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

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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 transfers

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

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.  

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