The electric motor

A simple electric motor can be built using a coil of wire that is free to rotate between two opposite magnetic poles. When an electric current flows through the coil, the coil experiences a force and moves.

The direction of the current must be reversed every half turn, otherwise the coil comes to a halt again. This is achieved using a conducting ring split in two, called a split ring or commutator. A coil of wire is used with lots of turns to increase the effect of the magnetic field.

A simple electric motor. A metal coil sits in a magnetic field and is connected to a cell via a commutator (also called a split ring). When current is applied the coil rotates.A simple electric motor

Speakers contain both a permanent magnet and an electromagnet. The two poles of the permanent magnet remain the same, but the poles of the electromagnet reverse when the direction of electric current reverses. For example, the end of the electromagnet which is its north pole will be attracted to the south pole of permanent magnet. This pulls the electromagnet towards the permanent magnet. When the electric current changes direction, the same end of the electromagnet becomes its south pole. The electromagnet is now repelled by the south pole of the permanent magnet, and moves away from it. The speaker cone is connected to the electromagnet, so it moves too. This creates vibrations in the air in front of the speaker, which is the sound we hear.

End of speaker cone is surrounded by a permanent magnet. A coil of wire which carries electrical signals is wrapped around the end of the cone. Movement generated causes the cone to vibrate.Cross section of a speaker

The left hand rule

Use Fleming’s left-hand rule to remember the direction of motion in an electric motor. It is shown in the following diagram.

The First finger points in the direction of the magnetic Field. The seCond finger points in the direction of the Current and is at right angle to the field. The thuMb points in the direction of the Motion. Remember to use your left hand, not your right!

Left hand, thumb, forefinger, second finger at 90 degrees with arrows. Thumb shows direction of movement. Forefinger shows field (north to south). Second finger shows current positive to negative.Fleming’s left hand rule
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