Class 6 Science Chapter 4: Exploring Magnets

A Story That Would Not Move

Reshma lives in a small coastal town in Kerala, and writes short stories. She was writing one for her grandmother's sixtieth birthday, about a ship carrying spices from Kerala in the olden days. Then she got stuck, and the reason she got stuck is the whole of this chapter.

Sailors and the Stars

Sailors in those days found their direction at night from the stars. Reshma knew that much. But she had put a storm into her story, with an overcast sky and no stars visible. So how would anybody on that ship know which way they were going?

The Device She Found

She looked in her school library and on the internet, and found the answer. Travellers used a device called a magnetic compass to find directions. That sent her back to something she had seen a hundred times and never looked at properly.

Magnets You Have Already Met

Pencil boxes and purses that hold themselves shut. The duster on a school writing board. Stickers. Toys. All of these have magnets in them, and almost nobody looks at them carefully. Reshma decided to.

The First Magnets Were Found, Not Made

The magnets those old sailors used rested on lodestones, which are magnets occurring in the ground and were known from ancient times. Nobody invented them. They were simply there in the ground, already behaving strangely.

Then People Learned to Make Them

Later, people worked out how to make a magnet out of a piece of iron. That changed everything, because a found object is limited and a made one is not. Nowadays we have magnets made of several different materials.

Artificial Magnets

The magnets in your school laboratory, and the ones in pencil boxes, stickers and toys, are all artificial magnets. Artificial here does not mean fake. It means made rather than found, and a made magnet works exactly as a found one does.

Three Shapes to Know

Magnets can be of various shapes. Three are worth naming. A bar magnet is a straight rectangular block. A U-shaped magnet is bent round so its two ends face the same way. A ring magnet is a flat circle with a hole in the middle.

The First Question Worth Asking

Now for the experiment. Gather up a handful of objects, each made of a different material, and find a magnet. Before you touch anything, predict which objects will stick, and write your prediction down. That step matters more than it looks.

Predict, Then Test

Now hold the magnet and bring it near each object in turn. Watch which ones stick. Write what actually happened beside what you predicted. Was your prediction right for all of them? Most people get at least one wrong, and that one is the useful result.

The Two Groups

Some objects felt a pull towards the magnet and clung to it. Others were not. That split gives us two names. A material the magnet pulls towards itself is called a magnetic material. One it ignores completely is called non-magnetic.

Which Metals Are Magnetic

Iron is a magnetic material, and it is the one everybody knows. It is not the only one. Two more metals behave the same way: nickel and cobalt. Certain mixtures of those three with other metals are pulled in as well.

And Which Are Not

Go back to your table and look at the objects that did not stick. Wood, rubber, plastic, glass: these are non-magnetic materials, and no amount of trying will change that. A wooden pencil is not going to stick to anything.

Not All Metals Are Magnetic

Here is the part that surprises people. Metal and magnetic are not the same word. Many metals are not attracted to a magnet at all. So the right question is never is it metal, but which metal is it.

What Comes Next

So a magnet pulls some materials and ignores others. The next question is whether the whole magnet pulls equally, or whether some parts of it pull harder than others. A handful of iron filings answers that in about ten seconds.

Scenario:

A class learns that lodestones occur in the ground and were known in ancient times. Arjun asks why anybody would care about a stone that picks up iron. Their teacher asks the class what a traveller far from home most needs to know. Direction. A lodestone hung on a thread settles the same way every time, and that makes it useful at sea where there are no landmarks. The class then considers what sailing without it would mean. You could follow a coast but not cross open water with confidence. Arjun changes his question. The stone mattered because it answered a problem that nothing else could answer.

Analysis:

A property looks like a curiosity until a need meets it. Settling north to south is only interesting to somebody who has lost their landmarks.

Activity:

Find out one way people found direction before the compass. Write three sentences on what it needed and when it failed.

Title:

Predict, then test

Safety:

Keep magnets away from a mobile phone, a television, a computer or a watch. Do not test anything electrical, and do not let a small child near small magnets.

What You Notice:

The wrong predictions cluster around metal objects, because people expect every metal to stick. A stainless steel spoon and an iron nail behave quite differently, and finding that out in your own kitchen is worth more than being told.

The magnets sailors used long ago were based on lodestones, which are magnets that occur naturally in the ground.

Iron is not the only magnetic metal. Nickel and cobalt are magnetic too.

The magnet on a school writing-board duster is doing the same job as the magnet in a pencil box.

The three magnetic metals — I-Ni-Co: Iron, Nickel, Cobalt — Three metals, and mixtures of them with others can be attracted as well.

Three shapes of magnet — Bar, U, ring — A straight one, a bent one and a round one. The shape changes nothing about the rules.

Does a Magnet Pull Everywhere?

