The force on a current-carrying conductor is given by F = BIL:
- F = force in Newtons (N)
- B = magnetic flux density in Tesla (T)
- I = current in Amperes (A)
- L = length of conductor in the field in metres (m)
This formula assumes the conductor is perpendicular to the field. If the conductor is at an angle θ to the field, the force becomes F = BIL sin(θ) — maximum at 90°, zero at 0°.
Worked example: A horizontal wire carrying 5 A lies in a vertical magnetic field of 0.4 T. The wire is 0.3 m long. Find the force: F = 0.4 × 5 × 0.3 = 0.6 N. Direction found using left-hand rule.
Motor torque: In a real motor, two sides of the coil experience equal but opposite forces — creating a turning effect (torque). More turns = more wire in the field = larger force = larger torque = faster or more powerful motor.
This higher tier covers Higher Tier: Force Calculation and Direction within The Motor Effect for GCSE Physics. Revise The Motor Effect in Magnetism for GCSE Physics with 19 exam-style questions and 12 flashcards. This topic appears regularly enough that it should still be part of a steady revision cycle. It is section 9 of 12 in this topic. This section is most useful once the core foundation idea is secure, because it adds the detail that pushes answers higher.
Practice questions for The Motor Effect
What is the motor effect?
Explain how Fleming's left-hand rule is used to find the direction of the force on a current-carrying conductor in a magnetic field.