Current
Alternating and direct current
An electric current flows either as a direct current or as an alternating current.
Direct current
On a voltage-time graph this would appear as a straight horizontal line at a constant voltage.
Car batteries, dry cells and solar cells all provide a direct current (dc) that only flows in one direction.
Alternating current
On a voltage-time graph, this would appear as a curve alternating between positive and negative voltages – the positive and negative values indicate the direction of current flow.
Power stations usually provide an alternating current (ac). In the UK, the mains electrical supply is generated at a frequency of 50 hertz (Hz) and is delivered to houses at about 230 volts (V).
Household electricity
In domestic devices, energy is transferred from the batteries or ac mains to the energy of the motors and heating devices.
Plugs
A plug connects a device to the mains electricity supply. The cable between the device and the three-pin plug contains three copper wires that are coated with plastic.
- copper wires are good conductors
- plastic is a good insulator
Each part of the plug has a function.
| Features of a plug | Function |
| Outer insulation | All three wires in the cable are bundled together and there is extra plastic insulation wrapped round them all for safety |
| Cable grip | This holds the cable tightly in place so that wires do not become loose |
| Live wire | Copper wire coated with brown plastic – this wire connects to the alternating potential difference pushing the current in the circuit |
| Fuse | A glass or ceramic canister containing a thin wire that melts if the current gets too high |
| Neutral wire | Copper wire coated with blue plastic – this wire is connected to a voltage close to zero, to ensure the live voltage always has a difference in potential to make the push for the current |
| Earth wire | Copper wire coated in striped plastic that provides a path for current to flow from the case of the device to the ground (also a zero voltage connection) if there is a fault |
Earthing
Without the earth wire, if a fault occurs and the live wire becomes loose, there is a danger that it will touch the case. The next person who uses the appliance could get electrocuted.
The earth wire is therefore connected to the case and is attached to a metal plate or water pipe underground. As the wire is made of copper, the earth wire provides a low resistance path to the ground. In the event of a fault, the live current passing through the case will follow this path to the ground instead of passing through a person. However, this would generate a very large current, leading to intense heating which could start a fire, so a fuse or a circuit breaker is also included in the circuit.
Fuses
A fuse provides a built-in fail-safe to the electrical circuit for a device. The fuse contains a thin wire that will melt if the current gets too high. If there is a fault that causes the casing of the device to become live, a large current will flow through the live wire and low-resistance earth wire. This high current will cause the fuse to melt.
Once the fuse has melted, the circuit is broken and no more current flows through the device. This means the case of the device is no longer live and there is no more risk of electrocution. A circuit breaker can serve the same function as a fuse but can be reset without the need for replacement if it trips.
The fuse or circuit breaker must be connected in the live wire side of a domestic circuit to ensure that it keeps high voltage from reaching the user, or surroundings, if a fault develops.
Electrical appliances
Appliances, power and energy
All electrical appliances transfer energy from one store to another, for example a chemical energy store in the fuel in power stations. This is transferred into a kinetic energy store of a fan or an internal energy store in a cooker.
The amount of energy transferred depends on the power (the energy transferred each second) and the amount of time the appliance is switched on for. The power of an appliance can be calculated using the equation:
P=E/t
This is when:
- energy (E) is measured in joules (J)
- power (P) is measured in watts (W)
- time (t) is measured in seconds (s)
One watt is the power when one joule of energy is transferred in one second.
Key fact
Time should be converted from minutes into seconds – this is done by multiplying the number of minutes by 60.
Power can also be calculated using the equation:
power = potential difference × current
P=VI
This is when:
- power (P) is measured in watts (W)
- potential difference (V) is measured in volts (V)
- current (I) is measured in amps (A)
This means that the energy transferred by an electrical appliance can also be calculated from a combination of the equations above:
E=VIt
This is when:
- potential difference (V) is measured in volts (V)
- current (I) is measured in amps (A)
- energy (E) is measured in joules (J)
- time (t) is measured in seconds (s)
Key fact
When working with mains electricity and appliances, the potential difference is 230 V.
