Magnetic Flux Density (AQA A Level Physics)

• A magnetic field is a region of space in which a magnetic pole will experience a force
• A magnetic field 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 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 magnetic flux lines around a bar magnet show the flux density is strongest at the two poles

• 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