P2bL8  Domestic Electricity

Key Words

Alternating current - current changing direction all the time.

Cable  - wire taking electricity from the plug to the appliance.

Cable grip - small clamp that holds the cable securely to the plug.

Cathode Ray Oscilloscope (CRO) - instrument that displays electrical waveforms.

Case - the body of the plug.

Direct current - current that flows one way only. 

Double insulation - live wires in an appliance have at least two layers of insulation separating them from the user.

Electricity - power transferred using electrical charge.

Earth - wire connected to the ground.

Fuse - weak link that melts if too big a current flows.

Live - the wire that carries the high mains voltage from the substation.

Neutral - the wire that is connected back to the substation.  It is also connected to Earth at the substation, so that the voltage at the neutral is zero.

Three-pin plug - plug with three brass conductors, live, neutral, earth.  Most plugs or of this pattern.

Twin and Earth - cable with three wires, live, neutral, and earth.

Twin core cable - wire with just the live and the neutral.

Test Yourself

Homework
Physics GCSE
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Grade E

Mains electricity is generated in massive power stations and distributed about the country using the national grid.  It arrives at our homes at a voltage of 230 V AC (alternating  current).  This means that the current changes direction all the time.  In Europe it changes direction at 50 times a second (50 Hz).  In the USA, the domestic voltage is 110 V at  60 Hz.  DC (direct current) is not used for the mains.  The diagram below shows the difference between AC and DC.

 

 

You can see that the DC has a constant level, while the AC is changing all the time

 

Both AC and DC can be used to heat things, light rooms, and run motors.  However transformers do not work with DC.  Electronic circuits need DC, but this comes from an AC power supply that has been rectified.

 

Electrical appliances are connected to the mains using a cable wired to a plug which is put into a socket.  The plug is wired like this:

 

              

 

The wire is three-core, having a live (brown), a neutral (blue),  and an earth (yellow and green) wire.  It is sometimes called a twin and earth (although I have never heard that expression, although I have bought electrical stuff on many occasions).

 

It is important to wire a plug properly.  To fail to do so can be extremely dangerous.

 

 

If in doubt, don't try it yourself; get a competent person to do it.

 

Grade C

You need to note the following about cables wired to plugs:

  • Each conductor is covered in a plastic insulating coating.  It is important that there are no nicks in this.  A short circuit could occur.

  • The three wires a covered in an outside insulating sheath.

  • The case of the plug is plastic for insulation.  The plug cover must be screwed on tight, otherwise it can come away in your hand, exposing live parts.

  • Make sure that the cable grip is tight onto the sheath, not the wires.

  • A fuse prevents too much current from damaging the appliance

Compare the two plugs in the box above.

 

 

A moulded plug prevents accidents arising from incompetent wiring; most appliances are provided with these.

 

There are different patterns of plugs in use throughout the world.  The different patters have evolved according to local custom.  The picture below shows moulded plugs.

 

 

The plug on the left is used extensively in Europe.  The plug on the right is the normal British pattern used in the UK and Ireland.

  • The European plug does not have a fuse in it, but it can be gripped more easily (important for elderly and frail people).

  • The British plug has a 5-amp fuse in a removable carrier.

  • The earth is wired to a socket on the European plug; the pin itself is in the socket into which the plug is inserted.  It is possible to plug this into an un-earthed socket (and the wrong way round) which could be hazardous.

  • With the British plug this is not possible.  The earth in opens shutters on the socket, preventing probing little fingers from touching live parts.

  • Close to the plug, the pins are insulated to prevent the risk of shock if the plug isn't put in properly.

The British plug is recognised internationally as being a particularly safe design, and many countries are adopting the pattern.

 

 

We can demonstrate AC waveforms using the cathode ray oscilloscope (CRO):

 

 

The read out is like this:

 

Note the following:

  • The wave has a shape called a sine wave.

  • The y axis (vertical) gives us the voltage.

  • The x-axis (horizontal) gives the time.

  • The centre red line is the zero volts level.

  • The straight green line is the DC equivalent of the alternating voltage.

  • There is a peak voltage (V0) both positive and negative.

The CRO takes a bit of interpreting.  However we can measure AC using a multimeter set to an AC range (much easier).

 

 

Do NOT , under any circumstances, try this at home

 

Note that this shows the AC voltage at 244 V.  Its range varies from 220 to 250 volts.

 

Grade A

Some appliances have cases made out of plastic.  This gives an extra layer of insulation so an earth  wire is not needed.  They are called double insulated, and the double insulation symbol shown below is on the case.

 

Can you see the symbol on the picture below?

 

Such appliances only need twin-core cable.

 

We can use the CRO trace to work out the frequency of the wave:

 

 

If we set the time base to 5 milliseconds per centimetre, we find that the period (time for one complete wave) is 15 ms = 15 ÷ 1000 = 0.015 s

 

The frequency of this wave is given by:

frequency = 1/period

 

frequency = 1 ÷ 0.015 = 67 Hz

 

For the waveform on this screen to be 50 Hz, the period would need to be 0.020 s, shown 4 cm on the screen.

 

  The horizontal axis shows the period, not the wavelength.