Fleming's left-hand rule (HT only): force, F, in newtons, N

The idea

This Higher Tier (HT) only statement identifies the quantity F in the equation F = B I l as force, measured in newtons (N). Force is a vector, so its direction matters as well as its size; in this context it acts on the current-carrying conductor and is perpendicular to both the magnetic field and the current. One newton is the force that gives a 1 kg mass an acceleration of 1 m/s². In calculations, F is found by multiplying magnetic flux density B in tesla, current I in amperes and length l in metres.

For example: 0.50 T × 3.0 A × 0.20 m gives 0.30 N. Fleming's left-hand rule gives the direction of this force, with the thumb indicating the force direction.

What earns the marks

• State that F represents force and that its SI unit is the newton (N).
• Recognise that force is a vector quantity, so both magnitude and direction are needed.
• Link the unit N to the equation F = B I l, where tesla × ampere × metre gives newtons.
• Use Fleming's left-hand rule to establish the direction of the force on the conductor.
• Quote calculated force values with the unit N and an appropriate number of significant figures.

Common mistakes and how to put them right

• Treating force as a scalar and giving only a number without direction.

  Put it right by stating the direction using Fleming's left-hand rule.

• Writing the unit as N m or N/m.

  Put it right by remembering that force is measured simply in newtons, N.

• Confusing force with magnetic flux density.

  Put it right by noting that B is measured in tesla while F is measured in newtons.

Exam tips

• Always include the unit N with a calculated force value.
• When asked for direction, refer explicitly to Fleming's left-hand rule and name the relevant fingers.
• Remember that questions involving F = B I l and Fleming's left-hand rule will only appear on Higher Tier papers.