Electromagnetism

The Motor Effect · F = BIL

Drag the current-carrying rod around the magnetic field. A force F = BIL acts on it, perpendicular to both the current and the field. The green arrow from the rod's centre shows that force — its length is proportional to the field strength, the current, and the rod's length.

Controls

Field strength B T

Sign sets direction: positive = into screen (⊗), negative = out (⊙).

Current I A

Magnitude of the current flowing through the rod.

Field region radius

Size of the circular magnetic field. The wire feels a force only where it passes through this region, so a bigger field means a longer effective L.

Current: → Wire: horizontal

Field

⊗ in

Effective L

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Practice question

Press “New question” to begin.

Force F = BIL ⊗ field into screen ⊙ field out of screen

Only the length of wire crossing the circular field feels a force, so growing the field increases the effective L. Press space to pause.

The physics behind it

A wire carrying current I in a magnetic field B feels a force F = BIL, where L is the length of wire in the field. The direction is given by Fleming's left-hand rule — it is perpendicular to both the current and the field. Here the field points into or out of the screen and the current runs along the rod, so the force pushes the rod sideways. The force does not depend on how fast you drag the rod — only on B, I and L — so the green arrow stays constant as you move it, changing only when you adjust a slider, flip the current, or reverse the field. This is the motor effect that makes electric motors turn.