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Every magnet has a north and a south pole; like poles repel and unlike poles attract. Magnetic poles cannot be isolated: cut a magnet and each piece has both poles, all the way down to single atoms.

Iron, cobalt, nickel and gadolinium are ferromagnetic. Inside them are domains, small regions that act like tiny bar magnets. In an unmagnetised piece the domains point in random directions; an external field makes them align and grow, and heating and cooling or tapping can lock the alignment in. Above the Curie temperature (1043 K for iron) a ferromagnet cannot be magnetised at all.

Electric current produces magnetic fields; in fact moving charge is the source of all magnetism. A current-carrying coil is an electromagnet: a temporary magnet whose strength rises with current and number of turns, and which an iron core strengthens greatly.

Experiment: What controls an electromagnet's strength?

1. Wind 20 turns on the core. Connect the coil, ammeter and rheostat in series with the supply.
2. Set the current to 0.5 A. Count how many paper clips the core lifts. Switch off between readings.
3. Repeat at 1.0 A, 1.5 A and 2.0 A with 20 turns.
4. Now keep the current at 1.0 A and test 10, 20, 30 and 40 turns.
5. Plot clips lifted against current, and against number of turns. Describe the trends.
6. Remove the core and repeat one reading. Compare.

Current (A)	Turns	Clips lifted, run 1	Clips lifted, run 2	Mean
0.5
1.0
1.5
2.0
1.0 (10 turns)
1.0 (40 turns)

Questions

1. Explain, using domains, how a piece of iron becomes magnetised and why heating can demagnetise it.

2. Describe and explain your results for current and for number of turns.

3. Why is Earth's magnetic pole near the geographic North Pole called a south pole?

4. A map from 2000 gives a declination for your town. Why might it be wrong today?

Sources for every fact on this sheet: echoesof.science/school/magnetism/sources/

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