Electromagnetism

When the magnetic field of a wire interacts with the magnetic field produced by magnets, it creates a turning force. This turning force is used to drive electric motors which is used in loads of appliances, including washing machines, elevators, refrigerators and cars.

 
 

Magnetic fields in a wire

An electric current moving through a conductor, such as a copper wire, generates its own magnetic field - this is called electromagnetism. The strength of the magnetic field is strongest closer to the wire and increases if the current is increased. The right hand grip rule determines the direction of the magnetic field produced by a current-carrying wire. The thumb points along the direction of the current while the other fingers determine the direction of the magnetic field.

Electromagnets

So a wire can generate a magnetic field when electricity is passed through it. We can make the magnetic field generated by a wire even stronger by twisting the wire into a spiral shape - this device is called a solenoid. This creates a strong, uniform magnetic field inside the solenoid. Outside the coil, the small magnetic fields cancel out one another and the magnetic field is much weaker. Overall, the magnetic field of a solenoid resembles a bar magnet. We can increase the magnetic field by increasing the number of turns on the coil, or by placing an iron core inside the solenoid. The magnetic field can be turned off by stopping the electric current.

 
 

The Motor Effect

The magnetic field generated by a wire can interact with the magnetic field produced from two magnets. This causes a force on the wire, causing the wire to move in a particular direction. This is known as the motor effect. This happens because charged particles, such as the electrons in the wire, experience a force when placed in a magnetic field, as long as it is not moving parallel to the magnetic field lines.

The force on a wire in a magnetic field increases if the current in the wire increases or the strength of the magnetic field increases. The force is always greatest if the wire is placed at 90o (perpendicular to) the magnetic field. There will be no motor effect if the current is parallel to the magnetic field.

When a force is exerted on the wire, it will cause the wire to move in a particular direction. The direction moved can be predicted using Fleming’s left hand rule. To do this, place your first finger in line with the magnetic field, pointing north to south. Then take your second finger and place it in the direction of the electric current. Whichever way your thumb is pointing should tell you the direction the wire moves.

Electric Motors

An electric motor (also called a DC motor) is made up of a coil of wire in between two permanent magnets. Direct current runs through the wire and experiences a force (the motor effect) exerted on it by the magnetic field. This force causes the coil of wire to turn. For every half-turn of the coil, the split-ring commutator causes the current to switch direction, ensuring that a turning effect is experienced for every half turn which keeps the motor turning.

The speed of the motor can be increased by:

  1. Increasing the current

  2. Increasing the strength of the magnetic current

  3. Increasing the number of turns on the coil

Electric motors are really useful in any appliance which needs rotating parts. This includes fans, drills, washing machines and electric whisks.

Loudspeakers

Loudspeakers also rely on the motor effect to work - they consist of a coil of wire between two permanent magnets which is wrapped around the base of a cone. The coil of wire experiences a force, causing the coil to move back and forth. Since the coil of wire is connected to the cone, these movements cause the cone to vibrate. The vibrations push air molecules around the cone, producing sound waves.