The National Grid
The National Grid is a system of cables and transformers linking power stations to consume.
Transformers
Transformers can either increase or decrease the potential difference.
Step-up transformer
Increases potential difference which decreases current which reduces heating of the transmission cables so less energy is wasted increasing the efficiency of the system.
Step-down transformer
Decreases potential difference which increases current to safe levels. These levels are suitable for domestic appliances.
The transmission cables
have a low resistance which reduces heating which reduces wasted energy and increases efficiency.
POWER
P = VI
power = potential difference × current
Power =
current x potential difference
Energy transferred =
Power x time
MAINS ELECTRICITY
The electricity supplied to homes in the UK is alternating current (AC).
a) State the potential difference of the UK mains supply.
230V
b) State the frequency of the UK mains supply.
50Hz.
c) Explain what is meant by alternating current.
an alternating current constantly changes direction
b) State the purpose of the earth wire in an electrical appliance.
Carries the electric current from the live wire to the earth if there is a fault, protecting the user from electric shock.
Explain why a person may get an electric shock if they touched the live wire (4 marks).
the live wire has a potential difference of 230V The person will have a potential difference of OV There is a large potential difference between the person and the device a current will flow through the person, causing them to be electrocuted.
A 2.2 MQ resistor passes a current of 50 uA, what is the power dissipated in the resistor?
More difficult Mixed Questions (you may need to use more than one equation….)
Alternating Potential Difference
A potential difference that constantly changes direction, which causes an alternating current to flow.
Direct Potential Difference
A potential difference in one direction causing current to flow in one direction.
Earth Wire
The green and yellow striped safety wire that prevents an appliance from becoming live.
Insulation
The coating around power cables that prevents electrocution and is colour coded to allow for easy identification.
Live Wire
The brown coloured wire that carries the 230V alternating current from the supply in a mains power supply.
Mains Electricity
An a.c supply, which in the UK has a frequency of 50Hz a value of 230V.
Neutral Wire
The blue coloured wire that completes the circuit in a mains power supply.
Alternating Potential Difference
A potential difference that constantly changes direction, which causes an alternating current to flow.
Direct Potential Difference
A potential difference in one direction causing current to flow in one direction.
