Energy stores and transfers

In a nutshell

Energy cannot be created or destroyed, it can only be transferred from one store to another. There are many different energy stores and four ways in which it can be transferred. You can use transfer or sankey diagrams to represent energy transfers.

Definitions

Keyword	Definition
Work done	Work done is the energy transferred to or from an object using force
System	A system is an object or several objects

Energy stores

Energy is the ability to do work and it can be stored and transferred. Energy can be stored in many ways, and the following are the stores you need to learn.

Energy Store	Description	Example
Electrostatic	Energy stored when positive and negative charges attract or repel.	Thunderstorms, nylon clothes, two balloons rubbed together
Nuclear	Energy stored in the nucleus of an atom.	Fusion in stars, fission reactions in nuclear power plants
Kinetic	Moving objects have kinetic energy.	Car, someone running, rolling a ball
Thermal	Objects with a temperature above $0\ K$ ( $-273\ \degree C$ ) have thermal (heat) energy.	Everything! Unless something has a temperature of $-273\ \degree C$
Gravitational potential	When an object is in a gravitational field it has this energy store. The amount of energy stored depends on height, mass and strength of gravitational field.	Skydiver, ball thrown in the air, car at the top of a ramp
Elastic potential	Energy stored when objects are squashed or stretched.	Elastic band, spring, slinky
Chemical	Energy stored between chemical bonds.	Food, batteries, fire logs
Magnetic	Energy stored when poles attract or repel.	Fridge magnets, compasses

Energy transfers

Energy can be transferred between the above stores in four different ways.

Energy Transfer	Description	Example
Mechanically	Energy transfers when a force is applied to an object	Pushing an object
Electrically	Energy transfers when charge flows	Electricity in a wire
By heating	Energy transfers from a hot region to a cold region	Water being boiled on a stove
By radiation	Energy is transferred as a wave	Sunlight

Conservation of energy

Energy cannot be created or destroyed, it can only be transferred from one store to another.

However, energy stores are not perfect, and some energy gets lost, for example, by heating. Therefore, the total energy put into a system is equal to useful energy and wasted energy combined.

Example

Describe the energy stores and transfers for a toy car going down a ramp.

At the top of a ramp a toy car will have gravitational potential energy. This will be mechanically transferred to kinetic energy as the toy car goes down the ramp. Some energy will transfer into the surroundings in the form of sound or heat.

Energy transfer diagrams

Energy transfers between stores can be represented in two different ways.

Transfer diagrams

Transfer diagrams can be used to represent energy transfers when energy values are unknown. This type of diagram uses rectangles to represent stores, and an arrow to show which energy transfer took place.

Example

Draw an energy transfer diagram describing the chemical energy store transfer from your muscles to the kinetic energy in a bike when you are pushing it.

$\boxed{chemical \space energy \space in \space muscles} \xrightarrow[mechanically] {energy \space transferred} \boxed {kinetic \space energy \space in \space bicycle}$

Sankey diagrams

Sankey diagrams are used when energy values are known. They are drawn using arrows to represent how much energy has been transferred as waste energy and useful energy.

Example

Draw and label a sankey diagram for a lightbulb that requires $60 \, J$ of energy. $50 \, J$ has been transferred to light energy, and $10 \, J$ has been transferred to heat energy.

Physics; Conservation of energy; KS4 Year 10; Energy stores and transfers

1.	$60 \, J$ of total energy for the lightbulb
2.	$50 \, J$ of useful light energy
3.	$10 \, J$ of wasted heat energy