# What is Electricity?

In metals, many of the electrons are free to drift about from one atom to another, and in some ways they can behave a little like a gas, although they will only flow about within the metal and can't normally escape into the air. If you move a magnet past a wire, it will move some of the electrons in the wire about, and the result will be that some parts of the wire will have more electrons in them than normal, while other parts have fewer than normal. The areas with more electrons will have a higher pressure of electrons in them, while the areas with fewer electrons will have a lower pressure of electrons, and naturally enough, this will result in a flow of electrons from the high pressure areas to the low pressure areas: you could call this a wind, but it's normally called a current, or indeed an electric current. You now understand what that means: it's just a flow of electrons from a high pressure area (with more electrons in it than the average) to a low pressure area (with fewer electrons than the average). If this pressure difference is small, then you have a small voltage and the electrons will drift gently across from the high-pressure zone to the low-pressure zone, but if the difference is great, you have a high voltage and the electrons will rip across into the low-pressure zone at high speed. This is exactly like the wind: a high pressure difference between two areas will cause a gale to blow from the high-pressure area to the low-pressure one, but if the difference is very small, you will only get a gentle breeze. You now understand what volts are: they are a measure of how strong the electron wind will be if it is allowed to flow from an area of high pressure to an area of low pressure, though it is better to think of it as a measure of how big that pressure difference is.
A battery is rather like a vacuum cleaner. Vacuum cleaners suck air in at one end and pump it out at the other, and batteries do exactly the same thing with electrons. This means that a battery creates a higher-than-normal pressure of electrons at one end, and a lower-than-normal electron pressure at the other end. If you take a piece of wire and attach its ends to different ends of the battery, you will create a circuit for the electrons to flow round, but this is not a good idea because it is likely to melt the battery and cause it to leak nasty chemicals: this is a "short circuit" with very little resistance to the flow of electrons, and so it will work the battery too fast and make it so hot that it will be damaged. When you make a circuit, you should always put a component into it with a suitable resistance for the battery you are using so that it will work the battery at a safe rate: a small light bulb would do, but you should first check that it can handle the voltage of the battery, because it it can't, its element will melt and break. Any normal torch bulb will be able to handle the voltage of a single 1.5 volt battery, so you can start with those (and any old piece of wire) and try making a circuit with them: hold the metal contact at the bottom of the bulb against one end of the battery, trap one end of the wire under the other end of the battery so that it is held in contact with it, and then touch the other end of the wire against the side casing of the bulb. There will now be a high pressure of electrons going into the bulb through one contact, and a low pressure of electrons at the other contact, and so the pressure difference will drive the electrons through the highly-resistant element of the bulb and make it glow.
If you have more than one bulb, you can try some interesting experiments, but to make things easier to explain, it would be worth imagining that the bulbs have been fitted into holders of some kind so that we can connect them into a circuit just by attaching wires to opposite ends of the holders, though I won't mention the holders again: I will just talk about connecting wires to opposite ends of the bulb. Look at the first circuit in the diagram below. There is a battery with wires attached to its ends. The other ends of the wires are connected to opposite ends of a bulb. The bulb is shown to be brightly lit, though in real life you'd probably need to use two batteries to light it to full brightness. Now Look at the circuit shown underneath with two bulbs instead of one. The two bulbs have been connected up in series (meaning that the electrons flow first through one of them and then on through the other), and they will both light up much more dimly than a single bulb would. These bulbs are dim because there is a smaller difference in electron pressure from one end of each bulb to the other. The difference in electron pressure across the two bulbs together is the same as it was across the single bulb in the first circuit, but the electron pressure between the bulbs is obviously going to be half way in between the pressures at the outside ends, so you only have half the difference in electron pressure across each bulb.

