P5 electricity

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Electricity in the home

The mains supply is an alternating current (ac current) and a battery supply is a direct current (dc current)

In an ac supply the current is constantly changing direction. They are produced by alternating potential difference in which positive and negative keep

alternating.

Uk mains supply is an ac supply at around 230V. The frequency of the supply is 50 cycles per second or 50Hz (hertz)

Cells and battery supply direct current. It always flows in the same direction and is created by direct potential difference.

Mains circuits

Every mains circuits has a live wire and a neutral wire. The current through a mains appliance alternates. This is because the mains supply provides an alternating potential difference between the two wires.

The neutral wire is earthed at the local electricity substation. Potential difference between the live and the earth wire is usually called the voltage of the wire.

NEUTRAL WIRE = BLUE carries electricity away. normal flow is in via live and out via neutral pd = 0v

LIVE WIRE = BROWN Provides alternating potential difference from mains supply pd = 230 v

EARTH WIRE = GREEN & YELLOW - doesn't normally carry current only when there is a fault pd= 0V

Touching a live wire will give you an electric shock a large potential difference is produced across your body and current flows through you

The National Grid

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.

They are used to make the size of the alternating current much bigger, typically from 25,000V-132,000V

Step down transformers are used to supply electricity from the National Grid to consumers.

Homes and offices in the Uk are supplied with 230V in direct current (the mains electricity)

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Plugs, sockets and cables

Live wire- brown- The live wire provides the alternating potential difference (at about 230V) from the mains supply

Neutral wire- blue- Completes the circuit and carries away current- electricity normally flows in through the live wire and out through the neutral wire. It is around 0V

Earth wire- green and yellow- It is for protecting the wiring. It stops the appliance casing from becoming live. It doesn’t usually carry a current- only when there’s a fault. It is also at 0V.

The longest pin of a three pin plug is designed to make contact with the earth of wire of a wall socket circuit. This means it is earthed when it is plugged in.

The pins are made of brass because brass is a good conductor and doesn’t rust or oxidise. Cooper isn’t as hard as brass even though it conducts better.

The inside of the case is shaped so pins can’t touch each other when the plug is sealed.

The plug contains a fuse between the live pin and wire. If too much current pass through the wire in the fuse it melts and cuts the wire off.

The brown wire is connected to the live pin.

The blue wire is connected to the neutral pin

The green and yellow wire is connected to the earth pin. A two core cable doesn’t have an earth wire.

Cables used for mains appliances are made out of two or three insulated copper wires surrounded by an outer layer of rubber or flexible plastic material.

Copper is used for the wires because it is a good electrical conductor and it bends easily.

Plastic is a good electrical insulator, so if you touch the cable you don’t get an electric shock.

Two core cables are used for appliances that have plastic cases.

cables of different thicknesses are used for different purposes. Cables that join together the wall sockets in your house must be much thicker than the cables join together in the light fittings.

Because the more current passes along wall socket cables than along lighting circuits, so wires in them have to be much thicker so they have less resistance. This stops the heating effect of the current making the wires too hot.

Short circuits

If a live wire touches a neutral wire, a very large current passes between the two wires at the point of contact.

This is known as a short circuit. Provided the fuse blows and it cuts off the current.

Peoples bodies are at 0V if someone touches a live wire, a big potential difference will act across their body. Causing a current flow through them and this could be lethal.

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Electrical Power and potential difference

The power supplied to a device is the energy transferred to it each second.

Calculating energy transferred from a power source:

Power P (Watts) = energy transferred E (Joules) / Time (seconds)

or E = P t

Calculating Power:

Power supplied P (Watts) = Current I (Amps) x Potential Difference V (Volts)

or P =I V

The correct rating for a fuse is P / V (Power / potential difference)

Electrical currents and energy transfer

Charge flow Q (Coulombs) = Current I (amps) x Time t (seconds)

Q = I x T

When charge flows through a resistor the energy transferred makes it hot. the energy transfered to a component is:

Energy transferred = Potential difference x Charge flow

E = V x Q

Appliances and Efficiency

efficiency = output power / input power x 100%