6.2 Electricity - Energy and uses

Power (P) is the rate at which energy is transferred. Electrical power is measured in watts (W).

An appliance with a high power rating transfers a lot of energy every second.

Power = Energy Transferred / Time

P = E / t

We can also calculate power using current and voltage:

Power = Current × Potential Difference

P = I × V

And by combining P=IV with V=IR, we get two more equations:

Power = (Current)² × Resistance (P = I²R)

Power = (Potential Difference)² / Resistance (P = V²/R)

AC and DC

• Direct Current (DC): The current flows in one direction only. This is supplied by batteries and cells.
• Alternating Current (AC): The current constantly changes direction. This is the electricity supplied by the mains.

In the UK, mains electricity is an AC supply with a frequency of 50 Hz and a voltage of about 230 V.

Cables and Plugs

Most mains appliances are connected with a three-core cable:

• Live Wire (Brown): Carries the high voltage (230 V).
• Neutral Wire (Blue): Completes the circuit. It is at or near 0 V.
• Earth Wire (Green & Yellow): This is a safety wire. It is connected to the metal casing of the appliance. If the live wire accidentally touches the casing, the large current flows down the earth wire, blows the fuse, and cuts off the electricity.

A fuse is a safety device with a thin wire inside. If the current gets too high, the wire melts and breaks the circuit.