Energy Stores and Systems

In physics a system is just the object, or group of objects, you choose to study. It might be a single ball, a kettle of water, or a car and the road together. Defining the system first makes it clear where energy is going.

Energy is measured in joules (J). It is never made or lost, but it is constantly moved around and stored in different ways. When a system changes, the way its energy is stored before the change is different from the way it is stored afterwards.

A system is an object or a group of objects. When a system changes, energy is transferred between stores; the total energy is conserved.

Thinking in terms of a system stops you losing track of energy. If a moving car brakes and stops, its kinetic energy does not vanish. It is transferred to the thermal store of the brakes, tyres and surrounding air, which warm up slightly. Track the system, and the energy always adds up.

Energy can be held in several different stores. You should be able to name them and recognise which store is involved in a given situation.

Energy store	Where you find it
Kinetic	Any moving object
Gravitational potential	An object raised above the ground
Elastic potential	A stretched or compressed spring or elastic band
Chemical	Fuels, foods and batteries
Thermal (internal)	A warm object; increases as temperature rises
Magnetic	Two magnets pushed together or pulled apart
Electrostatic	Charged objects held apart
Nuclear	The nucleus of an atom

The word store describes where the energy is; it does not tell you how the energy got there. Moving energy between stores is called a transfer, and that is the subject of the next slide.

A system's energy can be changed in exactly three ways at GCSE. Each one transfers energy between stores.

A system can be changed by heating, by work done by a force, or by work done when an electric current flows.

Work done means transferring energy using a force. When a force moves an object through a distance, work is done and energy is transferred. When an electric current flows in a component, work is done and energy is transferred electrically.

Way of changing the system	What happens	Everyday example
Heating	Energy transferred to the thermal store	A kettle heating water
Work done by a force	A force moves an object, transferring energy	Pushing a box across the floor
Work done by a current	Charge flows through a component, transferring energy	A current warming the element of a heater

Recognising which of the three is happening is the first step in describing any energy change accurately.

The specification names several everyday situations you must be able to describe. For each one, identify the store energy comes from and the store it goes to.

• An object projected upwards — the kinetic store falls as the object slows; the gravitational potential store rises as it climbs.
• A moving object hitting an obstacle — the kinetic store falls; energy is transferred (mainly to the thermal store) as the surfaces are deformed and warmed, often with a transfer by sound.
• An object accelerated by a constant force — work done by the force transfers energy to the object's kinetic store, so it speeds up.
• A vehicle slowing down — the brakes do work against friction; the kinetic store falls and the thermal store of the brakes and surroundings rises.
• Water brought to a boil in a kettle — the current does work on the heating element, which heats the water; the water's thermal store rises.

A good description names both stores and the way the transfer happens (heating, or work done by a force or current). Naming only one store rarely gains full marks.

Take a ball thrown straight up. Follow its energy through the motion, store by store.

At the moment of launch the ball has its greatest speed, so its kinetic store is at a maximum and its gravitational potential store is at a minimum.

As it rises, work is done against gravity. Energy is transferred from the kinetic store to the gravitational potential store, so the ball slows down.

At the highest point the ball is momentarily still: the kinetic store is at a minimum and the gravitational potential store is at a maximum.

Coming down, the transfer reverses: the gravitational potential store falls and the kinetic store rises, so the ball speeds up again.

If air resistance is ignored, the total energy stays the same throughout, simply shifting between the kinetic and gravitational potential stores. In reality a little energy is also transferred to the thermal store of the surrounding air, so the ball returns with slightly less kinetic energy than it started with.

Once you can describe a change, you can quantify it. The energy transferred by each of the three methods is found with its own equation, which you will meet in the following lessons.

Change to the system	Equation used	Recall or given?
Work done by a force	$W=Fs$	Recall and apply
Heating (temperature change)	$\Delta E=mc\Delta \theta$	Given on the equation sheet
Work done by a current	$E=Pt$	Recall and apply

Two of these must be recalled from memory ($W=Fs$, $E=Pt$). The heating equation $\Delta E=mc\Delta \theta$ is printed on the Physics equation sheet, so you rearrange it rather than memorise it.

For example, work done pushing a box with a force of 20 N through a distance of 3 m is $W=Fs=20\times 3=60\text{J}$. That 60 J is transferred from your chemical store to the kinetic and thermal stores of the box and floor.

1. Naming a "type of energy" instead of a store

Say the energy is stored in the kinetic store, not that there is "kinetic energy" moving around. The store-and-transfer language is what AQA now uses, and it earns the marks.

2. Forgetting to say how the transfer happens

A full description names the stores and the method: heating, work done by a force, or work done by a current. Naming stores alone often loses a mark.

3. Claiming energy is used up or lost

Energy is never destroyed. When a car stops, its kinetic energy is transferred to the thermal store of the brakes and surroundings, not lost.

4. Mixing up the store and the object

Gravitational potential energy is stored in the system of the object and the Earth together, because of the object's raised position. It is not "inside" the object like a fuel.

5. Using the wrong method for the change

Boiling water in a kettle is work done by a current heating the element, then heating the water. It is not "work done by a force". Match the description to the correct one of the three methods.