Chapter 1 of 8

All 8 chapters
  1. 1Magnetic fields, readunderstood
  2. 2Fields around wires and coils, readunderstood
  3. 3The motor effect, readunderstood
  4. 4Electric motors, readunderstood
  5. 5Electromagnetic induction, readunderstood
  6. 6Generators and dynamos, readunderstood
  7. 7Transformers, readunderstood
  8. 8Speakers and microphones, readunderstood

S4.13.1 Chapter 1

Magnetic fields

A magnet pulls a pin across the table without touching it. What is reaching out from the magnet, when there is nothing to see?

Your guess

Pick one before you read on. It is fine to be wrong.

Your guess

  • You picked A. Read on. At the end, the chapter comes back to your guess.

  • You picked B. Read on. At the end, the chapter comes back to your guess.

  • You picked C. Read on. At the end, the chapter comes back to your guess.

By the end you can

  • Draw field lines around a bar magnet and between two poles
  • Explain the difference between permanent and induced magnets

A magnet can pull a pin across a table without touching it. The pull reaches out into the space round the magnet. That space is the magnetic field. In a magnetic field, a magnet or a magnetic material feels a force.

You cannot see a field, but a small magnet can feel it. A compass needle is a small magnet. It turns until it lines up with the field. The way its N end points is the direction of the field there: the way a N pole would be pushed.

We draw a field with lines. Each field line shows the direction of the field with an arrow, and many lines together show where the field is strong. Draw one line at a time, and let a compass show you the way it bends.

Draw it first

Build the field of a bar magnet with a compass

  1. Three pictures of a bar magnet labelled N and S, with its N end on the left. Step 1: two field lines and one compass above the middle of the magnet. Step 2: three field lines on each side and two compasses. Step 3: four field lines on each side and three compasses. Every line has an arrow from the N end towards the S end, and every compass needle lies along the line near it.NS

    1 Put a small compass beside the magnet. Its needle lies along the field there. Draw a short line that follows the needle, with an arrow in the way the N end points.

  2. Three pictures of a bar magnet labelled N and S, with its N end on the left. Step 1: two field lines and one compass above the middle of the magnet. Step 2: three field lines on each side and two compasses. Step 3: four field lines on each side and three compasses. Every line has an arrow from the N end towards the S end, and every compass needle lies along the line near it.NS

    2 Move the compass a little way along and draw again. Join the short lines into curves. Every curve leaves the N pole and bends round.

  3. Three pictures of a bar magnet labelled N and S, with its N end on the left. Step 1: two field lines and one compass above the middle of the magnet. Step 2: three field lines on each side and two compasses. Step 3: four field lines on each side and three compasses. Every line has an arrow from the N end towards the S end, and every compass needle lies along the line near it.NS

    3 Add more lines. Outside the magnet each one runs into the S pole. The lines never cross, and they are closest together at the poles.

Here are the rules for drawing field lines. Outside a magnet, a line runs from the N pole to the S pole, and the arrow shows that way. Lines never cross, because the field has only one direction at each point. Lines are closer together where the field is stronger.

There is one more rule, and it is often missed. A field line has no start and no end. Outside the magnet it runs from N to S. Inside the magnet it carries on from S back to N. So every line is a closed loop.

A bar magnet labelled S on its left half and N on its right half, with field lines drawn as closed loops. One closed loop runs over the top of the magnet and one under it. Each runs out of the N end of the magnet, round the outside and back in at the S end, with its arrow pointing from the N end round to the S end. Two longer lines run through the middle of the magnet, one just above its axis and one just below, with arrows pointing from the S half to the N half. They curve away outside and leave the picture at both ends.SN
Every line is a closed loop: N to S outside the magnet, S to N inside it. The two lines through the middle are so big that they leave the picture and come round far away. Only some lines are drawn. The field is also there between the lines.

Look at where the lines are close together. They bunch near the poles, where the field is strongest. Far from the magnet they spread out and the field is weak. The field gets weaker with distance, but it never stops suddenly.

Now put two magnets end to end. When a N pole faces a S pole, the lines join across the gap. The field is strong in the gap, and the magnets pull together.

Two bar magnets end to end with a gap between them, each labelled S on its left end and N on its right end, so the N end of the left magnet faces the S end of the right magnet. Field lines leave the N end of the left magnet and run across the gap into the S end of the right magnet.SNSN
Unlike poles facing: the lines join across the gap and the magnets attract.

When two N poles face each other, the lines do not join. They bend away from each other. Between the poles there is one point where the two fields cancel and the field is zero. It is called the neutral point.

