When a wire carrying a current sits in a magnetic field, it experiences a force. The direction of that force is at right angles to both the current and the field, and can be found with Fleming's left-hand rule.
In a motor, a coil is placed between the poles of a magnet. Current flows one way along the top side of the coil and the opposite way along the bottom side, so the two sides feel forces in opposite directions, one pushed up and the other pushed down. This pair of forces creates a turning effect that spins the coil.
A part called the commutator reverses the current in the coil every half turn, so the push always keeps it rotating the same way instead of stopping halfway. This is the principle behind the motors in an LRT train, a fan or a blender.
In SPM you should explain motor action using the force on a current-carrying conductor and the role of the commutator.
Common misconceptions
- The motor turns because the magnets attract the coil -> It turns because of the force on the current-carrying coil, opposite on each side, giving a turning effect.
- The commutator supplies the current -> The commutator reverses the current each half turn so rotation continues in one direction.
- The force is along the current direction -> The force is at right angles to both the current and the magnetic field.
The physics behind it
A straight wire carrying a current in a magnetic field experiences a force. Its size is F = BIL, where B is the magnetic flux density in tesla (T), I is the current in amperes (A) and L is the length of wire in the field in metres (m); the force is in newtons (N).
Its direction is at right angles to both the current and the field, given by Fleming's left-hand rule. For a wire with B = 0.4 T, I = 2 A and L = 0.05 m, the force is F = BIL = 0.4 T × 2 A × 0.05 m = 0.04 N.
In a motor a rectangular coil sits between magnet poles. Current runs one way along the top side and the opposite way along the bottom side, so the two sides feel forces in opposite directions, one pushed up and one pushed down.
This pair of opposite forces produces a turning effect (a couple) that rotates the coil. A larger current, a stronger magnet or more turns of wire all increase the turning effect.
See it in daily life
Electric motors are everywhere once you look. A ceiling fan, a food blender, a washing machine drum, the windscreen wipers and power windows in a car, and the traction motors that drive a KL LRT train all turn because a current-carrying coil is forced to rotate in a magnetic field.
In each case electrical energy is converted into rotational kinetic energy.
A small toy car motor works in exactly the same way as a large industrial one, just with a weaker magnet and fewer turns, so it produces a smaller turning effect. You can even feel the principle in reverse: spin the shaft of a small motor by hand and it acts as a generator, lighting a small bulb.
The same interaction between a current and a magnetic field that spins a fan is what lets electricity do mechanical work throughout a modern home.
How this comes up in SPM
This belongs to the Form 5 Electromagnetism chapter, under the force on a current-carrying conductor in a magnetic field. Paper 2 asks you to State the factors that affect the force (current, flux density and length of conductor) and to use Fleming's left-hand rule to Determine the direction of the force, current or field.
You are often asked to Explain how a direct-current motor turns, describing the opposite forces on the two sides of the coil and the turning effect.
A key point examiners look for is the role of the commutator with its brushes: Explain how it reverses the current in the coil every half turn so the coil keeps rotating in the same direction. You may also State how to increase the speed of the motor.
Neighbouring standards include the magnetic field of a current, electromagnets and electromagnetic induction. Command words such as State, Explain, Determine and Describe recur, so link each effect to the force on the conductor.
Source: DSKP KSSM Physics Form 4 and 5 (Versi English) (Bahagian Pembangunan Kurikulum (BPK), KPM)