## 2008-11-05

### The force on a current-carrying conductor in a magnetic field

The force on a current-carrying conductor in a magnetic field

A moving coil loudspeaker is an application of the force acting on a current-carrying conductor in a magnetic field. When a varying electrical signal is sent to the coil, the coil is pushed in and out. This makes the cone vibrate, creating sound waves.

Magnetic Force

If we put two magnets near to each other, their magnetic fields will interact. Interact means that the magnets will experience forces on them as like poles will repel and unlike poles attract. It follows then that a wire in a field of a permanent magnet will experience a force when current flows through it. The magnetic field generated around the wire will interact with the field around the magnet. The two fields will produce a force.

Force produced by the combined magnetic field
“Catapult force”

The magnadur or slab-shaped magnets produce a uniform, parallel magnetic field around itself. When the two fields are combined, the pattern produced by iron filings indicates a complex field pattern, if free to move, it will be catapulted from the stronger field towards the weaker field or a neutral point.

The “catapult force” acts perpendicular to both the current and the magnetic field. If the wire carrying current is horizontal along the y-axis, and the magnetic field is horizontal along the x-axis, the force on the wire is vertical, up or down along the z-axis. Fleming’s left-hand rule neatly sums up this observation.

Fleming’s left-hand rule

Fleming’s left-hand rule says that if you hold the thumb and the first two fingers of your left hand at right angles, the thumb gives the direction of the force, the first finger shows the direction of the magnetic field (which is taken from north to south) while the centre finger points in the direction of the current (which is from the positive terminal to the negative terminal of the battery)

Turning force on a current-carrying coil in a magnetic field

Ammeter

The pointer of the ammeter is attached to a coil in a magnetic field. The higher the current through the meter, the farther the coil turns against the springs holding it. Thus the pointer moves farther along the scale, showing a bigger current.

Direct current motor

The catapult is used to make a simple electric motor. A wire is pushed in the opposite direction if the direction of the current through it is reversed.

The turning effect on a current-carrying coil in a magnetic field is used to make simple electric motors. This is why the turning effect is also known as the motor effect. The coil will turn in the opposite direction if the direction of the current through it is reversed. This principle is used in the electric drill to insert and remove screws when a coil is placed in a magnetic field, a magnetic force acts on each side of the coil. This produces a turning effect on a coil.

In a motor, the wire is wound around a central block called an armature. A spindle through the armature allows it to rotate. The current flows in opposite directions on each side of the armature, so one side is pushed while the other is pulled. This makes the armature to rotate.

After the coil has completed half a rotation, the current flowing on the side of the coil which is next to the north pole of the magnet is in the opposite direction to the first half of the coil switches direction. The same thing happens to the other side. As a result, the coil will start turning in the opposite direction producing a see-saw effect.

A commutator used in a DC motor to produce complete rotations of the coil. When the coil is in the upright position, there is no turning force trying to push it round. It is at this point that the commutator swaps over the contacts.

If the coil is already spinning, its inertia 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 pushed down and vice versa. As a result the coil keeps spinning in the same direction.

FACTORS AFFECTING THE SPEED OF ROTATION OF AN ELECTRIC MOTOR

The speed of rotation of the coil can be increased by:
- increasing the current
- using a stronger magnet
- increasing the number of turns on the coil