S-Cool Revision Summary

S-Cool Revision Summary

Magnetic Fields

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

Field Shapes

Permanent Magnets:

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

Magnetic fields can be represented by field lines, these lines are called lines of flux. These travel from North to South, i.e. in the direction an imaginary North pole would travel.

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.

You should be able to draw the shape of the magnetic field due to:

a bar magnet

a single wire

two wires near to each other

a single loop of wire

a solenoid.

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.

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

Right Hand Grip Rule

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.

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.

Magnetic field strength, B

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.

Ferrous Cores

Placing a ferrous core (i.e. iron or steel) in the solenoid will increase the magnetic field strength. (Iron more so than steel, although steel will stay magnetic when the coil is removed.) A magnet created by a current is an electromagnet and has the advantage of having a controllable field strength which will not reduce with time.

Relationship Between Current and Magnetic Field

The strength of a magnetic field is directly proportional to the current flowing.

B ∝ I

Therefore, if an alternating current is flowing, a magnetic field around the conductor will be produced, that is in phase with the alternating current.


Calculating Fields near Wires and Solenoids

You will already know that the magnetic field near a current gets weaker as you move further away from the current.

The magnetic field strength is inversely proportional to distance:

B µ Copyright S-cool

The magnetic field strength is proportional to current:

B µ I

Combining these relationships gives:

B µ Copyright S-cool so B = 'a constant' x Copyright S-cool

where the constant depends on the shape of the conductor.

Measuring Magnetic Field Strength

The strength of a magnetic field can be found using a device called the Hall probe. It works on a principle based on the Hall effect.