Representing Magnetic Fields (Cambridge (CIE) A Level Physics): Revision Note

Magnetic field definition

• A magnetic field is a field of force that is created either by:

  • moving electric charge

  • permanent magnets

• Permanent magnets are materials that produce a magnetic field

• A stationary charge will not produce a magnetic field

• A magnetic field is sometimes referred to as a B-field

• A magnetic field is created around a current-carrying wire due to the movement of electrons

• Although magnetic fields are invisible, they can be observed by the force that pulls on magnetic materials, such as iron or the movement of a needle in a plotting compass

Representing magnetic fields

• Like with electric fields, field lines are used to represent the direction and magnitude of a magnetic field

• In a magnetic field, field lines are always directed from the north pole to the south pole

The magnetic field lines around a bar magnet show the field is strongest at the two poles

• The simplest representation of magnetic field lines can be seen around bar magnets

  • These can be mapped using iron filings or plotting compasses

• The key aspects of drawing magnetic field lines are:

  • Arrows point out of a north pole and into a south pole

  • The direction of the field line shows the direction of the force that a free magnetic north pole would experience at that point

  • The field lines are stronger the closer the lines are together

  • The field lines are weaker the further apart the lines are

  • Magnetic field lines never cross

Magnetic field between two bar magnets

• When two bar magnets are pushed together, they either attract or repel each other:

  • Two like poles (north and north or south and south) repel each other

  • Two opposite poles (north and south) attract each other

Two opposite poles attract each other and two like poles repel each other

Uniform magnetic fields

• In a uniform magnetic field, the strength of the magnetic field is the same at all points

• This is represented by equally spaced parallel lines, just like electric fields

A uniform magnetic field has equally spaced field lines and is created when two opposite poles are held close together

The Earth's magnetic field

• On Earth, in the absence of any magnet or magnetic materials, a magnetic compass will always point north

• This is because the north pole of the compass is attracted to the Earth's magnetic south pole (which is the geographic north pole)

The Earth's magnetic field acts in a similar way to a bar magnet. A compass points to the Earth's magnetic south pole which is the geographic north pole