Motor Maker

Every electric motor, from a toy car to a train, works the same way. Send a current through a coil between two magnets and watch it spin.

Speed
0.0 rev/s
Current
0.00 A
Force per side
0.00 N
Back-EMF
0.00 V

The maths, live

Current I = (V − back-EMF) ÷ RI = (3.0 − 0.00) ÷ 1.10 = 0.00 A
Force on a side F = N × B × I × LF = 20 × 0.10 × 0.00 × 0.1 = 0.00 N
Turning effect τ = N × B × I × A × cos θτ = 20 × 0.10 × 0.00 × 0.008 × 1.00 = 0.0 mN·m
Back-EMF ε = N × B × A × ω × cos θε = 20 × 0.10 × 0.008 × 0.0 × 1.00 = 0.00 V

Use Fleming's left-hand rule: First finger = Field, seCond finger = Current, thuMb = Motion. Why does the coil need the split ring to keep turning?

How it works

A wire carrying a current across a magnetic field feels a force: F = B × I × L. Fleming's left-hand rule gives its direction. In a coil, the current goes one way along one side and the other way along the other side, so one side is pushed up and the other is pushed down, and the coil turns.

After half a turn the forces would push the coil back again, so a split-ring commutator swaps the current's direction every half turn and keeps it spinning the same way. As the coil spins it also makes a back-EMF, a voltage that fights the battery. That and friction stop it speeding up forever, so it settles at a steady speed.

Try this

  1. Reverse the current. Which way does the coil spin now?
  2. Which slider makes the biggest difference to the speed?
  3. Turn the voltage down until the motor stops. Can a flick get it going again?