Electricity and magnetism, Year 8: electromagnets and circuits

An electromagnet

提示It only works while the circuit is switched on.

为什么A 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

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

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

More

提示Each loop of wire adds to the field.

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

A bigger current

提示Think about what is flowing through the coil.

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

It can be switched off to drop the load

提示Think about what has to happen at the end of the lift.

为什么A 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

提示Several of them joined together make a battery.

为什么The 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

提示It is always connected in series, in the loop itself.

为什么It 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

提示Ask whether charge can leave the loop anywhere.

为什么Current 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

提示It is named after the unit it reads in.

为什么Its 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

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

为什么It 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.