10.2 The solenoid coil - its design and function

We have seen that a single current carrying wire produces a magnetic field of concentric lines of force.

This in itself is of little use, but, there are ways of increasing the magnetic field effect to produce something of use in many 'electromagnetic' applications.

Plot of the magnetic field created by a solenoid coil, with or without a soft iron core.

Left: The magnetic field produced by a solenoid coil, note the linear and denser concentration of the lines of force down the centre of the coil.

Right diagram above: A solenoid coil containing a soft iron core, around which is coiled insulated copper wire.

An effective solenoid needs to consist of hundreds of coils of finely wound insulated copper wire. 

Such an iron-cored solenoid coil can act as a temporary magnet as long as a d.c. current is flowing.

The principles of a functioning solenoid

If you coil the wire in a compact way (as in the diagrams above) you can greatly intensify the magnetic field effect.

The stretched out resulting current carrying coil is called a solenoid and can act as an electromagnet which can be switched on and off depending whether current is flowing or not i.e. acts as a temporary magnet.

You can 'construct' this magnetic field diagram using a plotting compass to map the magnetic field of a steel bar permanent magnet.

Inside the coils, the increase in field strength is due to all the lines of force lining up with each other and close together too - intensifying the magnetic field effect at what is effectively another north-south pole situation.

Remember - the closer the lines of force the greater the strength of the magnetic field at that point.

So, note the uniformity and intensity of the magnetic field inside the coil, which is much weaker outside the coil because lots of overlapping lines of force around each coil cancel each other out.

The magnetic field is overall weak except at the ends of the solenoid where it is very strong.

Note: The magnetic field pattern outside the solenoid is just the same as a bar magnet, with a north and south pole and the magnetic flux lines flowing from north to south.

You can work out the polarity of a solenoid by viewing the end of the solenoid and observing the way the current is flowing.

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