There are two different ways that electricity can be supplied; AC and DC.
DC stands for direct current. The current in DC electricity always flows in the same direction around the circuit. Cells and batteries produce a direct current, which is caused by a direct potential difference. The voltage-time graph for a DC supply is shown below.
DC stands for direct current. The current in DC electricity always flows in the same direction around the circuit. Cells and batteries produce a direct current, which is caused by a direct potential difference. The voltage-time graph for a DC supply is shown below.
The line is a horizontal line because the direction and voltage of a DC supply will always be the same.
AC stands for alternating current, which is where the direction of the current is constantly changing. The alternating current is produced by a potential difference that is constantly alternating (changing direction). Mains electricity in UK homes is an alternating current with a potential difference of 230 V and a frequency of 50 Hz (the direction of the current alternates/ changes 50 times a second). The voltage-time graph for an AC supply is shown below.
AC stands for alternating current, which is where the direction of the current is constantly changing. The alternating current is produced by a potential difference that is constantly alternating (changing direction). Mains electricity in UK homes is an alternating current with a potential difference of 230 V and a frequency of 50 Hz (the direction of the current alternates/ changes 50 times a second). The voltage-time graph for an AC supply is shown below.
The voltage (and current) for an AC supply is constantly alternating between positive and negative. This is why the curve is constantly switching between a positive and negative value for voltage.
Plugs
In the UK, most electrical appliances are connected to the mains by a three-pin plug. The cable that runs into the plug contains three wires that have a copper core coated in flexible coloured plastic. The wires have a copper core because copper is a good conductor of electricity. The flexible coloured plastic around the copper core is there because plastic is a good electrical insulator. The colour of the three wires indicates the role of each of the wires. The colours of the wires are the same for every electrical appliance so that it is easy to tell what each of the wires does. The three wires are shown in a plug below.
In the UK, most electrical appliances are connected to the mains by a three-pin plug. The cable that runs into the plug contains three wires that have a copper core coated in flexible coloured plastic. The wires have a copper core because copper is a good conductor of electricity. The flexible coloured plastic around the copper core is there because plastic is a good electrical insulator. The colour of the three wires indicates the role of each of the wires. The colours of the wires are the same for every electrical appliance so that it is easy to tell what each of the wires does. The three wires are shown in a plug below.
The roles of each of the three wires are:
We are now going to have a look at the structure of a plug in a bit more detail. Here is the diagram again with a few more labels.
- Live wire (brown). The live wire alternates between a high positive and negative potential difference. The live wire has an alternating potential difference of around 230 V. The live wire is how the current enters a device.
- Neutral wire (blue). This wire completes the circuit. A current leaves an appliance through the neutral wire. The neutral wire has a potential difference of 0 V.
- Earth wire (green and yellow). The earth wire is there for safety and it provides a pathway for the current to flow away from a device to the ground if the device has a fault. This stops the appliance (usually the case) becoming live. Normally there is no current flowing through the earth wire; there will only be a current flowing through the earth wire when the device has a fault. The earth wire has a potential difference of 0 V.
We are now going to have a look at the structure of a plug in a bit more detail. Here is the diagram again with a few more labels.
Here are the key details about a three-pin plug:
- The cable (which has three wires in it) comes into the bottom of the plug and is held in place by a cable grip.
- The neutral (blue) wire goes to the left pin.
The earth (green and yellow) wire goes to the middle pin.
The live (brown) wire goes to the right pin.
An easy way to remember which wire goes where is to look at the second letter of the colour of the wire.
- Blue goes left – neutral wire goes left
- Brown goes right – live wire goes right
- Striped goes top – earth wire goes top
- The live wire is connected to the live terminal. There is a fuse that connects the live terminal to the live pin. Fuses are there to protect the wiring of a device from a current that is too high (a current that is too high could break the wiring of a device).
- The case of a plug is made from plastic or rubber because these materials are good electrical insulators.
- The three pins are made from copper or brass because these materials are good conductors of electricity.
- The plug has a metal casing that the earth wire is attached to. Plugs have a metal casing to take the electricity away from the device to the earth if something goes wrong with the appliance.
Live Wire Electric Shocks
Our body has a potential difference of 0 V and the live wire has a potential difference of 230 V. This means that when you touch a live wire, a large potential difference is produced between the live wire and our body, which results in a current flowing from the live wire through us to the earth. This will produce a large electric shock that can injure or kill you.
An electric shock can be produced even if the live wire/ plug is switched off. This is because the live wire will still have a potential difference even though there is no current flowing through the live wire as the device is switched off. So, when you touch the live wire in a plug that is switched off, a large potential difference is still produced between the live wire and our body, which causes a current to flow from the wire through us to the earth.
Any connection between the live wire and the earth can be dangerous especially if the pathway has a low resistance. This is because a low resistance will mean that an extremely large current will flow from the live wire to earth. The flow of a large current from a device to the earth may result in a fire.
Our body has a potential difference of 0 V and the live wire has a potential difference of 230 V. This means that when you touch a live wire, a large potential difference is produced between the live wire and our body, which results in a current flowing from the live wire through us to the earth. This will produce a large electric shock that can injure or kill you.
An electric shock can be produced even if the live wire/ plug is switched off. This is because the live wire will still have a potential difference even though there is no current flowing through the live wire as the device is switched off. So, when you touch the live wire in a plug that is switched off, a large potential difference is still produced between the live wire and our body, which causes a current to flow from the wire through us to the earth.
Any connection between the live wire and the earth can be dangerous especially if the pathway has a low resistance. This is because a low resistance will mean that an extremely large current will flow from the live wire to earth. The flow of a large current from a device to the earth may result in a fire.