How an electric motor turns

A wire's current runs east. The magnetic field points straight down. Which way does the force push?

Fans, drills, blenders, electric cars: each one has a motor, and every motor is built on the same idea. A current-carrying coil between magnet poles, spinning. The motor effect puts a force on a straight wire. How does that force become a spin?

A rectangular coil of wire sits between magnet poles. The field runs from the north pole across to the south pole: here, toward the east. The coil's west side and east side both cross the field. A current flows around the coil.

Follow the current around the loop. Along the west side it flows north. Along the east side it flows back south.

Both sides sit in the same eastward field.

Fleming's left-hand rule for the west side: first finger east, along the field; second finger north, along the current. The thumb points down, so the west side is pushed down.

Fleming's left-hand rule for the east side: first finger east again; second finger south, along the current. The thumb points up, so the east side is pushed up.

Opposite currents, opposite forces: the coil turns.

The two sides carry current in opposite directions, so they feel forces in opposite directions.

One side is pushed down while the other is pushed up, so the coil turns.

That is how an electric motor turns: opposite currents on the coil's two sides give opposite forces, and the coil spins.

A motor coil's two sides carry current across the same field. How do the forces on the two sides compare?

In a drill's motor, why do the coil's two sides feel oppositely directed forces?

In a classroom motor, the coil's near side is pushed up and its far side is pushed down. The current is then reversed. What happens to the two forces?

Reversing the current reverses the force on each side: Fleming's left-hand rule.

The near side's force now points down; the far side's force now points up.

Both forces reverse, so the coil spins the other way.

The coil turns with the current, and with the current's direction.

Ravi says that if both sides were pushed the same way, the coil would not turn. Is Ravi correct?

Your turn

Why does an electric motor's coil spin?

In a motor, the coil's west side is pushed down. Which way is the east side pushed?

Four coils sit between magnet poles. Which coil turns?