1. SAFETY FEATURES USED IN THE CIRCUITS OF THE MAINS SUPPLY
Additional Science
Chapter 14
Physics 2

2. Electrical Power Additional Science Chapter 14 Remember:
Electrical Power (W) = Current (A) × Voltage (V) or
P = IV
Power = Current × Voltage
= × 240
= 60W
Current = 0.25A
Voltage = 240V
Additional Science
Chapter 14
Power = Current × Voltage
= × 240
= 3000W = 3kW
Current = 12.5A
Voltage = 240V

3. Electrical Power Additional Science Chapter 14 Current = 0.5A
Remember:
Electrical Power (W) = Current (A) × Voltage (V) or
P = IV
Current = 0.5A
Voltage = 240V
Power =?
Current = 2A
Voltage = 240V
Power =?
Answer: 480W
Answer: 120W
Additional Science
Current = 0.4A
Voltage = 240V
Power =?
Current = 3A
Voltage = 240V
Power =?
Chapter 14
Answer: 720W
Answer: 96W

4. Electrical Power Appliance Power(W) Current(A) Voltage(V)
You must remember:
Electrical Power (W) = Current (A) × Voltage (V)
or P = IV
Appliance
Power(W)
Current(A)
Voltage(V)
Hair dryer 3
240
heater
1200
iron
960
personal stereo
2.4
microwave
800
Additional Science
Chapter 14

5. Electrical Power Appliance Power(W) Current(A) Voltage(V)
You must remember:
Electrical Power (W) = Current (A) × Voltage (V)
or P = IV
Appliance
Power(W)
Current(A)
Voltage(V)
Hair dryer
720 3
240
heater
1200 5
iron
960 4
personal stereo
2.4
0.01
microwave
800
3.33
Additional Science
Chapter 14

6. Electrical plug Additional Science Chapter 14
Remember:
Te live wire carries the electricity
The fuse protects the appliance from damage
The earth wire protects us from danger from the circuit The plastic and the rubber is the material for the plug and wire cover
Earth
Live
Additional Science
Fuse
Neutral
Chapter 14
Cord grip
Cable

7. Electrical plug Which plug has been correctly wired?
A B C
Additional Science
D E F
Chapter 14

8. Electrical Power and the Fuse
You must remember:
Electrical Power (W) = Current (A) × Voltage (V)
or P = IV
Current = ?
Voltage = 240V
Power = 720W
Current = ?
Voltage = 240V
Power = 120W
Current = ?
Voltage = 240V
Power = 3000W
Current = ?
Voltage = 240V
Power = 480W
Additional Science
Chapter 14
What fuse must be used? 3A 5A
13A

9. The work of the fuse
Current travels along the live wire, through the appliance (bulb in this case) and back along the neutral wire. `
Additional Science
Chapter 14

10. The work of the fuse
This shows what happens when there is a fault in the circuit.
The current is too strong for the fuse. It blows causing a break in the circuit.
Additional Science
Chapter 14
Part of the live wire touches the side of the appliance.
A lot of current now flows along the earth wire.

11. Circuit Breaker Additional Science Chapter 14
Miniature circuit beakers (MCB) are used in modern consumer units. They replace fuses.
When the current exceeds the rated value, the strength of the electromagnet is enough to separate the contacts. The circuit is switched off.
Circuit breakers, like fuses, do not provide protection should you touch a live component.
Additional Science
Chapter 14
Advantages of the fuse:-
It works a lot faster
It is possible to reset it.
Click here for further information from the web

12. (Residual Current Devices RCDs)
These devices are designed to protect against electrical shock. This type of circuit breaker measures the current in the live and neutral wires. If everything is working correctly, then these two are equal.
If there’s a fault, you get a small electrical shock but not strong enough to kill you. You should always use a RCD every time you work with outdoor equipment.
If there’s a difference between the live wire and the neutral, the electromagnet in the live side will pull the iron bar downwards, pushing the button up to create a break in the circuit. The effect of this is to prevent electricity from reaching the appliance. Switch on the drill to see the RCD at work.
You must reset the switch before using the RCD again.
Additional Science
Break in the circuit B
Chapter 14 N
pivot
Iron bar
Wire coils (electromagnet)
Switch on the drill
Current out
Current in
Power supply
neutral
live

13. DC and AC Additional Science Chapter 14
Oscilloscope
DC and AC
Voltage (V)
Time (T)
DC means “Direct Current” – the current travels in one direction only e.g. in a battery
1.5V
1.5V
Oscilloscope
Voltage (V)
Time (T)
1 th/ s 50
Additional Science
AC means “Alternating Current” – the current changes 50 times per second (frequency = 50Hz). This is the frequency that comes from the mains supply.
230V
Chapter 14

14. Why 230V? Additional Science Chapter 14
The UK and European countries have used 230V since the start.
It would be very costly to change this to a lower voltage – appliances use 230V.
Many transformers would be required for our appliances to work at a lower voltage.
Thicker and more flexible cables would be required (lower voltage higher current to ensure the same power.
Many builders use tools that use 110V. This voltage reduces the risk of fatal electrical shocks.
Additional Science
Chapter 14
The USA has chosen a voltage of 120V for its mains supply. Because of this, some electrical appliance manufacturers include a switch so that the correct voltage can be chosen for the appliance – using the appliance with a voltage of 120V would be damaging in this country.