Physics 12.10 - 12.12 - Motor effect and force direction
A current-carrying wire and a magnet exert forces on one another through their magnetic fields. In this Higher-tier lesson, use Fleming’s left-hand rule to predict the force direction and distinguish the equal and opposite forces on the two objects.
The motor effect and interacting fields
Motor effect
The motor effect is the force experienced by a current-carrying conductor when it is in a magnetic field.
Take the system to be a straight conducting wire and a nearby permanent magnet. A current in the wire produces a magnetic field around the wire. This field overlaps with the permanent magnet's field, and the two magnetic fields interact. The interaction causes a force on the wire.
A simple observation makes the model testable. If a wire is placed in the field between two magnet poles and connected briefly to a low-voltage supply, the wire can kick sideways when the current is switched on. Reversing the current reverses the observed kick. This is evidence of a force; Fleming's rule, taught in the next part, predicts its direction.
The interaction involves both objects:
- the magnet exerts a force on the current-carrying conductor;
- the conductor exerts an equal-magnitude force in the opposite direction on the magnet.
The two forces do not cancel because they act on different objects. If the magnet is clamped, its support prevents visible movement, but the force on it still exists.
Fleming's left-hand rule
For the cases in this lesson, three directions are mutually perpendicular: the magnetic field, the conventional current and the force on the conductor. Fleming's left-hand rule links them.
Hold the left hand so that the thumb, first finger and second finger point at right angles to one another:
Continue this lesson.
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