Magnetic Field Shapes and Magnetic Field Strength, B

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Magnetic Field Shapes and Magnetic Field Strength, B

A magnetic field is a region in which a particle with magnetic properties experiences a force, and in which a moving charge experiences a force.

There are two main classes of magnet:

  1. Permanent magnets
  2. Electromagnets

Permanent Magnets:

Permanent magnets are common and are made of iron, cobalt or nickel alloys.

To represent the field around a magnet we use a diagram which needs to obey some rules (or conventions) so that whoever uses it can interpret it correctly.

Here is an example:

Magnetic Field Diagram

The points to note are:

We draw lines to represent magnetic fields. These lines are called lines of flux.

The arrow shows the direction of the force that a free north pole, for instance a North pole with no South pole (which doesn't exist!) would feel.

Field direction always goes from North to South. So pop a magnet at X in the field (see diagram) and it would align itself with its North pole pointing along the arrow.

The spacing between the lines of flux tells you about the strength of field - as the lines get closer together, the field becomes stronger - for example, near the poles.

Look at this field:

Magnetic Field Diagram

The region in between the poles shows equally spaced, parallel lines. This is called a uniform field. Field strength remains constant as you move around this area. Move out from the space between the poles and the field strength reduces. The lines of flux become further apart.

Temporary Magnets:

Around any conductor that has a current flowing through it there is a magnetic field. Switch off the current and the magnetic field disappears.

The shape of the field around a straight wire is shown below:

Magnetic Field Diagram

Note: The ⊗ means that conventional current is flowing through a wire into the page. (Think of an arrow - going away you see the flights, coming towards you see the point!)

Remember: Conventional current is the flow of positive charges. So conventional current goes in the opposite direction to the electron flow.

In a wire with conventional current flowing out of the page you get:

Magnetic Field Diagram

It's the same field shape as the one above, but the direction of the field is different.

Notice that in both cases the lines get further apart as you move away from the wire, this is because the magnetic field is getting weaker.

How do you remember which way the field goes (clockwise or anticlockwise)?

Answer: Use the corkscrew rule! The problem here is that as many of you are under 18 you won't have a clue what a corkscrew is... obviously! This will become easier once you're over 18 as you will be allowed to drink wine and will therefore have knowledge of a corkscrew.

Imagine you are screwing a corkscrew into or out of the page in the same direction as conventional current. The turning motion of the corkscrew is in the same direction as the field arrows need to be.

You can use the same idea to work out the shape of the field when the wire is coiled. Apply the corkscrew rule to different sections of the coil (below) and you should see that at all points, the field is to the right on the inside of the coil and to the left outside. In this example, we've cut a coil in half and are looking at it from the side - so the conventional current comes out of the page at the top and into the page at the bottom.

Magnetic Field Diagram

If you look at a long coil of wire (called a solenoid) the field shape becomes:

Magnetic Field Diagram

There is a uniform field inside the centre of the coil; outside the field is the same as the field around a bar magnet.

A quick way to work out the direction of the magnetic field in a solenoid is the right hand grip rule...

Make a fist and stick your thumb out (as if hitchhiking). Your fingers are wrapped in a circle, same as the coils in the solenoid. If you make your fingers point in the same direction as the conventional current around the coil - your thumb points towards the end of the solenoid that is the North pole.

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Magnetic Field Diagram

Neutral Points:

When two fields coincide they may cancel each other out and produce points where the magnetic field strength is zero. These points are called neutral points.

For Example:

Magnetic Field Diagram

Magnetic field strength is often called magnetic flux density and is given the symbol 'B' (obviously!?!).

Magnetic field strength is defined as the force acting per unit current in a wire of unit length, which is perpendicular to the field.

Magnetic field strength is measured in tesla, T.

A magnetic field has a strength of 1T if a wire of length 1 metre experiences a force of 1N when a current of 1A flows in the wire.

You can measure magnetic field with a current balance. Here is an example:

Magnetic Field Diagram

The rod is held in a fixed position, so when a force acts up or down on it due to the current in the wire between the magnets (see the 'motor effect'), the rod either:

  • remains still and the magnets push down on the scales, or

  • remains still and the magnets try to lift themselves up off the scales.

Either way, the reading on the scales will show how big this force is - so you have a measure of the force, F, due to the magnetic field and current.