## Physics-SchoolUK.com

### The UK site for KS3 and KS4 Physics

In the UK, as in all countries, mains electricity is Alternating p.d, whereas portable devices such as phones use Direct p.d.

These are what you must learn about in this section.

#### Direct and alternating p.d

What's the difference?

An Alternating p.d is, as the name suggests, a p.d that alters or varies continuously, but regularly, with time, like this:

A Direct p.d is one that doesn't alter with time but instead remains constant, like this:

Why do we have both types?

It is because the Alternating type is much easier to generate inside power stations and to distribute across countries.
So ALL mains electrical appliances like washing machines, kettles, TVs will use Alternating p.d.

But the Direct p.d is produced whenever we use batteries.
So ALL portable electrical appliances like phones, ipods, laptops etc, will use Direct p.d

The rest of this section is about mains electricity, so it is about p.d (alternating or direct p.d?)

Describing an Alternating P.d

Looking at the top graph above, there are just two labels to describe the alternating waveform.

The mains voltage varies, doesn't it. But it is found to have the same effect as a Direct voltage of about 230V, in the UK (it can be different in other countries).

So, in the UK, the mains p.d or voltage is quoted as 230V. You must remember this value.

The other value that you need to remember is related to the other label on the graph, time; it is called the frequency of the alternating waveform.
In the UK the mains frequency is 50Hz; this means that it alternates from one peak to the other peak 50 times every second!

So, in short, the mains p.d is very high (230V) and has a high frequency (50Hz); remember these two values.

#### Mains Electricity - Practical Details

The 3 Core Mains Cable

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 insulation covering each of the 3 wires and covering the bundle of 3 wires is, of course, plastic.
The material within each of the 3 wires is made from a conductor, copper.

So, one end of a 3 core cable is connected inside an appliance, eg a TV.
The other end is connected to a "3-pin plug" which is then plugged into a 3 pin mains socket, found mostly on the lower part of walls in most houses.

But it is critically important to know where to connect each of the 3 wires; this depends on knowing the roles of the "Live", "Neutral" and "Earth" wires.

The roles of the Live, Neutral and Earth wires

Think back to a basic electrical circuit consisting of a power source (eg a battery), a few wires and a lamp.
To get the lamp to light you would connect up the circuit as follows:

OK, if we replace our low voltage lamp and battery power source for a mains lamp and a mains power source, our circuit could be drawn like this:

So, you can see that the Live wire is "like" the wire from the positive side of a battery power source; it carries the electrical current from the alternating mains supply.
The Neutral wire is "like" the wire that leads back to the negative side of a battery power source; it completes the circuit, started by the Live wire, allowing the current to flow back to the alternating mains supply.
If either wire is missing then the circuit will not be complete and will not work, so both are equally important.

NB. Strictly speaking, it is incorrect to draw the above circuit with the current arrows because in a mains circuit the current is changing dirction 50 times each second. However, they help to illustrate the point, to get you to understand the role of the Live and Neutral wires.

Are both wires equally dangerous?

It is wise to treat them both as equally "very" dangerous.
However, if you have understood the roles of the two wires you will realise that the Live wire is really the most dangerous since it is the one that brings the alternating mains current from the socket, through the 3 core cable and to the appliance. In fact, the Live wire will be dangerous to touch even if the switch in the appliance is set to its "off" position; what happens when you touch such a wire is that YOU complete the circuit, becoming the return for the current; and since the current flows through you, you are electrocuted and risk death!

What about the 3rd wire, the Earth?

It seems from the above that the Earth wire is not needed !

This is such a bold statement, but its actually correct.
The Earth wire is not essential; in fact many appliances only use the Live and Neutral wires.
Examples of such appliance include all phone and laptop chargers, hand-held hair dryers, straighteners and many others. If you examine the 3 pin plugs attached to these appliances you will often find that one of the pins is made of plastic! That is the "false" Earth pin; there is nothing connected to it; its a dud.

So, two questions occur to us:
What is the role of the Earth wire, when it is used? and
Why do some appliances use an Earth wire but others don't?

What is the role of the Earth wire?

The Earth wire is a safety wire to stop the appliance becoming Live.

Sounds like a useful wire, so
why do some appliances use an Earth wire but others don't?

The safety Earth wire is only needed if the appliance has an external metal casing.
Only if the appliance has such a conductive casing can it ever become accidentally "Live" and pose a danger to users.
If it has a plastic, insulating, external casing, then it can't ever become "Live", so the Earth wire is not needed; it wouldn't be of use.
So, appliances such as phone chargers have completely plastic outer casings making an Earth wire unnecessary.

How does the Earth Wire do its very useful job?

A short answer: if the casing is made of metal and if it accidentally becomes Live (eg by a fault within the appliance), then the Earth wire carries a large current, very briefly, to ground, causing a fuse within the 3 pin plug to "blow" (to snap), cutting off the Live supply and making the appliance safe to touch, although it no longer works until it is repaired.

Notice:
1.the word ground in the above passage. The Earth wire is a wire that connects the case of a metal appliance, quite literally to the ground or to the Earth, often via the metal water pipes within a house that go under the ground. This ground or Earth is a point where the p.d is a very definite ZERO volts and a current will easily flow to it through an Earth wire.

2. The p.d between the Live wire and the Earth wire is approximately 230V, which we mentioned earlier. The p.d between the Neutral wire and the Earth wire is about zero volts, so similar to the Earth potential itself.

3. Although the p.d of the Neutral and Earth wires are about the same, we must never use the Earth wire in place of the Neutral (as the return wire for the current from the Live wire) because if we do then we lose the benefit of a fast acting safety wire.