Magnets and magnetic field lines

A needle that points north

Float a small magnet on water, or hang it from a thread. It turns until one end points north. This end is its north pole (the north-seeking pole). The other end is its south pole. A compass is a small magnet on a pivot, and it works in just this way.

What we know about magnets

• Every magnet has two poles, a north pole and a south pole. If you break a magnet in two, each piece has two poles. You cannot get a north pole alone.
• Like poles repel each other, and unlike poles attract each other.
• A magnet attracts magnetic materials: iron, nickel and cobalt, and many of their alloys. It does not attract copper, wood or plastic.
• A freely hanging magnet always comes to rest along the north–south line, because the Earth itself acts like a huge bar magnet.

The magnetic field

A magnet pushes and pulls on other magnets and on pieces of iron that are near it, without touching them. The space around a magnet in which this force can be felt is called its magnetic field.

The field has a direction at every point. The direction is the one in which the north pole of a small compass needle points when it is placed there.

Magnetic field lines

To draw a field, we use field lines. Place a sheet of paper over a bar magnet, and sprinkle iron filings on it. Tap the paper. The filings arrange themselves in curved lines. They show the field lines.

The field lines of a magnet follow these rules:

1. Direction. Outside the magnet, the lines go from the north pole to the south pole. Inside the magnet, they go from the south pole back to the north pole. So the lines are closed curves.
2. Strength. The field is stronger where the lines are closer together. The lines are closest near the poles, so the field is strongest there.
3. No crossing. Two field lines never cross. If they did, a compass needle at the crossing point would have to point in two directions at once, and this is not possible.

The arrow on a line shows the direction of the force on a north pole.

You should find that the compass always lies along a field line, and that the field near a pole is many times the field at P.

Practise with new numbers

Try it in Python

The program draws the field lines of a bar magnet, with a north pole and a south pole.

Remember these

• A magnet has two poles. Like poles repel and unlike poles attract.
• The field lines go from north to south outside the magnet, and they form closed loops.
• The field is stronger where the lines are closer. Field lines never cross.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Two magnets are held with a north pole facing a south pole. Do they attract or repel?

2. Where is the field of a bar magnet the strongest, and how do the field lines show it?

3. Why can two field lines never cross each other?

4. A compass is placed just beyond the south pole of a bar magnet, on the line of the magnet. Which way does its north end point?