2 The electromagnet

• An electromagnet consists of an insulated copper wire wound as a coil around a soft iron core.
• When current flows in the coil, a magnetic field is produced which magnetises the soft iron, and the whole device acts as a strong magnet.

Activity.

• Wind insulated copper wire evenly around a long soft-iron nail.
• Connect the ends in series with a battery, switch, rheostat and ammeter.
• Hold the nail above iron paper clips.
• Close the circuit: clips are attracted.
• Open it: the clips fall.
• So an electromagnet is a temporary magnet: it can be switched on and off by the current.
• A permanent magnet cannot be switched off.
• That is why a crane uses an electromagnet to pick up iron scrap and release it.

Factors affecting the strength of an electromagnet.

• Use the number of paper clips lifted as the indicator.
• Change one factor at a time (independent variable), measure the number of clips (dependent variable), and keep the others constant (controlled variables).

The strength increases with:

1. increasing the number of turns of the coil;
2. increasing the current intensity (reduce rheostat resistance, or add cells in series);
3. using a soft iron core (it is easily magnetised and loses its magnetism easily when the current stops; steel would stay magnetised).

Structure and function. The core is soft iron because it magnetises easily and demagnetises easily, which is what makes the electromagnet a temporary magnet.

(Both attract iron; they have north and south poles.)

Application: electric bell.

• When the button is pressed, current flows and the electromagnet magnetises its soft iron core and attracts a flexible metal strip, which brings the hammer against the gong.
• When the strip moves, it breaks the circuit at the contact screw, so the current stops and the electromagnet loses its magnetism.
• The strip springs back (elastic), closing the circuit again, and the cycle repeats rapidly, giving a continuous ring.

Application: sorting metal waste.

• Electromagnets separate iron and steel from mixed waste for recycling.
• Recycling metal needs far less energy than making new metal from ore.