How Can Something Push Without Touching?

You learned in Why Does a Rolling Ball Stop? that a force is a push or a pull, with a size and a direction. Most of the forces you meet every day need two things to touch: your foot kicking a football, friction between a tyre and the road, the floor holding you up. These are contact forces.

A few forces act across a gap, with nothing touching. They are non-contact forces, and there are three you can feel:

• Gravity pulls a falling mango down to the ground, though nothing holds it.
• A magnetic force pulls a pin up to a magnet, or pushes two magnets apart.
• An electric force pulls bits of paper up to a rubbed comb.

Non-contact forces still follow the rule you met in Pushes come in pairs. The magnet pulls the pin, and the pin pulls the magnet just as hard. Hang a magnet on a thread and bring a big iron nail near it: the magnet swings towards the nail.

In this topic we look at the electric and magnetic forces. (Gravity is far weaker; you need a whole planet to feel it.)

You learned in Grade 7 that rubbing two things together can move tiny particles called electrons from one to the other. Electrons carry a negative charge. The thing that gains electrons is left with a negative charge; the thing that loses them is left with a positive charge. When you rub a balloon on your hair, the balloon takes electrons from your hair. The balloon becomes negative, and your hair positive.

Here is the rule for charged things:

• Like charges repel: two positive things push each other apart, and so do two negative things.
• Unlike charges attract: a positive and a negative thing pull each other together.

You can see both, with nothing but a roll of sticky tape.

Why is the strip pulled towards your finger, or a comb towards bits of paper that have no charge? Paper is made of atoms, and every atom has electrons in it. When a negative comb comes near, it pushes the electrons in the paper a tiny distance away, to the far side of each atom. The near side of the paper is left a little positive, and the far side a little negative. The near side is closer, so its pull wins: the paper is pulled towards the comb. A charged thing attracts uncharged things in this way, whatever its own charge.

You know that a comb rubbed harder picks up more paper, and that it must be brought close. Scientists turn hunches like these into questions they can test: questions about how one thing changes another.

• “Does a bigger charge make the force stronger?”
• “How does the force change as the charged things get farther apart?”

To answer a question like this, you change one thing (the charge, or the distance) and keep everything else the same, just as you did in Plan an investigation. Then you measure what happens. But how do you measure a push that you can hardly feel?

One way is with a very sensitive balance, the kind used in a laboratory, which reads to a ten-thousandth of a gram. Put a small charged ball on a plastic rod on the balance, and set the reading to zero. Then hold a second charged ball above it. If the top ball pushes the bottom one down, the balance reads more. If it pulls it up, the balance reads less. The change in the reading tells you how strong the push or pull is.

If you change one thing at a time, you find two patterns.

More charge, more force. At 4 cm apart, ×1 charge on the top ball gives 0.0103 g; ×2 gives 0.0206 g; ×4 gives 0.0413 g. Twice the charge, twice the force. The force is proportional to the charge. (The same is true of the bottom ball’s charge: double either one, and the force doubles.)

Farther apart, much less force. With ×1 charge, at 3 cm the reading is 0.0183 g, and at 6 cm it is 0.0046 g. Twice as far, and the force is not a half but a quarter. Three times as far, a ninth. The force gets weaker very quickly as the distance grows.

The French engineer Charles-Augustin de Coulomb found this pattern in 1785, measuring the tiny twist of a thin wire. That is why the unit of charge, which you will meet in Grade 9, is called the coulomb. The charges in the widget are made up, but of a real size: a few billionths of a coulomb, less than a rubbed balloon carries.

You found in Grade 6 that an electromagnet gets stronger with more turns of wire, and with more cells pushing a bigger current through it. You met it again in Grade 8.

A permanent magnet has no turns or cells to change. So what makes its push stronger? You can find out with the same kind of balance: a kitchen scale is sensitive enough for magnets. Tape one small magnet on top of an upturned plastic cup on the scale, and set the scale to zero. Hold a stack of magnets above it, north pole facing north pole, so that they push the bottom magnet down.

These are powerful little magnets, so the readings are much bigger than for the charged balls. Look at what the table shows.

The gap matters most of all. One magnet above one magnet: at a 10 mm gap the scale reads 120.2 g; at 20 mm, only 19.3 g. Twice the gap, and the push is about a sixth. That is even faster than for charges. Close up, the push is huge: at a 5 mm gap it reads 427.9 g.

