Electricity at Home
We use two main sorts of electrical supplies, DC and AC.
DC - This is Direct Current. The current flows in one direction only (conventional current flow) and has a consistent value. Provided by batteries or DC adaptors/transformers that plug into the mains supply.
AC - This is Alternating Current. The current flows first one way then the other (imagine the charge being first pushed then pulled around the circuit). In normal mains circuits this happens 50 times a second, so we say it has a frequency of 50Hz (see "Waves" section). AC is what comes out of the mains sockets, usually at around 240V.
Why have DC and AC? Many things won't work with an AC supply, but, it is very easy to change the voltage of AC using a transformer (these don't work with DC).
You can compare the two types using an oscilloscope:
The Ring Main
This is the name given to the circuit in your home. You don't need to know too much about it except that it is a kind of parallel circuit (so the toaster doesn't switch the kettle off!) and that the lighting circuit is separate from the circuit for sockets.
This is the circuit that carries electricity all around the country, from the power stations to homes and businesses.
Producing the Power
See the energy section for the different types of power source. Most power stations are similar, once they have generated steam. The steam turns a turbine which is connected to a generator. The generator converts energy into electricity and the transformer then changes the current and voltage of the electricity.
Why The High Voltage?
This is to stop energy being wasted. You probably know that if you put too much current through a wire, it gets hot. Heat means wasted energy. If you think about the formula, Power = Voltage x Current, if you raise the voltage, you can transfer the same power with a lower current - less waste! This is why mains is AC, so that you can use transformers to change the voltage up and down. The 'Step up' and 'Step down' are the two types of transformer used. Step up transformers are located at power stations. Step down transformers are the ones you will see locally around your town or city.
Note: it is unlikely that your course requires you to know the voltages of the grid, but if so, check the values from your syllabus, in case they use different ones.
Normally you wouldn't sit down and leaf through your electricity bill, but for GCSE, you need to know a bit about how this is done. The main thing to get right is the unit used.
This is a unit of energy not power or time. It is the amount of energy if a 1kW appliance was left on for 1 hour. This means that:
I know what I said about always using base units, but 1 joule is hardly anything, particularly when you have lots of appliances going.
So, 1 unit of electricity is 1kWh of electrical energy. This costs around 6p, though it may change depending on your supplier or if you have a deal for cheap electricity at night. So a bill might break down like this:
Current Meter Reading: 25361.2
Previous Meter Reading: 24321.5
Units used: 1039.7 [25361.2-24321.5]
Cost per unit: 6p
Cost of electricity used: £62.38 [1039.7 x 0.06]
You may also have standing charges and VAT as well, but these just add onto the final total.
Because we use electricity everyday, we often forget how dangerous it can be. Mains supply can kill, but a bit of common sense can go a long way.
A common question is to give you a picture of domestic bliss and get you to identify the hazards, such as the person sticking their fingers in the toaster.
Things to look for are:
- bad wiring,
- water near appliances,
- too many double plugs/adaptors,
- frayed wires.
Just use your common sense and you should get some easy marks!
Wiring a Plug
One big problem used to be wiring plugs. By law now, all new appliances are fitted with one already, which helps, but you do need to know what's going on inside there:
The important things to do are to get the wires correct and the right length. Don't show too much bare wire and use the cable grip as shown.
Fuses help protect the circuit against faults. The key thing is to get the wire just thick enough to carry the current you want, but thin enough to melt if there is a current surge.
In the diagram above, the electric fire has a metal case. If there was a fault and the live wire touched the case, there would be no visible sign of a problem, except that it wouldn't work. Anyone touching it would complete a circuit with earth causing a potentially fatal current to flow. If it is earthed, the earth wire allows this current to flow easily. A larger current than normal flows, causing the fuse to blow - disaster is avoided.
Common sizes are 3, 5 and 13Amp fuses, but there are many others. Always choose one slightly higher than the current rating of the appliance, so that it doesn't blow under normal conditions.
For Example: A mains (240V) kettle has a power rating of 3kW. What fuse should be used?
Therefore, use a 13A fuse.
Fuses stop things getting too hot and catching fire but they don't always protect you! There might be a fault where there is not enough current to blow the fuse but more than enough to kill you.
The answer is a circuit breaker. Also called power breakers, MCB's and RCD's. They do exactly what the name suggests, they break the circuit if there is a problem. They automatically compare the current entering and leaving the circuit and even if there is the tiniest difference they 'trip' off.
What's this earth business? Well, although you would probably class a lump of soil as an insulator, the Earth (yes, I do mean our planet) is very good at soaking up loose charge. The earth in your house could be connected to the plumbing (if your water comes in metal pipes)or to a large metal spike in the ground somewhere. Yes, it really does work!
If something is completely cased in an insulator, like plastic, it is said to be double insulated, and does not need earthing. You can't get a shock from the case!