Physics · Moving Charges And Magnetism · NEET
They are the same physics. A single moving charge feels F = qv x B. A wire is just many charges drifting together. Add up qv x B for all the charges in a length L and you get F = IL x B. Use F = qvB for one particle, and F = BIL sinθ for a whole current-carrying wire.
Use Fleming's left-hand rule: point the first finger along B (field), the middle finger along I (current), and the thumb gives F (force). Or use the vector cross product IL x B directly. The force is always perpendicular to both the current and the field, so it pushes the wire sideways, never along itself.
F = BIL sinθ. When the current is parallel (θ = 0) or anti-parallel (θ = 180 degrees) to B, sinθ = 0, so F = 0. The cross product of two parallel vectors is zero. The force is largest when the wire is perpendicular to B (θ = 90 degrees), giving F = BIL.
Yes. L is a vector: its length equals the wire length and its direction is the direction of conventional current flow. That is why the cross product IL x B automatically gives both the size and the direction of the force. Always draw the current arrow first.
For a UNIFORM field, only the straight vector L joining the two ends matters, not the wiggly path. A bent or curved wire in uniform B feels the same force as a straight wire connecting its endpoints, because the field is constant and the vector segments add up head-to-tail.
A long straight wire of length 2 m and mass 250 g is suspended horizontally in a uniform horizontal magnetic field of 0.7 T. The amount of current flowing through the wire will be (g = 9.8 m/s^2)
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Newton (N). B is in tesla (T), I in ampere (A), and L in metre (m). One tesla equals one N/(A.m), so T x A x m = N.
When the wire is perpendicular to the magnetic field (θ = 90 degrees), because sin 90 = 1. Then F = BIL, the largest possible value.
The force reverses direction too. Reversing I flips the vector IL, so IL x B points the opposite way. The magnitude stays the same.
The magnetic force is always perpendicular to the wire and to B, so it cannot change the wire's speed by itself, but it can push a free wire and cause mechanical motion. This sideways push is what runs electric motors and moves galvanometer coils.
Each wire sits in the magnetic field made by the other. So each wire feels F = BIL from the neighbour's field. Parallel currents attract, opposite currents repel — this defines the ampere.
