Magnetic fields: (f) magnetic flux density; the unit tesla.

Magnetic flux density B describes the strength of a magnetic field: it is the force per unit current per unit length on a conductor placed at right angles to the field, so B = F / (IL). The SI unit is the tesla (T). One tesla is the flux density that produces a force of 1 newton on a 1 metre length of conductor carrying a current of 1 ampere perpendicular to the field, so 1 T = 1 N A⁻¹ m⁻¹. For example, a field of 0.50 T acting on a 2.0 m wire carrying 4.0 A at 90° gives F = 4.0 × 2.0 × 0.50 = 4.0 N. Flux density is a vector quantity, with direction given by the field lines. A strong answer earns marks for these points. Define magnetic flux density as the force per unit current per unit length on a conductor perpendicular to the field, B = F / (IL). State that the SI unit is the tesla (T) and express it as 1 T = 1 N A⁻¹ m⁻¹. Explain that flux density is a vector quantity whose direction is the direction of the magnetic field lines. Use B = F / (IL) to calculate flux density or force, ensuring the conductor is perpendicular to the field. A common slip: Confusing magnetic flux density with magnetic flux. Instead, flux density B is force per unit current per unit length, while flux is the product of flux density and area. Exam tip: Learn the defining equation B = F / (IL) and the unit equivalence 1 T = 1 N A⁻¹ m⁻¹.

Some boxes have lost their names. Pick the right name for each number.

• 1Box 1

  1 T = 1 N A⁻¹ m⁻¹Direction given by the field linesForce per unit current per unit length on a conductor perpendicular to the field

• 2Box 2

  1 T = 1 N A⁻¹ m⁻¹Direction given by the field linesForce per unit current per unit length on a conductor perpendicular to the field

• 3Box 3

  1 T = 1 N A⁻¹ m⁻¹Direction given by the field linesForce per unit current per unit length on a conductor perpendicular to the field