Two bar magnets end to end with a gap between them. The left magnet is labelled S on its left end and N on its right end. The right magnet is labelled N on its left end and S on its right end, so the two N ends face each other. The field lines from the two N ends bend away from each other. A cross between the magnets marks the neutral point, with the label neutral point.SNNSneutral point
Like poles facing: the lines push apart and the magnets repel. The field is zero at the neutral point.

Not every metal is magnetic. A magnetic material is pulled by a magnet. Of the common metals, only iron, nickel and cobalt are magnetic. Most steels are magnetic too, because steel is mostly iron. Some stainless steels, such as many kitchen spoons, are not. Copper, aluminium, gold, wood, plastic and glass are not magnetic.

A table with the columns Material and Pulled? meaning pulled by a magnet. Rows: iron, Yes. most steel, Yes. nickel, Yes. cobalt, Yes. copper, No. aluminium, No.MaterialPulled?ironYesmost steelYesnickelYescobaltYescopperNoaluminiumNo
Only a few metals are magnetic. Most metals are not. A kitchen spoon test is fair: many stainless steel spoons are not pulled.

There are two kinds of magnet. A permanent magnet keeps its magnetism all the time. A bar magnet of steel is one. An induced magnet is a piece of magnetic material that is a magnet only while it is in a magnetic field. An iron nail near a magnet is an induced magnet.

Why can iron become a magnet? Inside the metal are tiny regions called domains. Each domain is a very small magnet. In a piece of iron that is not magnetised, the domains point in all directions, and their effects cancel. In a magnet they are lined up.

See it three ways

As a picture

Two boxes of small arrows, one above the other. The top box is labelled not a magnet, and its arrows point in many different directions. The bottom box is labelled magnet, and all its arrows point to the right.not a magnetmagnet

In words

In a magnet the domains line up. In unmagnetised iron they point in all directions and cancel. A field from outside can turn them to line up.

Bring a N pole of a magnet up to a soft iron nail. The field turns the domains in the nail, and the nail becomes a magnet. The end of the nail nearest the N pole becomes a S pole. The far end becomes a N pole. A S pole is nearest the N pole, so they attract.

A bar magnet labelled S on its left half and N on its right half touches the left end of an iron nail. On the iron nail, the left end is labelled S and the right end is labelled N. Three pins hang in a chain from the right end of the nail. Labels: iron nail, pins.SNSNiron nailpins
The nail is an induced magnet while the bar magnet is there. Its S pole is nearest the magnet’s N pole.

An induced magnet is always attracted to the magnet that makes it. The pole nearest the magnet is always the opposite pole. Take the magnet away. Now the two materials differ. Soft iron loses almost all its magnetism at once, and the pins drop. Steel keeps its magnetism, so steel makes a permanent magnet. Most ordinary nails are mild steel and keep a little magnetism. Pure soft iron keeps almost none.

An iron nail with no poles marked, and three pins lying on a table below it. Labels: iron nail, pins.iron nailpins
Soft iron: the magnet has gone, so the nail has almost no magnetism left and the pins have fallen.
A steel nail with its left end labelled S and its right end labelled N, and three pins hanging in a chain from its right end. Labels: steel nail, pins.SNsteel nailpins
Steel: the magnet has gone, but the nail kept its magnetism and the pins still hang.

Try it

Map a magnetic field with pins

You need: a bar magnet, if you can get one, a sheet of thin paper, a few small bits of steel wool or short iron pins, a small compass, if you have one.

Be safe: Steel wool and pins can cut or prick. Tear off tiny pieces with a cloth round your fingers, keep pins in a box, and keep both away from small children. Keep a strong magnet away from phones and bank cards.

  1. Lay the magnet on a table and put the paper flat over it.
  2. Sprinkle a few bits of steel wool, or lay pins, on the paper near the magnet.
  3. Tap the paper gently. Look at the way the bits line up.
  4. If you have a compass, move it round the magnet on the paper and watch its needle.

What you will see: The bits line up along curved lines that run from one pole to the other. They bunch near the poles, where the field is strong. Each bit becomes a tiny induced magnet, and it lines up with the field. The compass needle follows the same curves. A flat fridge magnet gives thin stripes instead. A speaker magnet gives rings. Their poles are placed differently.

If you cannot try it, this is what happens.
A bar magnet labelled S on its left half and N on its right half, with four curved field lines on each side joining the two ends. What the bits of steel wool show.NS

A wrong idea many people have

The magnetic field is only along the lines that are drawn.

Why it feels right: A picture of field lines shows empty gaps between the lines, and it is easy to think the field is missing there.

