Electromagnetism · Experiment

Motor & Generator

The same machine, run both ways: feed a current through a loop in a magnetic field and it spins (motor); crank the loop by hand and it makes electricity (generator). One device, one physics — F = BIL and Faraday's law.

force F = N·B·I·L on each sideω(t) — motorEMF(t) — generator⊙ current out of page · ⊗ into page

Controls

Torque τ—
Angular velocity ω—
Steady ω—
Spin rate—
Spinning up: the commutator flips the current every half-turn, so the torque always pushes the same way.
About this experiment ›

What you are looking at

A rectangular coil (10 cm × 10 cm, N turns) sits between the poles of a magnet, pivoted on a horizontal axle. The two circled symbols are the coil's active sides seen end-on: ⊙ means current flowing out of the page, ⊗ into the page. The gold arrows are the magnetic forces on those sides, and the graph on the right records the machine's output over time.

Motor: current in, motion out

A wire of length L carrying current I across a field B feels a sideways force
F = B·I·L
The two sides of the loop carry current in opposite directions, so their forces point opposite ways — a couple that twists the loop. The torque depends on the loop's orientation θ (angle between the coil's normal and B):
τ = N·I·A·B·sin θ
Left alone, that torque would reverse every half-turn and the loop would just wobble. The split-ring commutator fixes this: it swaps the current direction through the coil exactly when the loop passes the flip point, so the torque always drives rotation the same way. With a little friction the spin settles where drive and drag balance, at
ω_steady = N·I·A·B·(2/π) / b
(2/π is the average of |sin θ| over a turn, and b is the friction coefficient).

Generator: motion in, current out

Crank the same loop at angular speed ω and the flux through it, Φ = N·B·A·cos ωt, keeps changing. Faraday's law turns that change into an EMF:
EMF = N·B·A·ω·sin(ωt)
With slip rings (each terminal stays on its own ring) the output keeps its natural alternation — pure AC. Note ω appears twice: cranking faster makes the sine both taller and faster. The peak is N·B·A·ω and the useful "RMS" value is that divided by √2.

One machine, two directions

Motor and generator are the same device run in opposite directions — the deep reason is that the motor effect (F = qv×B pushing charges sideways) and the generator effect (the same force pushing charges along the wire when you move it) are one and the same Lorentz force. Every real motor generates a "back-EMF" as it spins, and every generator feels a resisting torque when you draw current: energy conservation, enforced by the physics itself.

Things to try

In motor mode, double the current and watch the steady ω double — then watch the spin-up get snappier too. Switch to generator mode and crank slowly, then quickly: the EMF sine grows taller and squeezes together at the same time. Raise N or B in either mode and everything scales — τ and EMF are both proportional to N·B·A.