You know now that a magnet attracts iron. Here is the next question, and it is a fair one. Does every part of the magnet pull equally hard, or is the pull stronger in some places than others? Guessing is no use. You need to see it.

Making the Pull Visible

The trick is to use something light enough to be moved. Iron filings are very small pieces of iron, and each one weighs almost nothing. Scatter some over a sheet of paper, lay a bar magnet on top of them, and tap the paper gently.

What the Filings Do

Look carefully at how they stick. They do not coat the magnet evenly. The greatest number gather near the two ends of the bar, while very few cling to the long stretch in between. The pattern is unmistakable and it is the same every time.

Two Places, One Name

Those two crowded ends have a name. They are called the poles of the magnet. One is the North pole and the other is the South pole. So a magnet is not uniformly magnetic. Its pull is concentrated at two spots.

Does the Shape Change It?

Fair question. Try the same thing with a U-shaped magnet or a ring magnet instead of a bar. The answer is no: most of the iron filings gather at the poles any magnet carries, whatever its shape. The shape moves the poles around, but it does not remove them.

Now the Strange Part

Here is a question worth sitting with. A magnet has two poles. Could you somehow get hold of just one? Cut a bar magnet in half, throw away the South half, and keep a piece that is only North. It sounds perfectly reasonable.

It Does Not Work

There is no way to end up holding a magnet that has only one pole. Break a magnet into smaller pieces and North and South poles still exist in pairs, in every piece. Break those pieces again and it happens again. Neither a lone North nor a lone South can exist.

Think About What That Means

You cannot cut your way to half a magnet. The moment you make the cut, the two new pieces each grow the pole they were missing. However small the piece gets, it arrives complete, with both poles present.

Reading a Real Magnet

Pick up a magnet from a laboratory and you will usually find its poles marked. On some, the letters N and S are written on the ends. On others the North pole is shown by a white dot. On others again the North pole is painted red and the South pole blue.

Why That Marking Matters

Without a marking, the two ends of a bar magnet look identical. They are not, and the difference decides what happens when two magnets meet. That is why manufacturers bother, and why an unmarked magnet is a small nuisance to work with.

A Question From the Exercises

Atharv rolls a bar magnet over a heap of steel U-clips and counts how many stick at three positions along the magnet. Before reading on, predict his numbers. Then check your prediction against what the iron filings already showed you.

The Answer, and Why

He should pick up many clips at each end and very few in the middle. Something like ten, two, ten. A result of ten, ten, two would be wrong, and so would ten everywhere. The filings told you the shape of the answer before he ever started counting.

Testing a Claim Yourself

This is a pattern worth keeping. When somebody tells you a magnet is strongest in the middle, you do not have to argue. Spread filings, lay the magnet down, tap the paper, and look. The magnet settles the question itself.

What Is Still Unanswered

So a magnet has two poles, always two, and they are at the ends. But the names North and South have not been explained yet. Why should an end of a piece of metal be called North? The next topic answers that, and the answer involves the whole planet.

Scenario:

A class rolls a bar magnet through iron filings and counts what sticks at three places. Near one end they count a thick cluster, at the middle almost nothing, and near the other end another thick cluster. Meera suggests the middle of the magnet is broken. Their teacher asks the class to repeat the count on a second magnet, and the pattern comes out the same. Two magnets cannot both be broken in the same place. The class records what the counts show instead. The pull is strongest near the two ends and weakest in between, and that is true of the magnet rather than of any one sample.

Analysis:

One magnet gives a result and two give a pattern. Repeating the count is what rules out a fault in the sample.

Activity:

Roll a magnet through iron filings or pins. Count what sticks at five points along it and draw a bar chart of the counts.

Title:

Counting pins along a magnet

Safety:

Steel pins are sharp. Count them out and count them back in, and keep them away from younger children. Keep the magnet away from phones, watches and computers.

What You Notice:

The two ends give roughly equal counts and the middle gives very few. Repeating it is the part worth doing, because a pattern that holds three times is a finding, while one that happens once could be luck.

A magnet of any shape, not just a bar, collects most of its iron filings at the poles.

Some magnets show the North pole with a white dot instead of the letter N.

On some magnets the North pole is painted red and the South pole blue.

Where the filings go — Ends crowded, middle bare — The pattern is the whole result. Most at the two ends, very few in between.

The rule about poles — Never one, always two — Break it as small as you like and a pair of poles comes back each time.

Hang It Up and Let Go

Tie a thread around the middle of a bar magnet and hang it so it swings free. You may need to slide the thread along until the magnet balances level. Now turn it gently with a finger and let it come to rest. Watch where it stops.

Mark the Line It Chooses

Mark the ground under each end of the magnet and join the two marks with a line. That line is the direction the magnet chose. Now give it another gentle push, wait until it stops again, and check. Does it come back to the same line? It does, every time.

