1. Learning Objective Describe how a plug and mains electricity works
To be able to:
Describe how a plug and mains electricity works
Key Words:
Alternating potential difference, insulation

2. Exam question Wednesday, 31 July 2019 Mains Electricity
Date and title in books. You need a pen, pencil & ruler.
Starter Activity – 5 minutes
Exam question

3. Label a plug, describe the role of each component
Target 9-1
Describe the role of a plug and identify the three colours of wire
(F/H)
Target 4-9
Describe the key differences between alternative p.d. and direct p.d.
(H)
Target 5-9
Label a plug, describe the role of each component
Extension: Application of Knowledge
Target 8/9
Explain why the live wire is not needed for some appliances
Learning Outcomes

4. Direct current
Direct current (DC) is electric current that always flows in one direction. It is produced by cells and batteries.
Electronic circuits such as those in computers and stereos need direct current electricity in order to work.
Photo credit: © Marisha Sha, Shutterstock.com 2011
Direct current cannot be transferred efficiently over large distances.

5. Alternating current
Alternating current (AC) is electric current that is constantly changing direction. It is produced by most generators and is used in mains electricity.
Motors often work using AC, and AC voltage can easily be converted with a transformer. Many other electronic devices need to convert AC to DC to make them work.
Photo credit: © Henry Nowick, Shutterstock.com 2011
AC can be transferred efficiently over large distances, which is why it is generated for mains electricity instead of DC.

6. What is the difference between DC and AC?
The difference between DC and AC can be seen using a CRO. The oscilloscope trace shows how the voltage varies with time. DC
time
voltage AC
time
voltage
peak forward
peak reverse
The voltage of a DC supply is steady and always in the same direction.
The voltage of an AC supply follows a repeated pattern: it rises to a peak, returns to zero and changes direction. 6

7. Frequency of AC
The frequency of AC electricity is the number of complete cycles per second, which is measured in hertz (Hz).
1 complete cycle AC
time
voltage
peak forward
peak reverse
The AC frequency can be determined from an oscilloscope by counting the number of complete waves per unit time.
If the frequency is increased, the number of complete waves shown on the screen increases. For example, if the frequency is doubled, the number of waves doubles. 7

8. In the UK Mains electricity is an ac supply.
In the United Kingdom the domestic electricity supply has a frequency of 50 Hz and is about 230 V.

9. Why is electricity dangerous?
Warning signs are used to alert people to the presence of high voltage electricity.
When a current passes through living material, it causes an electric shock.
This interferes with nerve signals from the brain to the body, and can cause a heart attack, muscle spasms and breathing difficulties.
Other dangers include overheated and faulty appliances, which may lead to electrical fires.

10. What does each wire do?
Each of the wires in a cable performs a specific function.
The neutral wire completes the circuit. It is kept at a zero voltage
by the electricity company.
The live wire carries the high voltage.
Photo credit: © valzan, Shutterstock.com 2011
The earth wire is a safety wire that is needed to earth appliances with a metal case. This makes it safer to touch the appliance if it develops a fault.

11. Which wire is which?
Image credit: © Keo, Shutterstock.com 2011

12. live terminal earth terminal fuse neutral terminal cable grip cable
Inside a plug
The three pins in an electrical plug are connected to the three wires inside a cable.
live terminal
earth
terminal
fuse
neutral
terminal
Teacher notes
The virtual experiment on the next slide gives students an opportunity to show that they know where each of the wires should go in a three-pin plug.
cable
grip
cable
Could you wire a plug?

13. The Live Wire
The earth wire is a safety wire to stop the appliance becoming live.
The potential difference between the live wire and earth (0 V) is about 230 V. The neutral wire is at, or close to, earth potential (0 V).
The earth wire is at 0 V, it only carries a current if there is a fault.