Electricity in the home

Alternating current

Direct current (d.c) is when the current goes around a circuit in one direction; alternating current (a.c) is when the electrons rapidly change direction (they repeatedly reverse their direction and flowing in one way then the opposite way in successive cycles). Frequency is the number of cycles it passes through each second.

In the UK, the mains frequency is 50 cycles per second (50 Hertz).  To work out the time period (time taken for one cycle), use 1 ÷   f. The time taken for a cycle is 0.02s ( 1 ÷  50).

Every mains circuit has a live wire and a neutral wire. A mains appliance has alternating current because the mains supply provides an alternating potential difference (voltage) between the live wire and neutral wire (the potential difference's polarity repeatedly reverses its direction). Potential differences in direct current circuits do not change direction.

The neutral wire is earthed at the local electricity substation. The potential difference betwen the live wire and the earth is called the potential difference of the live wire which is dangerous because its potential difference rapidly changes every cycle. It reaches about 325V in each direction in UK homes.

When you use mains appliances, electricity is supplied from the power stations to buildings through the National Grid - a nationwide network of cables and transformers:

• Step-up transformers are used at power stations to transfer electricity to the National Grid and make the size of the alternating current much bigger so less current is needed to transfer the same amount of power and power loss due to the resistance heatings in the cables is reduced significantly. As a result, the National Grid is an efficient way to transfer electricity.
• Step-down transformers are used to supply electricity from the National Grid to consumers.

An oscilloscope shows how an alternating potential difference changes with time.

• The waves on the oscilloscope screen are caused by the potential difference increasing and decreasing continuously. Adjusting the 'Y-gain' control changes the waves' height; adjusting the 'time-base' control changes how many waves fit across the screen.
• The peak potential difference is the difference between the highest level and middle level of the waves in volts. Increasing the potential difference of the alternating current supply makes the waves taller.
• Increasing the frequency of the alternating current supply increases the number of cycles you see on the screen, so the waves on the screen get squashed together.

Cables and plugs

Study tip: make sure you know what is in a three pin plug and the colour of each wire.

The metal case is automatically connected to earth through the earth wire which stops the metal case becoming live if the live wire breaks and touches the case. This prevents electrocution. An appliance that has a plastic case is double insulated and it has no earth wire connection. 

Outer casings of plugs, sockets, and cables of all mains circuits and appliances are made of hard-wearing electrical insulators because…