy changes when a system changes

Heating: transferring energy to the thermal store

1. When a system is heated, energy is transferred to its thermal store.
2. The temperature rises if that energy increases the average kinetic energy of the particles.
3. For heating calculations, use $\Delta E=mc\Delta \theta$.
4. Here $\Delta E$ is the change in thermal energy in $J$, $m$ is the mass in $kg$, $c$ is the specific heat capacity in ${J/kg}^{\circ }C$, and $\Delta \theta$ is the temperature change in ${}^{\circ }C$.

Work done by forces: force times distance

1. When a force does work on an object, energy is transferred.
2. Pushing a trolley transfers energy to its kinetic store, and lifting a box transfers energy to its gravitational potential store.
3. To calculate the work done, use $W=Fs$.
4. Here $W$ is the work done in $J$, $F$ is the force in $N$, and $s$ is the distance moved in the direction of the force in $m$.
5. Because work done is an energy transfer, $1J$ is the same as $1N\; m$.

Work done when a current flows: transferring energy electrically

1. When a current flows, energy is transferred electrically because charges move through a potential difference.
2. That energy may go to a useful store, such as the thermal store of the water in a kettle, or be dissipated to the surroundings.
3. To find the energy transferred, use $E=Pt$.
4. Here $E$ is the energy transferred in $J$, $P$ is the power in $W$, and $t$ is the time in $s$.
5. You can also use $E=QV$, where $Q$ is the charge flow in $C$ and $V$ is the potential difference in $V$.
6. If a question gives you the equation, use it; otherwise you choose the right equation and rearrange it yourself.

Conservation: energy is redistributed, not lost

1. Energy is conserved: it cannot be created or destroyed.
2. When a system changes, the energy is redistributed between stores or transferred to the surroundings.
3. Showing this on a common scale means comparing every energy value in the same unit, usually joules, so the transfers can be set side by side.
4. For example, if a kettle transfers $100000J$ electrically and $85000J$ raises the thermal store of the water, the energy dissipated is $100000-85000=15000J$.
5. The total is still $100000J$, now split as $85000J$ to the thermal store of the water and $15000J$ to the thermal store of the kettle and surroundings.