3. Energy transformations in circuits
Energy cannot be created or destroyed.
In all devices and machines energy is transformed from one type to another.
Batteries supply energy to a circuit. When the circuit is connected, the chemical energy stored in the battery is transformed, via electrical energy, to heat and light energy in the bulbs.
The total amount of heat and light energy is the same as the amount of chemical energy lost from the battery.

4. Explaining energy transfers in circuits
A model can help us to understand how current works in an electric circuit. In this model, the moped riders represent the flow of charge and the pizzas represent the electrical energy carried around the circuit.
When the pizzas are eaten, they are transformed into other useful nutrients in peoples’ bodies.
Teacher Notes
This alternative model of an electric circuit uses the following representations:
moped riders = electric charge (i.e. electrons)
pizza = electrical energy
pizza shop = source of electrical energy
house = bulb (or other device) that uses electrical energy
1 rider leaving the pizza shop each second = e.g. 1 ampere
If the mopeds travelled faster, the current would increase.
change in number of pizzas carried by moped rider = potential difference
If each rider carried more pizzas, the potential difference would increase.
In a similar way, electrical energy is transformed into light and heat energy by the lamp.

5. Conventional current
When electricity was first discovered it was believed that the particles moving around circuits had a positive charge.
Scientists thought that the particles must be moving from the positive to the negative terminal of a battery as the particles were positive.
current direction
Movement of electric charge in this direction is called conventional current.

6. Electron flow
We now know what these particles are – they are called electrons.
They are negative so they move away from the negative and towards the positive terminal of a battery. This movement is called electron flow.
electron flow
Some people still think of electricity in terms of conventional current, not electron flow. This can be confusing, so it’s important to understand the difference between the two. 6

7. Energy transfer in circuits summary

9. What is the energy transformation?
Batteries can power many electrical devices.
Photo credit (torch, drill): © 2008 Jupiterimages Corporation
What kinds of energy do these devices transform electrical energy into?

10. What do we use electrical energy for?
Electrical energy is very useful to us because it is easily changed into other kinds of energy.
For example, a television changes electrical energy into light and sound, which help us communicate. In this case, electrical energy is also converted into energy that is not so useful – heat energy.
light and sound energy
electrical energy
heat energy

11. Energy transfers in circuits

12. Energy transfers in everyday objects

14. Energy transformations in circuits
What happens to the electrical energy received by a lamp in a circuit?
5 J transformed into light energy
electrical energy from battery 100 J
95 J transformed into heat energy
Most of the energy from the battery does not produce light – most of it is wasted as heat!

15. Calculating energy efficiency
The efficiency of an energy transfer can be calculated using this formula:
total energy input
useful energy output
% efficiency =
× 100
This bulb converts 200 J of electrical energy from the circuit into 10 J of useful light energy:
Worksheet 1 accompanies this slide.
200 10
% efficiency =
× 100 5% =

16. Energy efficiency activity

17. Energy efficiency summary

19. Why should electricity usage be reduced?
Most of our electricity is generated by burning fossil fuels in power stations.
These emit massive amounts of carbon dioxide into the air which causes global warming.
We also have to pay for the electricity we use. If we can spend less money on electricity, then we have more to spend on the things we enjoy.
Worksheet 2 accompanies this slide.
This gives pupils the opportunity to write to their local MPs about reducing electricity usage.
Photo credit: © 2008 Jupiterimages Corporation

20. Reducing our electricity bills
There are several ways that you can help to reduce the amount of electricity you use every day.
Using electrical appliances less. This is the most effective way to reduce the amount of energy you use – think about whether the electricity that you’re using is necessary.
Photo credit: © 2008 Jupiterimages Corporation
Making sure devices are turned off. Simply leaving electrical equipment, such as televisions, on stand-by can waste energy.

21. Reducing our electricity bills
There are several ways to make your house more energy efficient.
Energy efficient light bulbs These bulbs are more efficient than normal light bulbs.
Insulation This minimizes the amount of heat lost from a house, meaning that less energy is needed to keep it warm.
Photo credit (lightbulb): stock.xchng
Photo credit (insulation): © 2008 Jupiterimages Corporation
Teacher notes
See the Physics Around Us presentation to find out more about ecohousing.
Some modern houses have been designed to reduce the amount of energy they use. 21

23. Glossary
circuit – A complete loop of conducting components that electricity flows around.
current – The flow of electricity, measured in amps (A).
efficiency – A measure of how much energy is changed into a useful form during an energy transfer.
electrons – Tiny particles that flow around electrical circuits.
energy transformation – The process of changing energy from one form to another.
resistor – A component that makes it difficult for electricity to flow and so reduces the current. It converts electrical energy to heat energy.
wasted energy – Energy in a form that is not wanted or is not used.

24. Anagrams

25. True or false?

26. Multiple–choice quiz