6 · IGCSE Physics
The motor effect
Choose the directions of the magnetic field and the current and see the force on a wire predicted by Fleming's left-hand rule. Switch to a simple DC motor to see how a split-ring commutator keeps the coil turning.
Fleming's left-hand rule: First finger = Field (N → S), seCond finger = Current, thuMb = Motion (force).
F = B I L
Try this
- In Straight wire view, reverse the current, then the field, then both. Predict the force direction with your left hand before you look.
- Double B, then I, then L, and check that the force F = BIL doubles each time.
- In DC motor view, set the commutator to None and explain why the coil only rocks; switch it back and read the torque as the coil passes the vertical.
Exam tip: Always use the left hand for the motor effect, and say that the force is largest when the current is at 90° to the field and zero when it is parallel to it.