Magnetic Flux Density (CIE A Level Physics)

• A current-carrying conductor produces its own magnetic field
  • When interacting with an external magnetic field, it will experience a force

• A current-carrying conductor will only experience a force if the current through it is perpendicular to the direction of the magnetic field lines
• A simple situation would be a copper rod placed within a uniform magnetic field
• When current is passed through the copper rod, it experiences a force which makes it move

Force on a copper rod experiment setup

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

• The magnetic flux density B is defined as:

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

• Rearranging the equation for magnetic force on a wire, the magnetic flux density is defined by the equation:

• Where:
  • B = magnetic flux density (T)
  • F = force (N)
  • I = current (A)
  • L = length of the conductor in the field (m)

• Note: this equation is only relevant when the B field is perpendicular to the current
• Magnetic flux density is measured in units of tesla, which is defined as:

A straight conductor carrying a current of 1A normal to a magnetic field of flux density of 1 T with force per unit length of the conductor of 1 N m^-1

• 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