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Magnetic forces on a current-carrying coil

A coil of wire (ABCD) is placed in a magnetic field created by two permanent magnets. Note that only the relevant poles are shown in the diagram.

Direct current is passed clockwise through the coil.

This image just shows one loop of the wire. In most situations, there would be several loops of the coil.
This image just shows one loop of the wire. In most situations, there would be several loops of the coil.

The wire from A to B experiences an upwards force by Fleming's left hand rule.

The current flowing through the wire from C to D is in the opposite direction so it experiences a downwards force.

These two equal and opposite forces create two anti-clockwise moments about the axis.

The coil rotates about the axis in the anti-clockwise direction.

The turning force on the coil can be increased by increasing:

  • the current in the coil
  • the number of turns on the coil
  • the strength of the magnetic field.

A d.c. motor uses the magnetic force on a coil carrying d.c. current to produce continuous rotational motion.

The motor consists of a coil of wire placed between two opposite poles.

A split-ring commutator is attached to the coil as shown in the diagram. This is a ring of metal that has been split into two.

The direction of the current has to be reversed each time the coil turns. If the direction of the current did not change, the coil would not keep rotating in the same direction continuously.

The split-ring commutator connects the coil to the power supply and means that each time the coil rotates half a turn, the current in the coil reverses direction.

Most motors also wind the coil onto a cylinder of soft iron (not shown in this diagram).

The iron is easily magnetised so it increases the magnetic field strength and produces a greater turning force on the coil.

The d.c. motor passes a direct current through a coil of wire to produce a continuous rotation motion.

A 2D representation of a d.c. motor
A 2D representation of a d.c. motor
  • The current on the left hand side of the coil flows out of the page.

    The left hand side of the coil experiences an upward force.

  • The current on the right hand side of the coil flows into the page.

    The right hand side of the coil experiences a downwards force.

  • This causes the coil to rotate clockwise.
  • When the coil is vertical, there is no turning force. However, the inertia of the coil keeps it rotating.
  • As the coil passes the vertical, the commutator switches the direction of the current to keep the coil rotating clockwise.