Which Line Is It?

Knowing where the Sun rises and sets gives you a rough idea of east and west, so the line can be identified. A magnet free to turn settles along the line running north to south. That is not a tendency. It is what happens on every attempt.

Now the Names Make Sense

Whichever end of the magnet comes to face north goes by the name North-seeking pole, or the North pole for short. The other end, facing south, is the South-seeking pole, or the South pole. The names describe what the ends do.

But Why Should It Care?

A hanging piece of metal has no reason to prefer one direction over another. Unless something enormous is pulling on it. A magnet left free to turn settles north to south because our planet is itself one huge magnet.

The Control Experiment

Now do the same thing with a small iron bar instead of the magnet. Hang it, spin it, let it stop, and note the line. Then spin it again. Does it always settle north-south? It does not. It can come to rest along any direction at all.

What That Comparison Gives You

Only magnets settle north-south. An iron bar does not, although it is made of a magnetic material and sticks to magnets happily. So this activity is also a test: hang a piece of metal freely and watch, and you learn whether it is a magnet.

Turning a Property Into a Tool

A thing that always points the same way is useful to anybody who is lost. That is exactly what was done. In olden days a small device for finding directions was built around this property, and called the magnetic compass.

What Is Inside a Compass

A magnetic compass is usually a small round box, covered with something you can see through. Inside it is a needle-shaped magnet, balanced so it can turn, set on a pin that stands up from the floor of the box. The needle is balanced on that pin so finely that it turns with almost no effort.

The Red End and the Dial

The end of the needle that rests towards the north is usually painted red, so you can tell the two ends apart at a glance. Below the needle is a dial with the directions marked on it. Those two details are what make a compass readable.

How to Use One

Put the compass down where you want to know the directions. Give it a moment: after some while the needle settles north-south. Now gently rotate the whole box until north and south on the dial line up with the needle. Every direction at that place can now be read off the dial.

Make One Yourself: Magnetising

Collect a cork, an iron sewing needle, a bar magnet, a glass bowl and water. Lay the needle on a wooden table. Set one pole of the magnet against one end of the needle and slide it the whole way along. At the far end, lift the magnet clear.

The Part People Get Wrong

Now bring that same pole back to the same end you began from, and stroke along again in the same direction. Always the same pole, always the same way. Repeat at least thirty to forty times. Then bring a pinch of iron filings, or a few steel pins, close to the needle. If they are drawn in, it has become a magnet.

Floating It

Push the needle sideways through the cork. Set the cork afloat in a bowl of water, keeping the needle clear of the water itself. When it stops moving, your compass is ready. Note which way each end points, then spin the cork gently and check that it returns to the same line.

The Indian Instrument

Much before the modern compass came into wide use, Indians navigating at sea used a device very like the floating needle you have just made. It was an iron piece shaped like a fish and magnetised, floating in a bowl of oil. It was called the matsya-yantra, or machchh-yantra.

Scenario:

Reshma is writing a story set on a ship carrying spices from Kerala in the olden days. Her sailors steer by the stars each night. Then she puts a storm into the story. The sky stays overcast for three days and no star is visible at any hour. She wants the ship to reach port rather than be lost, and she wants the way it happens to be true rather than invented. Her grandmother, who is turning sixty, will certainly ask how the sailors managed. Reshma has found that travellers of that time used a magnetic compass, and she has also read about an older Indian instrument. Now she has to decide what to put on the deck of her ship.

Analysis:

The storm has removed the sailors' only reference and left them with the sea, the ship and whatever they carry. The question is what object could still tell them north, and why the sky being covered makes no difference to it.

Activity:

Find out how your own town lies relative to the sea, a river or a main road, then use a compass, or a magnet hung on a thread, to work out which direction your classroom window faces. Write down how you decided, and ask somebody who has travelled by boat how they knew which way they were going.

Title:

A compass in a bowl

Safety:

The needle is sharp: ask an adult to push it through the cork and keep it away from small children. Do not do this near a phone, a watch or a computer.

What You Notice:

The needle comes back to the same line every time you spin it, which is the whole point. If it does not settle at all, it has probably not been stroked enough times, and another twenty strokes usually fixes it.

A magnet free to turn always settles north to south, because the Earth is itself one enormous magnet.

The matsya-yantra, a magnetised fish-shaped iron piece floating in oil, served Indian sailors for finding their way at sea.

On a compass, the end of the needle that rests towards north is usually painted red.

Why the poles have those names — North-seeking, so North — The end that seeks north is named for what it does, not for where it is.

Testing whether metal is a magnet — Hang it. Only a magnet settles north-south. — An iron bar will rest in any direction at all. A magnet never does.

