Magnetic Flux Density (AQA A Level Physics): Revision Note

Magnetic fields

• A magnetic field is a region of space in which a magnetic pole will experience a force

• A magnetic field is created either by:

  • a moving electric charge

  • a permanent magnet

• 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 acts on magnetic materials, such as iron

Magnetic flux density

• The strength of a magnetic field can be described by the density of its flux lines

• The magnetic flux density $B$ of a field is defined as

The number of magnetic flux lines passing through a region of space per unit area

• Magnetic flux density is measured in teslas (T) 

• One tesla, 1 T, is defined as

The flux density that causes a force of 1 N on a 1 m wire carrying a current of 1 A at right angles to the flux

• The higher the flux density, the stronger the magnetic field i.e. regions where flux lines are closer together

• The lower the flux density, the weaker the magnetic field i.e. regions where flux lines are further apart 

Representing magnetic flux

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

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

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

• 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 flux lines shows the direction of the force that a free magnetic north pole would experience at that point

  • Flux lines are drawn closer together to represent where the field is stronger

  • Flux lines are drawn further apart to represent where the field is weaker

  • Magnetic flux lines never cross