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Magnetic fields

Electricity and magnetism · level 6, lesson 3 of 6

A question to start

Sprinkle iron filings on a sheet of paper over a bar magnet and tap it. The filings line up into curved lines from end to end. What are they showing?

The idea

A magnet has a magnetic field around it. The iron filings line up along the field lines.

  • Outside the magnet, the lines run from the north pole round to the south pole.
  • They are closest together, so the field is strongest, near the poles.
  • A compass needle lines up along the field, its north end pointing along the lines.

The strength of a magnetic field, \(B\), is measured in tesla (T). A tesla is a large field, so we often use millitesla (mT, a thousandth) and microtesla (µT, a millionth).

Field Strength, about
Earth's field at the surface 24 to 66 µT
A fridge magnet, close up a few mT
A hospital MRI scanner 1.5 to 3 T
NS

Worked examples

1. Comparing. An MRI at 3 T against Earth's field at 50 µT: \(3/(50\times10^{-6}) = 60{,}000\) times stronger.

2. Reading the lines. Two magnets with north poles facing: the lines from each turn away from the other, leaving a gap with almost no field in the middle, where they push apart.

3. A compass near a magnet. Close to a fridge magnet, its field of a few mT is about a hundred times Earth's, so a compass points to the magnet.

Try it

  1. Which way do field lines run outside a magnet?
  2. Where is a bar magnet's field strongest?
  3. How many microtesla is 2 mT?
  4. Why does a compass near a magnet stop pointing north?
Answers
  1. From the north pole round to the south pole.
  2. Near the poles, where the lines are closest.
  3. 2,000 µT.
  4. The magnet's field there is far stronger than Earth's.

Common mistake

"Field lines start at north and end at south." Outside the magnet they run north to south, but they carry on through the magnet from south back to north. Magnetic field lines always make closed loops.

Where it leads