There is a second way of putting two bulbs into a circuit which will allow them both to be as bright as the bulb in a circuit on its own: the bulbs have to go in parallel, as shown in the circuit on the right. (I will just take a moment to describe this for people who can't see the diagram. Imagine the original circuit with the battery at the top, one bulb at the bottom, and wires at either side connecting the ends of the battery to the ends of the bulb. Now attach two more wires to the ends of the battery and connect the other ends of these new wires to opposite ends of a second bulb. This is not quite the way I have drawn it in the diagram, because I have actually shown the new wires attached to the original wires close to the first bulb and a long way out from the battery, but this makes no practical difference to the circuit.) In this circuit, both the bulbs are bright, and the reason for this is easy enough to see: the difference in electron pressure between the wires at opposite sides of the circuit is the full pressure difference generated by the battery, and so both bulbs have that same high difference in pressure across them. You could actually put several more bulbs into the circuit in parallel in the same way and they would all be brightly lit, whereas if you put them all in series they would get dimmer and dimmer with each bulb added. When I was taught about this at school, it wasn't explained in terms of electron pressure, so the whole thing was a complete mystery as to how it actually worked: we just had to learn that bulbs in series were dim while bulbs in parallel were bright. I actually had to work out for myself what is going on in the wires so that I could explain it to you properly. The extra brightness you get by having two bulbs in parallel doesn't come without a cost: it will drain the battery twice as quickly as a single bulb, and three bulbs in parallel will drain it three times as fast. WARNING: it is important that you don't put too many bulbs into a circuit in parallel because they can run the battery down so fast that it behaves like a short circuit, melting the battery.
Difference in electron pressure is measured in volts, while current is measured in amps. Current is simply a measure of the amount of electrons moving through a point in a circuit in a given length of time. When only one bulb is in a circuit, the current will be the same at all points in the circuit, but when there are two bulbs in the circuit and they are wired up in parallel, the current will be twice as high in the parts of the wires connected to the battery and it will remain high all the way to the point where the wires branch to go to the two bulbs. Beyond the branching point, the current will go down to the same level as in the circuit with a single bulb and it will remain at that level on the journey through each bulb. If you have a circuit with two bulbs in series, the current will be the same at all points in the circuit, but it will also be lower than in the circuit with just a single bulb: by having two bulbs in series there will be more resistance to the flow of electircity than in the circuit with a single bulb, so the two bulbs are not only dimmed because of the reduced difference in electron pressure across them, but this is made still worse by the higher resistance.
It isn't always necessary to have a circuit for electricity to flow: all you need is a difference in electron pressure between two areas and a connection between them through which electrons can flow. A flashgun on a camera works by storing up electrons at high pressure in a capacitor and then allowing them all to flow through the flash in an instant, creating a very powerful flash of light. Lightning is similar to this: electrons are stored up in a cloud until the pressure difference between the cloud and the ground is so high that the electrons are able to jump across the gap between them. The electrons actually make the journey by turning atoms in the air into ions: an ion is an atom in which the number of protons and electrons do not match, so the atom either has a positive or negative charge. Electricity can flow through ions by moving the ions around and swapping electrons between them to try to give them all a neutral charge (by getting the number of protons and electrons to match). Electricity can also flow through water if the water contains ions, and a simple way to put ions in water so that it can conduct electricity easily is to disolve salt into it. If you put a battery into fresh water, nothing much will happen, but if you put it into salt water (like sea water), it will short circuit the battery, behaving just like a wire and connecting the two ends of the battery together: electronics and sea water don't mix well for this reason. Salt is a mixture of two kinds of atom: sodium and chlorine, although they are actually sodium and chlorine ions, some being positively charged and the others being negatively charged. When salt is added to water it breaks up into individual ions which find gaps to sit in between molecules of water, and because water molecules don't pack together as perfectly as they might, there are plenty of gaps between them just waiting to be filled, and so you can add quite a lot of salt to water without the volume of water increasing at all. The result of this is that salt water is more dense than fresh water (it's heavier because it contains more material in the same amount of space) and so fresh water tends to float on top.
Batteries contain ions too, ions which are desperate to stop being ions by either getting rid of an electron or by gaining one, but they are cleverly packed into the battery in such a way that the only way an electron can be transferred from a negative ion to a positive one is by taking the long way round from one end of the battery to the other by an external route. This desire of negative ions to get rid of an electron and of positive ions to gain one is what drives the current round the circuit. Another way to power a circuit is to use a dynamo: magnets are moved past a wire in such a way that they drive electrons along the wire in one direction only, so a higher pressure of electrons is built up at one end of the dynamo and a lower pressure is generated at the other end: this pressure difference can then be used to drive electrons round a circuit.
Electricity is sent from power stations to your house without using a complete circuit: the dynamos in the power station push electrons one way for a fraction of a second, and then they push them back the other way for the next fraction of a second, and they keep changing the direction of the flow (this is known as alternating current). Only one wire is needed between the power station and a substation near your house. Waves of high and low electron pressure are sent along the single wire, and when they reach the end of the wire they are simply sent into the ground, though not before they've gone through your house. If the electric appliances in your house have two-pin plugs, one of them connects to the live wire that leads first to the substation and then on to the power station, while the other pin connects to the neutral wire that leads back to the substation where it is then fed into the ground. At the point where the wire goes down into the ground, it pumps electrons into the ground and then sucks them back in again, and it does this fifty or sixty times a second. If you have three-pin plugs, the third pin connects to the earth wire, and that leads straight down into the ground at your own house: this is a safety feature designed to redirect current away from you if the outer casing of an appliance goes live and risks electrocuting you.

Try to remember the following points:-

• Voltage is the difference in electron pressure between two points in a circuit.
• Amps is a measure of the amount of electrons passing a single point in a circuit in a set time.
• Electricity flows best through metals, plasmas (flames) and ion-rich liquids such as salt water.
• Direct current sends electrons one way round a circuit.
• Alternating current changes the direction of electron flow many times a second.
• Moving a magnet past a wire will make electrons move in the wire, thus generating a current.

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