A solenoid is a long coil of wire. The word originates from Greek sōlēnoeidēs meaning 'pipe-shaped' which comes from the Greek for pipe: sōlēn. It is pipe shaped!

It's field pattern is like that of a bar magnet, the only difference is that you must extend the field lines through the centre of the coil... they therefore form complete loops instead of starting and ending on 'a pole'.

It is a really useful piece of equipment. It is a magnet that you can turn 'off' and 'on' whenever you want to!

In class you may come across one like this - designed to allow you to see the field pattern within the coil:

Practical solenoids have many turns on them (and you cannot see inside of them!). Many pieces of electrical equipment have solenoids within them. A solenoid with a soft iron core inside it is a powerful magnet that you can switch 'off' or 'on'.

Hints on drawing a diagram for a solenoid

Field lines are virtually parallel through the centre of the coil.

Field lines never meet or cross.

Don't draw too many or you'll find it difficult to keep the diagram symmetrical and correct.

Draw field lines in a different colour from the wiring.

Ensure the circuit is complete. (If the switch in your diagram is open no current would flow and there would be no magnetic field!)

Don't forget to put in the current direction and then find out which end of the coil acts like which pole of a magnet. Mark these 'poles' clearly on your diagram. .

Finally put in the field line direction.

You can take part in an Animated Java Tutorial on this by clicking here.

The solenoid acts like an electromagnet

The strength of the field depends upon:

the current - the larger the current the stronger the field,

the number of turns - the larger the number of turns (windings), the stronger the field

the presence of a soft iron core. This increases the strength of the field substantially as the core becomes magnetised in a way that reinforces the field from the coil. The domains line up when the current is switched on to make the overall field stronger.

Remember that the stronger the field is the closer the field lines will be when represented on a diagram..

A couple more points to remember - they are favourites for examiners to ask you about!

The core must be soft otherwise when the current is switched off the core would still be magnetised. A soft iron core will demagnetise as soon as the field from the coil is removed - as soon as the current is switched off.

A d.c. power source (e.g. a battery) must be used so that the current only flows in one direction. If an a.c. current (mains supply) is used the direction that current flows changes so many times in a second that the domains in the core do not have time to line up in one direction before they are pulled into the opposite direction. This results in a random arrangement of domains and a net zero magnetic field in the core.

Making an electromagnet

See the electromagnet experiment

Uses of the electromagnet

To pick up ferromagnetic materials in a scrap yard. Attached to a crane it is useful to pick up scrap iron and steel. There is no need for careful positioning of a hook and you simply switch the current off to drop scrap (disentangling a hook can be tedious).

To sort ferromagnetic materials from non-magnetic materials such as aluminium in a scrap yard.

In circuits like the electric bell.