Electromagnetic Effects: Question 3

Worked solution

Part (a): Why the coil turns

The coil is a loop, so current flows along its two long sides in opposite directions to each other (in at one side, along, and back out the other).

Both sides sit in the same magnetic field, so by the motor effect (Fleming’s left-hand rule) each side experiences a force. Because the currents in the two sides point opposite ways while the field is the same, the forces on the two sides also point in opposite directions. For example, one side is pushed up and the other pushed down.

These two equal-sized, opposite forces act on opposite sides of the coil rather than along the same line, so together they form a couple. A couple produces a turning effect (torque), which is why the coil rotates about its axis.

Part (b): The split-ring commutator

As the coil spins, each side swaps from one pole to the other every half turn. If the current stayed flowing the same way through the coil, the force on each side would reverse direction every half turn, and past the point where the coil is vertical (in the plane at right angles to the field), the couple would act to turn the coil backward instead of continuing to push it round the same way.

The split-ring commutator prevents this: it reverses the direction of the current in the coil at exactly the moment the coil is vertical, each half turn. This reverses the force on each side at the same instant, so the couple keeps acting in the same rotational sense throughout, and the coil keeps spinning continuously in one direction.

If continuous slip rings (with no split) were used instead, the current in the coil would never reverse. The coil would turn correctly for the first half turn, but once past vertical the couple would reverse and act to turn it back the way it came. Instead of spinning continuously, the coil would simply oscillate back and forth around the vertical position.

Part (c): Increasing the turning effect

The turning effect (torque) on the coil can be increased by, for example:

• increasing the current flowing through the coil, or
• increasing the number of turns on the coil, or
• using a stronger magnet (or adding a soft-iron core inside the coil to strengthen the field).

Any two of these are valid, since each increases the force on the sides of the coil (or the number of turns each producing a force), increasing the total turning effect.

Final answers

• (a) The two sides carry current in opposite directions, so the motor effect gives them opposite forces, which form a couple that turns the coil.
• (b) The commutator reverses the current every half turn, keeping the turning effect in the same direction; with continuous slip rings the coil would oscillate instead of spinning continuously.
• (c) Any two of: increase the current; increase the number of turns; use a stronger magnet (or add a soft-iron core).