A stronger magnet pushes harder, but stacking isn’t simple. At a 10 mm gap, a stack of two magnets reads 164.2 g and a stack of three 183.5 g. More magnets give a bigger push, but two magnets don’t push twice as hard as one. The second magnet in the stack is farther from the bottom magnet, and you have just seen how quickly the push weakens with distance.

The numbers in this widget were worked out from the laws of magnetism for real magnets: small neodymium discs (the strongest kind of magnet sold), 10 mm across and 5 mm thick. Your own measurements with other magnets will be different, but the pattern will be the same.

The same factors make electric motors stronger. A motor turns because its magnets push on the current in its coil (you will learn how in Grade 10: The electric motor). A bigger current, more turns in the coil, or stronger magnets give a bigger turning push, so the motor can turn a heavier load.

How does a magnet “know” that a pin is there, across empty space? Scientists explain it like this. A magnet changes the space around it, even when nothing else is there. The space round a magnet, where a magnetic thing would feel a force, is its magnetic field. You met this name in Grade 8. When a pin is put in the field, the field pushes or pulls on it.

How can you find out where a field is, and which way it points, if you can’t see it? Use a test object: a small thing that feels the force, and shows it. For a magnetic field, the test object is a small compass. Its needle is a tiny magnet, so it turns to line up with the field wherever you put it. Move it to a new place, and the needle shows the field there. Carry it all round a magnet, marking the direction each time, and you have a map of the field.

Here is what a map like this shows.

• Close to the magnet, the needle follows the magnet’s field. The arrows curve out from the N pole and round into the S pole.
• Far from the magnet, the needle swings back towards north. The Earth is a giant magnet too Grade 8: Geomagnetism, and its field is everywhere. Far away, the magnet’s field is weaker than the Earth’s.
• With two magnets, N facing S, the arrows run straight across the gap. With N facing N, they turn away from the gap.
• Now search the top half of the sheet, up and to one side of the magnet (or near the top corners, with N facing N). Somewhere there, the magnet’s field points south, and is exactly as strong as the Earth’s field pointing north. The two cancel completely, and the needle has no direction to point at all: a neutral point Grade 8: Evidence that the Earth is a magnet. Can you find one?

An electric field

A charged thing changes the space round it too. The space round a charged object, where another charge would feel a force, is its electric field. To map it, the test object is a tiny charged thing: scientists use a tiny positive charge, and draw the direction it would be pushed.

Around a positively charged ball, a tiny positive test charge is pushed straight outwards (like charges repel), in every direction. Around a negative ball, it is pulled straight inwards. Join up the directions, and you get field lines. Where the lines are crowded together, the field is strong.

Gilbert’s versorium was a test object for electric fields too. The field pushes the electrons in its metal needle a little towards one end, so the needle lines up along the field, just as a compass needle lines up in a magnetic field. (It can’t show which way along the line the field points: for that you need a charged test object.)

Maybe the field is only an idea, a way of drawing pictures. How could you test whether there really is something in the space between a magnet and a pin? Here is an experiment you can do at home.

This experiment gives evidence that a field exists in the gap. The clip feels a force where nothing touches it, and the force is still there when the space is filled with paper or plastic. But a good scientist also asks: is the experiment fair, and could anything else explain the result? This is called evaluating an experiment.

• Is it really the magnet? Take the magnet away, and the clip falls. This is a control: the same set-up without the thing you are testing.
• Could the thread be holding it up? No. The thread can only pull down, towards the table. Something else pulls up.
• Could it be air pushing? Try it inside a box with no draughts: it still works. And magnets work just as well with the air pumped out. The Earth’s magnetic field reaches far out into space, where there is no air at all, and turns aside the charged particles that stream out from the Sun.
• Could the materials have been tested unfairly? Each sheet should go into the same gap, at the same place, without touching or moving the clip. Thicker things fill more of the gap, so a test with a thick book and a test with thin paper aren’t quite the same. And one try isn’t enough: repeat each test, to make sure the result happens every time.

• Who Feels the Bigger Push in a Crash? A magnet and a pin pull on each other equally, like every pair of forces. See what equal pushes do to things of different mass.
• Why Does a Rolling Ball Stop? What any push, touching or not, does to the motion of the thing it pushes: balanced and unbalanced forces.
• Where Does a Lifted Stone Keep Its Energy? Pull two magnets apart and you store energy in their field, as you do when you lift a stone.
• Going further still, in Grades 9–12: How Do Magnets Push Without Touching? The energy stored in magnetic and electric fields, and where it is.