In our daily life, electrical energy is needed in order to do various jobs. Electric lamps are needed in order to run the electrical appliances used in the home. Electromagnets are also made by using electricity. An object that has the property of attracting iron dust or pins towards itself is called a magnet. Electromagnets are also used in a power house in order to generate electrical energy.
Electrical appliances such as a bulb, a computer, a projector, a television, a fan, a heater and a fridge need electricity in order to be used. The electrical energy needed to run those things is obtained from a source of electricity. Through conducting wires it is carried as far as houses and industries. The electric current that can be transmitted in this way is current electricity.
You must know that lightning flashes and thunder falls in the sky. In the same way, you must have seen tiny sparks when taking off woollen or nylon clothes in the dark. This too is one form of electricity. The electricity produced in this way is static electricity.
What is inside every substance
All substances are made of molecules, and atoms join together to make molecules. In an atom there are two kinds of charge: the central part, which is called the nucleus and has a positive charge, and the electron, which has a negative charge and goes round the outside.
Read that paragraph once more and notice what it is quietly telling you. Every object in the room, including you, is packed with positive charge and negative charge already. A wooden desk is not waiting to be given electricity; it is stuffed with the stuff. So why does the desk not shock you? Because in an ordinary object the two kinds of charge are present in equal amounts, and equal amounts of opposite charge cancel out, so from the outside the object behaves as though it has no charge at all. We call that neutral. This one fact rearranges the whole topic. The question is never how to make electricity out of nothing. The question is only how to upset the balance.
How rubbing produces a charge
When two substances rub against each other, electrons move across from one to the other. In the substance the electrons move into, the number of electrons becomes greater and it becomes negatively charged. In the substance from which the electrons move away, there is a shortage of electrons and it becomes positively charged.
| The object that GAINS electrons | The object that LOSES electrons |
|---|---|
| Now has more electrons than protons | Now has fewer electrons than protons |
| Becomes NEGATIVELY charged | Becomes POSITIVELY charged |
| Example: the comb, after rubbing in hair | Example: the hair the comb was rubbed in |
Look carefully at that table, because the two columns are not two separate events. They are one event described from both ends. Rubbing does not manufacture any electricity. It only picks up electrons from one object and puts them down on the other, and every electron that arrives on the comb is an electron that has left the hair. So the negative charge on the comb is exactly as big as the positive charge on the hair, no more and no less, and if you added the two together you would get zero again. Nothing was created. Nothing was destroyed. Something was moved. This is why the two objects always end up with opposite charges rather than both the same, a fact that would be quite mysterious if you thought rubbing was making electricity out of nowhere.
That is why charge is produced when friction happens in various non-conducting substances such as plastic, wool, nylon, polyester and acrylic. When such clothes are worn, friction takes place between the clothes and the body and charge is produced, and when the clothes are taken off the charges move and a crackling sound is heard. In the dark, something like a spark of fire is also seen. In this process there is an increase or decrease in the amount of charge, and that is what produces the electricity.
The electricity produced in a non-conducting substance by an increase or decrease in the amount of opposite charge, because of friction, is called static electricity.
Two words in that definition are doing real work. The first is non-conducting. Rubbing moves electrons on a metal spoon just as well as it does on a plastic comb, so why does the book only list plastic, wool, nylon, polyester and acrylic? Because on a metal the extra electrons do not stay put. Metal lets charge travel freely through it, so the moment you produce a surplus it spreads out, runs down your arm and away into the ground, and there is nothing left to notice. On an insulator, charge cannot travel, so every electron stays exactly where it landed. The second word is static, which simply means not moving. That is the whole difference between this topic and the next one. In static electricity the charge is stuck in one place until something makes it jump. In current electricity the charge is flowing steadily along a wire.
Why the comb picks up paper that has no charge
When a comb is rubbed in the hair, the electrons that were in the hair move across into the comb. At the start the pieces of paper have no charge on them. When the comb, which now has a negative charge, is brought near the pieces of paper, the effect of the negative charge on the comb produces a positive charge on the part of the paper that is nearest to it, and attraction takes place. Because attraction happens between opposite charges, the pieces of paper stick to the comb.
The force produced in this way, when a comb and hair are rubbed together, is the electrostatic force.
That explanation is correct but it stops one step early, and the missing step is the interesting one. Think about what happens inside a piece of paper when the negative comb comes close. The paper is neutral, so it contains just as much positive charge as negative. The comb pushes the paper's own electrons away, so the far side of the paper becomes slightly negative and the near side is left slightly positive. Fine. But now the comb is attracting the near side and pushing away the far side at the same time, so why does it not simply cancel out? Because of distance. The positive side it is pulling is closer to the comb than the negative side it is pushing, and electrical forces get weaker as things get further apart. The pull therefore beats the push, and the paper moves. This is worth understanding once, because it explains a whole family of everyday effects: a charged balloon sticking to a wall that has no charge, dust flying onto a television screen, and a thin stream of water bending towards a rubbed pen.
The two rules of charge
| Situation | What happens | Everyday example |
|---|---|---|
| Opposite charges, one positive and one negative | They attract each other | Paper jumping up to a rubbed comb |
| Like charges, both positive or both negative | They push each other away | Hair standing apart after being combed on a dry day |
These two rules explain almost everything in this topic. Notice the second one at work in something you have seen many times. On a dry day, combing your hair moves electrons out of every strand at once, so each strand is left with the same kind of charge as all the others. Like charges push each other away, so the strands try to get as far from one another as they can, which is why freshly combed hair stands out and refuses to lie flat.
One more everyday detail is worth knowing. All of these effects work far better on a dry day than on a wet one. Damp air carries a thin film of water onto every surface, and water conducts, so the charge you build up leaks quietly away before you can use it. That is why the comb and paper trick often fails during the monsoon and works beautifully in the dry cold of winter.
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