Electricity and Magnetism

We use electricity all day: to light bulbs, run fans and refrigerators, charge phones, and work computers, projectors and televisions. In Nepal most of it is made in hydropower stations, where flowing water turns a machine called a generator. Inside the generator, magnets (often electromagnets, magnets made with electricity) spin past coils of wire, and that makes the electric current. Electricity and magnetism always go together.

A magnet is a substance that attracts iron things, like iron dust and pins, towards it. An electromagnet is made by winding insulated wire round a piece of iron and passing an electric current through the wire: while the current flows, the iron becomes a magnet.

Electricity that flows along wires from a source, such as a power station or a cell, to the place where it is used is called current electricity. Wires carry it from power stations to houses and factories all over the country.

But there is another kind. You may have seen lightning, or heard the crackle and seen the sparks when you take off a wool or nylon sweater in the dark. This is electricity too, but it doesn’t flow along a wire; it builds up on an object and stays there until it jumps away in a spark. It is called static electricity (static means “not moving”).

The same thing happens when you take off clothes made of polyester, nylon, acrylic or wool: the cloth rubs against your body and hair, charge builds up, and it jumps across in tiny sparks that you can see in the dark and hear crackle. To understand why, we need to look inside matter.

Inside an atom

Everything is made of tiny particles called atoms (and molecules, which are groups of atoms joined together). An atom is far too small to see, but scientists know what is inside it:

• In the middle is the nucleus. It holds protons, which have a positive (+) charge, and neutrons, which have no charge.
• Round the nucleus move electrons, which have a negative (−) charge.

An ordinary atom has the same number of protons and electrons, so the positive and negative charges cancel. It is neutral. Most things around you, made of neutral atoms, have no charge.

The protons are held tightly in the nucleus and never move from one object to another when you rub them. But some electrons, on the outside of the atoms, can be rubbed off one material and onto another.

Rubbing moves electrons

When two different materials are rubbed together, electrons move from one to the other:

• The material that gains electrons has more electrons than protons. It becomes negatively charged (−).
• The material that loses electrons has fewer electrons than protons. It becomes positively charged (+).

When a balloon is rubbed on wool, electrons move from the wool to the balloon: the balloon becomes negative and the wool positive. When you comb your hair, electrons move from your hair to the comb: the comb becomes negative, your hair positive.

This charge is made in insulators, materials that don’t let electricity flow through them easily: plastic, rubber, wool, nylon, polyester, acrylic, glass, dry hair. Because the charge can’t flow away through an insulator, it stays where it was made. Electricity produced in insulators by rubbing (friction), when they gain or lose electrons, and which stays on them, is called static electricity.

Why the comb picks up paper

A piece of paper has no charge to begin with. So why does it jump to a charged comb?

When the negatively charged comb comes near, its negative charge pushes the electrons in the paper a tiny bit away (like charges push each other away). The side of the paper nearest the comb is left slightly positive, and the far side slightly negative. The near, positive side is attracted to the negative comb more strongly than the far side is pushed away, because it is closer. So the paper is pulled up to the comb.

Charged objects push and pull each other:

• Opposite charges attract (+ and − pull together).
• Like charges repel (+ and +, or − and −, push apart).

The force of attraction or repulsion between charged objects is called the electrostatic force. Rub two balloons on wool and hang them side by side on threads: both have the same (negative) charge, and they swing apart.

Static electricity can be a nuisance, or even dangerous. Its harmful effects in daily life include:

1. Lightning and thunder: a giant spark of static electricity (section 8.3).
2. Fires in fuel tankers: petrol sloshing and flowing through pipes builds up static charge. A spark near petrol vapor can start a fire.
3. Damage to computer chips: a spark of static from your finger, too small to feel, can destroy the tiny parts of a computer chip. Repair workers wear a strap on the wrist that leads the charge safely away.
4. Dust and hair sticking: charged TV and computer screens and phone screens attract dust; clothes attract fluff and hair.

But static electricity is also useful:

1. Photocopiers and laser printers: a drum inside is charged in the pattern of the page, and the powdered ink (toner) sticks only where it is charged. The toner is then pressed onto the paper and melted in place.
2. Paint sprayers: the paint drops are given a charge, and the object being painted (a car body, a bicycle frame) the opposite charge. The paint is pulled onto it evenly, even round the back, and less is wasted.
3. Air filters: in some air cleaners, and in the chimneys of factories, smoke and dust are charged and then caught on plates of the opposite charge, so cleaner air comes out.
4. Dusting equipment: some dusters are made of materials that become charged when wiped, so they pull dust in.

