Magnetic Flux Density (Cambridge (CIE) A Level Physics): Revision Note

Force on a current-carrying conductor

• A current-carrying conductor produces its own magnetic field

  • When interacting with an external magnetic field, it will experience a force

• The force due to a magnetic field can be observed by

  • placing a copper rod in a uniform magnetic field

  • connecting the copper rod to a circuit

• When current is passed through the copper rod, it experiences a force

  • This causes it to accelerate in the direction of the force

Observing the force on a current-carrying conductor

A copper rod moves within a magnetic field when current is passed through it

Magnetic flux density definition

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

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

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

• This is described by the magnetic flux density B of a field, which is defined as:

  The force exerted per unit current per unit length on a straight current-carrying conductor placed perpendicular to the magnetic field

• 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 field

• To put this into perspective, the Earth's magnetic flux density is around 0.032 mT and an ordinary fridge magnet is around 5 mT