Domestic Electricity

Energy Transfer

Electrical devices are all examples of energy transfer devices, and like all such devices they produce useful energy changes and some waste energy changes too. The waste energy change will result in the production of heat, mainly from the heating effect of a current.

Examples of devices and their useful energy changes

Kettle: Electrical to Heat and some light

TV : Electrical to light and sound

Laptop: Electrical to light, sound, electromagnetic energy for WiFi etc

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Fridge: Electrical to kinetic of the pump, heat

The rate at which these transfers occur depend upon the power rating of the device.

Devices can gain their initial energy from batteries that supply direct current or the mains that supplies an alternating current. Some may also use solar cells, such as calculator and some watches. Solar cells provide a direct current supplies of electricity.

As with all energy transfers the total energy supplied by the electrical power supply must equal the total of all the energy types produced by the device. This is known as the Law of Conservation of Energy, so the more energy that is converted to waste heat the less efficient the device will be.

New electrical devices sold within the UK and the EU have an energy rating indicating how efficient they are.

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Types of Current

Batteries, cells and solar panels all generate a constant potential difference across the device, they maintain a constant positive and negative side to their terminals. This results in the current produced flowing in one direction, from the positive terminal to the negative terminal. This type of current is called Direct Current (DC) and is caused by a Direct Voltage.

[Note that although current flows from positive to negative, the actual electrons flow from the negative to the positive.]

This symbol is used in a circuit diagram to indicate that a power supply is a direct voltage and direct current supply.

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This symbol is used in a circuit diagram to indicate that a power supply is a direct voltage and direct current supply.

The electricity supplied by generators and from our mains supply is Alternating Current (AC) from an alternating voltage supply. In this type of supply the polarity of the potential difference (voltage) is reversed several times a second. This results in the current changing its direction of flow as well. In the UK this happens in the mains supply 50 times a second. Some countries, such as the USA use an AC supply that switches 60 times a second.

This is the circuit diagram symbol used for an AC power supply.

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UK Domestic Supply

The standard domestic power supply to all UK homes, schools and offices etc is set at:

230 V AC

50 Hertz (Hz)

This is the same across the whole of Europe and has been since 2003, before then the UK supply was 240 V AC and Europe 220 V AC.

Many larger consumers, such as factories and hospitals also have a supply up 500V AC, but all are at 50 Hz AC.

A consistent supply to all houses in this way always manufacturers of devices to design equipment with set power ratings. It also ensures a safe consistent supply to all houses.

Wires in a Plug

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The three pin plug is a safety device with several features in its design.

The longer top pin pushes aside a safety plate inside the socket that covers the power cable contacts to prevent anyone accidentally touching the live power supply when the socket is not in use.

Inside the Plug

Brown Wire - Live Wire: This wire supplies the voltage to the device, the level alternates to cause the alternating current.

Blue Wire - Neutral Wire: This wire completes the circuit and allows the current to flow, this wire is NOT at zero volts and current is flowing in this wire.

Green/Yellow Wire - Earth Wire: Safety wire that provides an alternative route for the current in an emergency. The earth wire is connected to the case of the device. This is only required on devices with a metal conductive casing. Should the live wires in the device come into contact with the case the current will flow along the earth wire causing a large rise in the current that will blow the fuse and cut off the power supply. The earth wire in the socket is connected to an earthing point in the house. Normally this is the mains water pipe, this is metal and allows the sudden excess current to flow harmlessly into the earth.

The Fuse: This safety device is connected to the live wire that carries the AC voltage, they are designed to ‘blow’ if the current in the live wire exceeds a certain level. Most devices in the home have either 3 or 5 Amp fuses.

A fuse has a metal cap at either end and between them a thin wire, if the current is too high the heating effect will melt the wire and break the circuit.

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Electrical Safety

The plug with its earth wire and the fuse are two of the safety devices built into modern domestic supplies to prevent electrocution and fires.

The fuse in a plug must be rated for the device it is connected to, if the amp rating is too high the fuse wire may not melt in an emergency, potential resulting in electrocution of anyone touching the device or in the device overheating and catching fire.

At the point in a house where the mains supply enters there will be a fuse board. These ‘fuses’ act in the same way as a fuse in the plug, but in modern houses the fuse has been replaced with a trip switch or circuit breaker that cuts off the supply if it is too high. These have to be connected to the live wire, as this wire carries the AC voltage supply. If these trip switches do trip, it is important to fix the problem that caused it to trip before resetting this switch. This should be done by a qualified electrician.

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Note that trip switches or circuit breakers are also known as RCD’s Residual Circuit Breakers.

These systems have been designed to ensure people’s safety, they should not be altered. For example replacing a fuse with a strip of metal or allowing the earth wire and the live wire to touch, this can cause large currents to flow and risk fires or electrocution.

Explain the difference between the live wire and the earth wire in a plug.
Your answer should include: AC / voltage supply / safety wire
Explanation: The live wire carries the AC voltage supply to the device and has a current flowing in it when the device is on. The Earth wire normally has no current flowing in it. It is a safety wire that provides a safe route for current if the live wire touches the casing of the device.
Why is the fuse connected to the live wire and not the neutral wire in the plug?
alternative current
Explanation: The live wire carries the alternative current so the fuse is connected to this wire so that in an emergency the power supply will be cut off.
What is the standard domestic electrical supply in the UK?
Your answer should include: 230V / 50Hz / 230 / 50
Explanation: All house in the UK are supplied with electricity at 230 V and 50 Hz AC.