How a thundercloud becomes charged

On a hot, humid day, warm, wet air rises fast. As it rises it cools, and its water vapor turns into tiny water droplets, making a cloud. In a tall storm cloud, the air rises so high (often over 10 km) that the drops freeze into ice crystals and soft lumps of ice called graupel (soft hail). Strong winds called updrafts carry the small ice crystals up, while the heavier graupel falls down through them.

The ice crystals and the graupel crash into each other millions of times, and each collision moves a little charge, rather like rubbing a balloon on wool. The light ice crystals become positive and are carried to the top of the cloud. The heavier graupel becomes negative and gathers in the lower part of the cloud. So a thundercloud usually has a positive top and a negative bottom.

When the charge becomes huge, the air can no longer hold it apart, and electrons rush through the air in a giant spark. This sudden flow of charge through the air is called an electric discharge. It may jump:

• inside one cloud, from its negative part to its positive part, or between two clouds (most lightning is like this);
• from the cloud to the ground. The negative base of the cloud pushes electrons in the ground away, so the ground under the cloud becomes positive. The spark usually strikes the highest point nearby: a tree, a tower, a tall building or a person standing in an open field.

A flash of lightning is a giant electric discharge from a cloud. It produces a huge amount of light, heat and sound. The air in its path is heated to about 30,000 °C, five times hotter than the surface of the Sun, in a few millionths of a second. The hot air expands so suddenly that it makes a mighty bang: thunder. Lightning can set trees on fire, split them, damage buildings, and kill people and animals.

Lightning rod

How can a building be protected? With a lightning rod (also called a lightning conductor). A pointed rod of copper is fixed on the highest point of the building. A thick copper strip or wire runs from it down the outside wall and into the ground, where it is joined to a copper plate or rod buried in a pit. This joining to the ground is called earthing.

If lightning strikes, it hits the rod, the highest point, and the huge current runs down the copper strip into the ground, instead of through the building. Copper is used because it conducts electricity very well. The pit is filled with charcoal and salt, which hold water and keep the soil round the buried plate moist, so that the charge can spread easily into the ground.

Many new buildings in Nepal, such as schools built to resist earthquakes, apartments, shopping centers and cinema halls, are fitted with lightning rods.

Where does the electricity in your home and school come from? Is it the same kind as the electricity in a torch?

The electricity in your home comes along wires from power stations, mostly hydropower stations. A torch, a clock or a TV remote gets its electricity from cells. Both are current electricity: electricity that flows.

For electricity to flow, four things are needed:

1. a source of electricity, such as a cell, a battery or the mains supply;
2. conducting wires, which carry the electricity (copper wire covered with plastic);
3. a switch, to start and stop the flow;
4. a load, the device that uses the electricity, such as a bulb, a fan or a radio.

When these are joined so that there is a complete, unbroken path from one end of the source, through the load, and back to the other end, the electricity flows. The complete path along which an electric current flows is called an electric circuit. If the path is broken anywhere (the switch is off, or a wire comes loose), the flow stops.

In a metal wire, the flow of electricity is a flow of electrons. The continuous flow of electrons through a conductor, from one end to the other, is called current electricity. The electrons leave the negative terminal of the cell, go round the circuit, and come back into the positive terminal.

Electrical appliances change electrical energy into other forms of energy: a bulb into light (and heat), a heater or an iron into heat, a radio or a speaker into sound, a fan or a water pump into motion (using the magnetic effect of the current in its motor).

Combination of cells

A cell is a source of electricity. It turns the energy stored in chemicals into electrical energy. Every cell has two ends, or terminals: a positive terminal (+), usually the end with the small metal cap, and a negative terminal (−), the flat end. Cells come in different kinds, shapes and sizes: the simple cell made in laboratories, the dry cells in torches and remotes, the lead–acid cell in car and inverter batteries, and the flat button cells in watches.

A group of cells joined together is called a battery. (In daily life we call a single dry cell a “battery” too.) An ordinary dry cell gives 1.5 volts (V); the volt tells how hard the cell pushes the electricity round the circuit.

Using two or more cells together is called a combination of cells. There are two ways to join them: series combination and parallel combination.

Series combination

In a series combination, the cells are joined end to end, in a line: the negative terminal of the first cell is joined to the positive terminal of the second, and the negative terminal of the second to the positive terminal of the third, and so on. The free + of one end and the free − of the other end are joined to the circuit.

Uses of series combination:

• to get a higher voltage, and so a brighter bulb or a stronger current, by adding more cells;
• in torches, radios, remotes, clocks, toys and many other appliances.

But the brighter bulb uses up the cells faster.

Parallel combination

In a parallel combination, the positive terminals of all the cells are joined together at one point, and all the negative terminals together at another point. These two points are joined to the circuit.

Uses of parallel combination:

• to make the cells last longer;
• to supply a steady, small current for a long time.

1. Choose the best option

2. Differentiate between

3. Give reasons

4. Answer the following questions