Misconception 1: "Use the right hand for the motor effect"
Use the LEFT hand for motors (force on a current in a field). The RIGHT hand is used for generators (or the right-hand rule for field direction around a wire). Remember: LEFT for force on conductor (motors), RIGHT for induced current (generators).
Misconception 2: "The motor would work fine without the commutator"
Without the commutator, the coil would rotate 90° to the vertical position (where forces are equal and opposite), then stop — or oscillate back and forth. The commutator reverses current every half turn, keeping the forces always pushing in the same rotational direction for continuous spinning.
Misconception 3: "Force is greatest when current is parallel to the field"
The opposite is true. Force is MAXIMUM when the current is at 90° (perpendicular) to the field. When the current is parallel to the field, the force is ZERO. This is why the motor coil has zero force at the 90° position (coil plane parallel to field) and maximum force when the coil is horizontal.
Quick Check: State the three variables that affect the size of the force on a current-carrying wire in a magnetic field.
Force F = B × I × L, so the three variables are: (1) magnetic flux density B (strength of field), (2) current I (size of current through wire), (3) length L (length of wire in the magnetic field). Increasing any of these increases the force.
This common misconceptions covers Common Misconceptions 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 7 of 12 in this topic. Use this common misconceptions to connect the idea to the wider topic before moving on to questions and flashcards.
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.