The electric motor effect is what makes a motor spin. We can see it work on a single piece of copper wire.
Picture 3 Electric catapult effect.
Look at picture 3. It shows a loose piece of copper wire on some rails. The loose piece of wire is between the poles of a magnet. The rails are attached to a power supply. What will happen if we switch on the voltage?
The wire is catapulted to the right. The magnetic field made it move but only when there was an electric current in the wire.
Which way are they facing?
The magnetic field points from the north pole of the magnet to its south pole. Notice that the field is at right angles to the current. This arrangement produces the biggest force and makes the wire move out.
A current in a wire at right angles to a magnetic field produces a force on the wire.
Picture 4. Flemings Left Hand Motor Rule. Roll your cursor over each of the purple letters to see what they remind you of.
Which way does it move?
The wire moves at right angles to both the magnetic field and the current. We can remember which way it moves using Fleming's Left Hand Motor Rule. Arrange your left hand like the one in picture 6. The three digits represent the three quantities as shown in the table below.
Picture 5. Showing the forces on a coil.
Making it spin
We can understand a motor in the same way. As the current flows round the coil:
Together, these two forces make the coil turn on its axis.
When to swap the current
When the coil is in the upright position, there is no turning force trying to push it round. The two forces are trying to pull the two side of the coil outwards. It is at this point that the commutator swaps over the contacts.
If the coil was already spinning, its momentum will carry it through this upright position. When the contacts are reconnected, the commutator has reversed the current. So the side of the coil that was being pulled up before is now being pulled down. And vice versa.
Therefore the coil keeps spinning in the same direction.