Electricity and magnetism, Year 8: electromagnets and circuits

An electromagnet

HintIt only works while the circuit is switched on.

The whyA current in a wire produces a magnetic field. Winding the wire into a coil, usually round an iron core, concentrates the field into a useful magnet.

Iron

HintIt is magnetic, but loses its magnetism as soon as the current stops.

The whyThe core becomes magnetised by the coil and makes the field far stronger. Iron is chosen because it does not stay magnetised afterwards.

More

HintEach loop of wire adds to the field.

The whyEvery turn of the coil carries the current round again and adds its own contribution, so more turns give a stronger magnet.

A bigger current

HintThink about what is flowing through the coil.

The whyThe magnetic field is produced by the current, so increasing the current increases the field.

It can be switched off to drop the load

HintThink about what has to happen at the end of the lift.

The whyA permanent magnet strong enough to lift a car could never let go of it. Cutting the current releases the load exactly where it is wanted.

A cell

HintSeveral of them joined together make a battery.

The whyThe long line is the positive terminal and the short thick line is the negative one. Two or more of the symbols in a row stand for a battery.

An ammeter

HintIt is always connected in series, in the loop itself.

The whyIt measures current, in amps. All of the current being measured has to flow through it, which is why it sits in the loop.

0.3 A

HintAsk whether charge can leave the loop anywhere.

The whyCurrent is a flow of charge, and charge is not used up. What the bulb takes from the circuit is energy, so the current is equal at every point in the loop.

A voltmeter

HintIt is named after the unit it reads in.

The whyIts symbol is a circle with a V inside. It compares the two sides of a component to find the push across it.

In parallel with the bulb

HintUnlike an ammeter, it is not placed in the main loop.

The whyIt needs one lead on each side of the component because it measures a difference between two points. It is added after the main circuit is built.