In the previous topic you saw something surprising: a wire carrying current (, measured in ampere, A) makes its own magnetic field in the space around it. Now flip the situation. What if a current-carrying wire sits inside a magnetic field that is already there? Today you will find that the wire gets pushed. And once you can predict which way it is pushed, you understand the exact thing that spins the motor in a ceiling fan.
1. Present the physical scene
Picture this on your school lab bench. A strong U-shaped horseshoe magnet lies on the table, its two poles facing each other with a gap in between. Across that gap, resting loosely, is a short, light aluminium rod, connected by flexible wires to a cell. The rod is free to roll.
Two quick words before we go on. A conductor here just means the rod — a metal that lets charge flow through it. A magnetic field is the invisible region of magnetic influence between the two poles; we write its strength as and measure it in tesla (T). The current through the rod is , in ampere (A).
Right now the cell is off. No current. The rod just sits there.
Stop scrolling. Try it in your head before reading on. The moment you switch the cell on and current flows through the rod, what do you think the rod does — stay still, get hot, or move?
(Answer: it suddenly jumps — the rod rolls and shoots out of the gap between the poles. Not because it got hot, not because the magnet pulls iron, but because a current-carrying conductor in a magnetic field feels a real, mechanical force — a push. Switch the current off and the push vanishes.)
You can now name today's effect: a wire carrying current, placed in a magnetic field, gets pushed by a force.