Unit 2 — Electricity and Magnetism

Science — Third Preparatory

3.2 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:

increasing the number of turns of the coil;
increasing the current intensity (reduce rheostat resistance, or add cells in series);
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.

Comparison aspect	Permanent magnet	Electromagnet
Magnetism	Always present	Present only while current flows
Strength control	Cannot be controlled	Can be changed by turns, current and core
Example of use	Compass needle, fridge door	Crane lifting scrap, 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.

Unit 2 — Electricity and Magnetism · 16 / 20