2.3 Domestic uses of electricity

Question 1

Direct current

Answer 1

A direct current (d.c.) is defined as, A current that is steady, constantly flowing in the same direction in a circuit, from positive to negative.
The potential difference across a cell in a d.c. circuit is in one direction only.
A d.c. power supply has a fixed positive terminal and a fixed negative terminal
Electric cells, or batteries, produce direct current (d.c.).

Question 2

Alternating current

Answer 2

An alternating current (a.c.) is defined as.
A current that continuously changes its direction, going back and forth around a circuit.
On an oscilloscope, direct current and alternating current are represented in the following way:
An a.c. power supply has two identical terminals.
The potential difference between the terminals varies as a sine curve, or sinusoidally.
The frequency of an alternating current is the number of times the current changes direction back and forth each second.
In the UK, mains electricity is an alternating current with a frequency of 50 Hz and a potential difference of around 230 V.

Question 3

Mains electricity

Answer 3

Mains electricity is the electricity generated by power stations and transported around the country through the National Grid.
Everyone connects to the mains when plugging in an appliance such as a phone charger or kettle.
Mains electricity is an alternating current (a.c.) supply.
In the UK, the domestic electricity supply has a frequency of 50 Hz and a potential difference of about 230 V.
A frequency of 50 Hz means the direction of the current changes back and forth 50 times every second.
Mains electricity, being an alternating current, does not have positive and negative sides to the power source.
The equivalent to positive and negative are called live and neutral and these form either end of the electrical circuit.

Question 4

Live wire

Answer 4

Carries the alternating potential difference from the mains supply to a circuit.
It is the most dangerous of the three wires.
If it touches the appliance without the Earth wire, it can cause electrocution.

Question 5

Neutral wire

Answer 5

Forms the opposite end of the circuit to the live wire to complete the circuit.
Because of its lower voltage, it is much less dangerous than the live wire.

Question 6

Earth wire

Answer 6

Acts as a safety wire to stop the appliance from becoming live.
This prevents electric shocks from occurring if the appliance malfunctions or the live wire breaks off and touches the case of the plug.

Question 7

Potential Difference of Live, Neutral and Earth Wires

Answer 7

The live wire carries most of the power to the circuit and, as a result, it is the most dangerous wire.
It has a voltage of around 230 V.
The neutral wire is much safer than the live wire, although it can deliver a small shock.
It has a voltage close to 0 V.
The earth wire only carries a current to the ground if there is a fault in the appliance.
It has a voltage of 0 V.

Question 8

Dangers of mains electricity

Answer 8

Normally, the earth wire does not carry any electricity – it is there for safety only.
As a result, in most circumstances, its potential difference is 0 volts.
Many electrical appliances have metal cases that pose a potential safety hazard:
If a live wire (inside the appliance) came into contact with the case, the case would become electrified.
Therefore if anyone touched it, they would risk electrocution.
The earth wire is an additional safety wire that can reduce this risk
If this happens:
The earth wire provides a low resistance path to the Earth.
This causes a surge of current in the earth wire and hence also in the live wire.
The high current through the fuse causes it to melt and break.
This cuts off the supply of electricity to the appliance, making it safe to touch.