IGCSE Physics / Electric motors and electromagnetic induction
The motor effect revision notes
Edexcel International GCSE Physics (4PH1), chapter 21: Electric motors and electromagnetic induction, specification point 6.12, 6.14. Free lesson.
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The motor effect is the force on a wire carrying a current in a magnetic field. It happens because the magnetic field made by the current interacts with the magnet’s field.
How the wire feels a force
A current in a wire makes a magnetic field that circles the wire. When the wire is placed in a magnet’s field, the two fields overlap. On one side they add together, making a stronger field. On the other side they partly cancel, making a weaker field. The wire is pushed from the strong side towards the weak side. This is the motor effect. Between two flat north and south poles, the field can be uniform: its strength and direction are the same throughout the region. The force is caused by the interaction of the two fields, not by the wire cutting through field lines.
What changes the size and direction of the force
The force’s size and direction depend on the current, the magnetic field and the angle between the wire and the field. The force always acts at right angles to both the wire and the magnetic field. A bigger current gives a bigger force. Reversing the current reverses the force. A stronger magnetic field gives a bigger force. Swapping the poles reverses the field and the force. Reversing both the current and the field leaves the force acting in the same direction as before. The force is biggest when the wire is at 90° to the field, and zero when the wire is parallel to it.
Why cutting field lines is not the explanation
The wire is not moving to begin with, so it cannot be moving through the field and cutting field lines to cause the force. Cutting field lines is associated with induction, which makes a voltage. For the motor effect, it is the current’s magnetic field interacting with the magnet’s field that produces the force.
How to change the force
For a wire already carrying a current at right angles to a uniform magnetic field, you can increase the force by changing either the current or the strength of the field. You can reverse the force by reversing just one of them.