8.2.2 Turning Effect of a Current Carrying Coil in a Magnetic Field

1. If a current carrying coil is placed in a magnetic field (As shown in diagram above), a pair of forces will be produced on the coil. This is due to the interaction of the magnetic field of the permanent magnet and the magnetic filed of the current carrying coil.
2. The diagram below shows the catapult field produced.

3. The direction of the force can be determined by Fleming's left hand rule.
4. Since the current in both sides of the coil flow in opposite direction, the forces produced are also in opposite direction. The 2 forces in opposite direction constitute a couple which produces a turning effect to make the coil rotate.
5. Examples of electric equipment whose operation is based on this turning effect are
   1. the direct current motor
   2. the moving coil meter.

Moving Coil Meter

Light Indicator

A light indicator which has lower inertia  is used to increase the sensitivity of the meter.

Linear Scale

1. Due to the radial magnetic field and the cylindrical soft-iron core, a linear scale is produced.
2. A linear scale is more accurate and easier to be read.

Mirror

1. A mirror is used to prevent parallax error.
2. When the observer's eye is exactly above the indicator, the indicator will cover its own image on the mirror. 
3. This can used to prevent parallax error.

Curved Permanent Magnet

1. A curved permanent magnet is used to produce a radial field.
2. A radial field is a magnetic field where the field lines are either pointing away or toward the center of the field.
3. A radial can be focused by a cylindrical soft-iron core.

Rectangular Coils

1. When a current flows through the coils, a force will be generated due to the interaction between the magnetic field of the permanent magnet and the coil.
2. The force will turn the coils, which in turn move the indicator.

Cylindrical Soft-Iron Core

1. A cylindrical soft iron core is placed inside the radial field produced by the curved magnet.
2. A soft-iron core can focus the magnetic field of the permanent magnet.

Hair Spring

1. The deflection of the coil and the indicator stops when the force is balanced by the opposing force from the hair spring.
2. The angle of deflection is directly proportional to the magnitude of the current in the coil.

Loud Speaker

1. The loud speaker contains a cylindrical coil which is free to move in a radial magnetic field set up by a strong cylindrical permanent magnet.
2. The magnet has a central South Pole and a surrounding North Pole. The field lines are therefore radial and at right angles to the turns of the-coil.
3. When varying the current flows through the coil, a force of varying magnitudes will act on the coil. This will cause the coil to move to and fro according to the magnitude of the force.
4. The paper cone then vibrates to produce sound waves.