4.2.3 Domestic Uses and Safety

In the UK, the electricity supplied to our homes is very different from the electricity supplied by a battery. Understanding how we use this energy safely is a vital part of the AQA specification.

1. Direct and Alternating Current

There are two types of electrical current:

1. Direct Current (DC): The current flows in one direction only. Cells and batteries supply DC.
2. Alternating Current (AC): The current constantly changes direction.

UK Mains Supply Details:

1. It is an AC supply.
2. It has a frequency of 50 Hz (it changes direction 50 times per second).
3. It has a potential difference of approximately 230 V.

2. Mains Cables and the Three-Pin Plug

Most electrical appliances are connected to the mains using a three-core cable. Each wire inside the cable is color-coded for identification:

3. Electrical Safety and the Earth Wire

The Live wire is the most dangerous. Even if a switch is open, it can still provide a lethal electric shock if you touch it, as you would provide a path to the earth (0V).

How the Earth Wire and Fuse Work Together:

1. If the live wire breaks and touches the metal casing of an appliance, the casing becomes "live."
2. The Earth wire provides a low-resistance path to the ground.
3. A huge current flows through the live wire to the earth.
4. This surge in current melts the fuse in the plug, breaking the circuit and making the appliance safe.

Edge Case: Double Insulation. Some appliances have plastic casings and no metal parts showing. These are "double insulated" and do not require an earth wire. They are marked with a symbol of a square within a square.

4. Power and Efficiency in Domestic Appliances

Appliances transfer energy from the mains to the device. The amount of energy transferred depends on the power of the appliance and how long it is switched on.

Calculating Power

Power is the rate at which energy is transferred. For any electrical component:

$P=V\times I$

$P=I^2\times R$

Where:

1. P is power in watts (W)
2. V is potential difference in volts (V)
3. I is current in amperes (A)
4. R is resistance in ohms ($\Omega$)

5. Mathematical Example

Question: A vacuum cleaner is connected to the 230V mains. The current flowing through it is 4A. Calculate the power of the vacuum cleaner and suggest a suitable fuse (3A, 5A, or 13A).

Step 1: Calculate Power ($P = VI$)

$P=230V\times 4A=920W$

Step 2: Choose a Fuse

A fuse must have a rating slightly higher than the normal operating current so it doesn't melt during regular use, but low enough to melt if there is a fault.

1. Operating current = 4A.
2. Next available size = 5A.

Answer: The power is 920W and a 5A fuse should be used.

6. The National Grid

The National Grid is a system of cables and transformers linking power stations to consumers.

1. Step-up transformers: Increase the potential difference (to about 400,000V), which decreases the current. This reduces energy lost as heat in the cables, making the grid efficient.
2. Step-down transformers: Decrease the potential difference to a safer 230V for domestic use.