Two Magnets, One Question

Everything so far has been one magnet against something else. Now put two magnets together. Take a pair of bar magnets with their poles marked, call them A and B, and find out what they do to each other.

Setting It Up Fairly

Lay magnet A on its long side across five or six round pencils, so that the slightest push will roll it. That is the trick of the experiment: a magnet that can move tells you what it is feeling. Then bring one end of B near an end of A, without letting them touch.

The First Result

Magnet A rolls. Note carefully which way, because it matters. Now bring the other end of B towards the same end of A, and it rolls the other way. The same magnet, the same spot, and two opposite results depending on which end is presented.

The Rule

Unlike poles of two magnets, meaning the North of one and the South of the other, attract each other. Like poles, meaning two Norths or two Souths, repel each other. Four words: unlike attract, like repel.

Now Swap In an Iron Bar

Do it again, but swap one of the two magnets for a plain iron bar. Bring the North pole of the magnet to one end of the bar: attraction. Bring the South pole: attraction again. Try the other end of the bar: attraction both times.

Four Results, All the Same

Both ends of the iron bar are attracted by both poles of the magnet. There is no combination that pushes. That is a completely different pattern from the one two magnets gave, and the difference is worth more than it first looks.

Why a Pull Proves Nothing

If you feel a pull, you know nothing. Two magnets can pull. A magnet and an iron bar can pull. A magnet and a steel pin can pull. Attraction is what any magnetic material does.

And Why a Push Proves Everything

But a push is different. Nothing is repelled by a magnet except another magnet. So repulsion is what identifies a magnet. If two objects push each other apart, both of them are magnets, and no further test is needed.

A Question You Can Now Answer

Reshma buys three identical metal bars. Two are magnets and one is plain iron, and she has nothing else to test with. Try every pair, end to end, both ways round. The one bar that never pushes anything away is the iron one.

The Compass Needle Is a Magnet Too

A compass needle is itself a magnet, so it should follow the same rule. Bring the North pole of a bar magnet slowly towards the North pole of a resting compass needle. The needle moves away. Bring the South pole instead, and the needle moves closer.

Now Put Something in the Way

Here is a better question. Set up the magnet and compass as before, then stand a piece of wood upright between them without disturbing anything. Does the needle still swing? Record what you see, then try again with cardboard, then thin plastic, then thin glass.

Nothing Blocks It

The needle goes on swinging by much the same amount with any of those sheets in the way. The conclusion is a large one: magnetism reaches straight through materials that are not magnetic. A magnet reaches an object without touching it and without a clear path.

Which Is Why the Games Work

That fact is behind every trick at Reshma's school fair. Steel balls are steered around a maze by a magnet moved under the cardboard tray. A steel paper clip is lifted out of water without wetting your fingers or the magnet. A magnet can shift certain objects with nothing between at all.

Two Cars and a Prediction

Fix a magnet on each of two matchbox cars with like poles facing each other, and roll them together. Ask before you look: do they rush towards each other, or run away? You already know the rule, so you already know the answer.

Treating a Magnet Properly

A magnet can be spoiled. Do not heat it, do not drop it and do not hammer it, and keep it away from mobile phones and remote controls. Store magnets in twos, unlike poles kept to the same side, a piece of wood in between, and two pieces of soft iron laid across the ends.

Scenario:

Two pupils bring magnets together and get different results. Nita's pair pulls together and Imran's pushes apart. Imran decides his magnets are faulty. Their teacher asks both to mark the ends and try every combination. The class builds a table of four trials. A North with a South attracts, and so does a South with a North. A North with a North repels, and so do two Souths. Imran sees that nothing was faulty. His magnets had simply been held like poles facing. The class writes the rule from its own table, and notes that the result depends on which ends meet, not on which magnet is used.

Analysis:

Four trials turn two confusing results into a rule. The table is what shows that the ends decide the outcome.

Activity:

Mark the ends of two magnets. Try all four combinations, record attract or repel for each, and write the rule you find.

Title:

The clip that comes out dry

Safety:

Use a plastic or steel bowl rather than glass, and mop up spills at once so nobody slips. Keep the magnet well away from a phone or a watch on the table.

What You Notice:

The clip climbs the inside of the bowl while the magnet climbs the outside, with the bowl wall between them the whole time. The plastic sheet changes nothing, which is the same result the compass gave with wood, cardboard and glass.

Repulsion is the mark of a magnet, because nothing but a magnet is ever pushed away by one.

Magnets should be stored in twos, unlike poles kept to the same side, a piece of wood between them, and soft iron across the ends.

Heating a magnet, dropping it or hammering it can spoil it.

What happens between poles — Unlike attract, like repel — Two words each. North and South pull; North and North push.

Which test proves a magnet — A pull proves nothing, a push proves everything — Iron is attracted too. Only another magnet is pushed away.