The test that breaks it: Put a compass between two of the lines. Its needle still lines up. A pin placed in a gap is still pulled. The field is there too.

What is true: The field is everywhere near the magnet. Field lines are a drawing. They show the direction of the field, and by how close they are, how strong it is.

A wrong idea many people have

If a magnet pulls something, that thing is a magnet too.

Why it feels right: Two magnets attract each other, so it seems that anything attracted must be one.

The test that breaks it: A magnet pulls a plain iron nail from either end. Turn the magnet round, and the nail is still pulled. A real magnet would push away from one of the two ends.

What is true: An induced magnet is always attracted, so attraction does not prove a magnet. Only repulsion proves it: a thing that is pushed away by a pole is a magnet.

A wrong idea many people have

Field lines start at the N pole and stop at the S pole, and they can cross.

Why it feels right: Drawings of the field outside a magnet show lines that seem to begin at one pole and end at the other.

The test that breaks it: Inside the magnet the field runs from S to N. The lines go on through the magnet and join up. If two lines crossed, the field would have two directions at one point, which cannot be.

What is true: Field lines are closed loops with no start and no end. Outside the magnet they run N to S, inside they run S to N, and they never cross.

Key words

magnetic field
the space round a magnet where it pushes or pulls on magnets and magnetic materials.
field line
a line that shows the direction of the field. Closer lines mean a stronger field.
neutral point
a place where two fields cancel and the field is zero.
permanent magnet
a magnet that keeps its magnetism, such as a steel bar magnet.
induced magnet
a magnetic material that is a magnet only while it is in a magnetic field.
magnetic material
a material that a magnet pulls: iron, nickel, cobalt, most steels and some alloys.
soft iron
iron that is easily magnetised and loses almost all its magnetism when the field is removed.

Back to your guess

The answer is B: The push and pull reach all round the magnet, strongest near the poles.

A magnet has a magnetic field all round it. The field is strongest near the poles and gets weaker with distance, but it is not zero anywhere near the magnet. A pin is pulled because it is inside the field. The field is not a thing you can see, so we draw it with lines.

Check yourself

Try each one before you look. A wrong answer tells you where to go next.

  1. Outside a bar magnet, which way do the field lines run?

    Tap your answer.

    1. Yes, that is right. The direction of a field is the way a N pole would be pushed. A N pole is pushed away from the magnet’s N pole and towards its S pole.

    2. Not this one. That is the way they run inside the magnet. Outside, the arrow shows the way a N pole would be pushed: away from N and towards S. Try another.

    3. Not this one. Circles round a middle are the field of a current in a wire. Outside a bar magnet the lines run from the N pole round to the S pole. Try another.

    4. Not this one. A N pole pushes a N pole away, so the lines leave the N pole. They do not run towards it. Try another.

    See the answer

    From the N pole to the S pole.

    The direction of a field is the way a N pole would be pushed. A N pole is pushed away from the magnet’s N pole and towards its S pole.

    • If you chose B: That is the way they run inside the magnet. Outside, the arrow shows the way a N pole would be pushed: away from N and towards S.
    • If you chose C: Circles round a middle are the field of a current in a wire. Outside a bar magnet the lines run from the N pole round to the S pole.
    • If you chose D: A N pole pushes a N pole away, so the lines leave the N pole. They do not run towards it.

  2. Which of these will a magnet pull?

    Tap your answer.

    1. Not this one. Copper is not magnetic. Only iron, nickel, cobalt, most steels and some alloys are. Try another.

    2. Not this one. Plastic is not magnetic. A magnet does not pull it. Try another.

    3. Not this one. Aluminium is a metal, but it is not a magnetic one. A magnet does not pull it. Try another.

    4. Yes, that is right. Steel is mostly iron, and iron is a magnetic material. Copper, plastic and aluminium are not magnetic.

    See the answer

    A steel pin.

    Steel is mostly iron, and iron is a magnetic material. Copper, plastic and aluminium are not magnetic.

    • If you chose A: Copper is not magnetic. Only iron, nickel, cobalt, most steels and some alloys are.
    • If you chose B: Plastic is not magnetic. A magnet does not pull it.
    • If you chose C: Aluminium is a metal, but it is not a magnetic one. A magnet does not pull it.

  3. Two bar magnets face each other with their N poles. What do the field lines between them do?

    Tap your answer.

    1. Not this one. That is what happens when a N pole faces a S pole. Two N poles push their lines apart. Try another.

    2. Not this one. Field lines never cross. A crossing would mean two directions of field at one point. Try another.

    3. Yes, that is right. Like poles repel, and their lines push apart. At the neutral point the two fields are equal and opposite, so the field there is zero.

