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Mains Electricity

Electricity supplied to the home is an alternating current (AC) . The current changes direction regularly.

In the UK the frequency of the mains supply is 50 Hertz.

Electricity supplied by a battery is a direct current (DC). The current only flows in one direction.

Question: Mains electricity is an ac supply.
Explain the difference between direct and alternating potential difference. (2 marks)

Most electrical appliances are connected to the mains using three-core cable.

The insulation covering each wire is colour coded for easy identification:

live wire – brown

neutral wire – blue

earth wire – green and yellow stripes.

The live wire carries the alternating potential difference from the supply.

The neutral wire completes the circuit. The earth wire is a safety wire to stop the appliance becoming live.

The potential difference between the live wire and earth (0 V) is about 230 V. The neutral wire is at, or close to, earth potential (0 V).

The earth wire is at 0 V, it only carries a current if there is a fault.

Wiring inside a UK plug

The fuse is always connected in series with the live wire; the fuse is designed to melt if too much current flows through it.

A live wire may be dangerous even when a switch in the mains circuit is open, because it could cause electric shock

 A short circuit is any connection between the live wire and earth. A short circuit could cause electric shock to users or it could cause the appliance to overheat (because too much current is flowing through it) and possibly catch fire.

Question: A toaster is plugged in to the mains electricity supply.
What is the potential difference between the live and neutral wires?

Power and Work Done

Power is the rate of energy transfer. It is measured in Watts

learn this definition

Power transfer in any circuit device is related to the potential difference across it and the current through it, and to the energy changes over time:

power = potential difference × current

power = current²  × resistance

Question: The electric motor in a car is powered by a battery.
To charge the battery, the car is plugged into the mains supply at 230 V
The power used to charge the battery is 6.9 kW
Calculate the current used to charge the battery.

Question: What is the power loss due to heating in a 50 Ohm resistor when the current is 0.5A?

Everyday electrical appliances are designed to bring about energy transfers. For example:

  • In a battery powered torch: the chemical energy of the battery is transferred electrically to the thermal energy store inside the bulb
  • In a mains powered washing machine: the energy is transferred electrically to the kinetic energy store of the motor

The amount of energy an appliance transfers depends on how long the appliance is switched on for and the power of the appliance.

Work is done when charge flows in a circuit. Work done equivalent to energy transferred

The amount of energy transferred (in Joules) by electrical work can be calculated using the equations:

energy transferred = power × time

energy transferred = charge flow × potential difference

Question: A heater has a power rating of 2500 W.
The heater is turned on for 180 seconds.
Calculate the energy transferred by the heater.

Question: The mains electricity supply is at 230 V.
A different heater transfers 4200 J of energy.
Calculate the charge flow through the heater.

Domestic appliances all have a power rating. The power rating tells the user how much energy is transferred by the device each second. For example:

  • A 3000W kettle transfers 3000J each second electrically into the thermal energy store of the heating element
  • A 4.5kW oven transfers 4500J each second into the thermal energy store of the heating element
  • A 2.5kW hairdryer transfers 2500 J each second electrically into the kinetic energy store of the motor

Question: Four of the appliances in Figure 1, including the fan heater, are designed to transform electrical energy into heat.
Name the other three appliances designed to transform electrical energy into heat.

Figure 1

The National Grid

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

Electrical power is transferred from power stations to consumers using the National Grid.

Step-up transformers are used to increase the potential difference from the power station to the transmission cables then step-down transformers are used to decrease, to a much lower value, the potential difference for domestic use.

The National Grid system is an efficient way to transfer energy. The low current in the cables reduces energy losses into the surroundings by heating.

Question:Explain why step-up transformers are used in the National Grid.