The big idea: Switch on a fan or a cordless drill and something inside spins — a coil of wire carrying a current, sitting in a magnet's field, gets shoved sideways. That sideways push on a current-carrying wire is the motor effect.
The push is strongest when the current flows at right angles to the field, and there is no push when the current runs along the field.
The motor effect: a wire carrying a current I across a magnetic field B feels a push, the force F. The three directions — field B, current I and force F — are mutually perpendicular (at right angles to each other). Fleming's left-hand rule lines them up.
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Spot it: Three things meet at the wire and they are all at right angles to each other: the field B, the current I, and the force F.
Reverse the current or reverse the field and the force flips the other way.
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The size of the force on the wire depends on the field strength B, the current I, the length of wire in the field L, and the angle between the current and the field:
- force on the wire (N)
- magnetic field strength / flux density (T, tesla)
- current in the wire (A)
- length of wire in the field (m)
- angle between the current and the magnetic field
The most common case: Most questions set the wire at right angles to the field, so θ = 90° and sin θ = 1.
The equation then becomes the simpler F = BIL — that is the form you usually use.
When the wire is perpendicular to the field (sin θ = 1): F = B × I × L. Cover the one you want — letters side by side → multiply (F = B I L); one above the others → divide (e.g. B = F ÷ (I L)).
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A straight wire of length 0.25 m carries a current of 4.0 A at right angles to a magnetic field of strength 0.30 T. Find the force on the wire.
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How this is tested — the motor effect shows up two ways in exams:
Paper 1A
- Size: F = BIL — often rearranged to find B from how the force changes with the current.
- Direction: Fleming's left-hand rule for the force on a wire, rod or coil in a field.
Paper 2
- Extended F = BIL calculations — finding B, the current, or the length of wire in the field.
The classic trap: If the current runs parallel to the field (θ = 0°, sin 0° = 0) the force is zero — not maximum.
Fleming's left-hand rule: Hold the left hand with thumb and first two fingers at right angles:
- First finger → Field (B), from north to south. - SeCond finger → Current (I). - ThuMb → Motion / force (F).
Reverse the current or the field, and the force reverses.
A horizontal rod lies across two rails and carries a current flowing from left to right, with a magnetic field pointing straight out of the page toward you. (a) Use the left-hand rule to state the direction of the force on the rod. (b) A 0.20 m length of the same rod carries 5.0 A at right angles to the field and feels a force of 0.60 N. Find the magnetic field strength B.
Model answer plan
See the mark-by-mark plan — for / against / judgement, with marking guidance — in study mode.