    4. Not this one. There is a field in the gap. It is only zero at the one neutral point, where the two fields cancel. Try another.

    See the answer

    They bend away from each other, with a neutral point between the magnets.

    Like poles repel, and their lines push apart. At the neutral point the two fields are equal and opposite, so the field there is zero.

    • If you chose A: That is what happens when a N pole faces a S pole. Two N poles push their lines apart.
    • If you chose B: Field lines never cross. A crossing would mean two directions of field at one point.
    • If you chose D: There is a field in the gap. It is only zero at the one neutral point, where the two fields cancel.

  4. A soft iron nail and a steel needle are both held against a bar magnet. The magnet is then taken away. What happens?

    Tap your answer.

    1. Not this one. Steel does, but soft iron loses almost all of it once the field has gone. Try another.

    2. Yes, that is right. Soft iron is an induced magnet only while the field is there. Steel keeps its lined up domains, so it is a permanent magnet.

    3. Not this one. The iron does. Steel is hard to magnetise and keeps its magnetism, which is why it makes permanent magnets. Try another.

    See the answer

    The iron nail loses almost all of its magnetism. The steel needle keeps it.

    Soft iron is an induced magnet only while the field is there. Steel keeps its lined up domains, so it is a permanent magnet.

    • If you chose A: Steel does, but soft iron loses almost all of it once the field has gone.
    • If you chose C: The iron does. Steel is hard to magnetise and keeps its magnetism, which is why it makes permanent magnets.

  5. A bar is pulled towards the N pole of a magnet. It is pulled again when turned end for end. Is it sure to be a magnet?

    Tap your answer.

    1. Not this one. An unmagnetised piece of iron is also attracted to either pole, because it becomes an induced magnet. Try another.

    2. Not this one. Many metals, such as copper and aluminium, are not attracted at all. Being a metal does not make a magnet. Try another.

    3. Yes, that is right. An induced magnet is always attracted. Only repulsion proves a magnet, because an induced magnet is never pushed away.

    See the answer

    No, plain iron would be attracted to the pole at either end too.

    An induced magnet is always attracted. Only repulsion proves a magnet, because an induced magnet is never pushed away.

    • If you chose A: An unmagnetised piece of iron is also attracted to either pole, because it becomes an induced magnet.
    • If you chose B: Many metals, such as copper and aluminium, are not attracted at all. Being a metal does not make a magnet.

  6. What does it mean when field lines are drawn close together?

    Tap your answer.

    1. Not this one. It is the opposite: where the lines bunch up, the field is stronger. Try another.

    2. Not this one. The arrow on each line shows the direction. How close the lines are shows only the strength. Try another.

    3. Yes, that is right. The spacing of the lines shows the strength of the field. Close lines, such as near a pole, mean a strong field.

    4. Not this one. At a neutral point the field is zero, and no line runs through it. Close lines mean a strong field. Try another.

    See the answer

    The field is stronger there.

    The spacing of the lines shows the strength of the field. Close lines, such as near a pole, mean a strong field.

    • If you chose A: It is the opposite: where the lines bunch up, the field is stronger.
    • If you chose B: The arrow on each line shows the direction. How close the lines are shows only the strength.
    • If you chose D: At a neutral point the field is zero, and no line runs through it. Close lines mean a strong field.

  7. Inside a bar magnet, which way do the field lines run?

    Tap your answer.

    1. Yes, that is right. Field lines are closed loops. They run N to S outside the magnet, so inside they must run S to N to complete the loop.

    2. Not this one. That is the way they run outside. They must run the other way inside, or the lines would not join up. Try another.

    3. Not this one. There is a field inside a magnet. The lines go on through it and close up as loops. Try another.

    4. Not this one. The lines inside run along the magnet, not across it, from the S end to the N end. Try another.

    See the answer

    From the S pole to the N pole.

    Field lines are closed loops. They run N to S outside the magnet, so inside they must run S to N to complete the loop.

    • If you chose B: That is the way they run outside. They must run the other way inside, or the lines would not join up.
    • If you chose C: There is a field inside a magnet. The lines go on through it and close up as loops.
    • If you chose D: The lines inside run along the magnet, not across it, from the S end to the N end.

Keep it

Copy this into your exercise book.

Field lines run N to S outside a magnet and S to N inside: closed loops, never crossing. Close lines mean a strong field. Soft iron is an induced magnet. Steel is permanent.

Draw: A bar magnet with three loops of field line and arrows on them.

Reading with a child? A short page for the adult: what this chapter teaches and three questions to ask.

No internet tomorrow? Save the whole book as one file (913 KB). It opens on any phone.