MAGNETISM
Permanent and induced magnets
Permanent magnets - produces its own magnetic field
Induced magnets - has to be inside a magnetic field in order to act as a magnet
force of repulsion - like poles repel
force of attraction - unlike poles attract
Generator effect
Magnetic fields
a region around a magnet where a force acts on another magnet or magnetic material
strength of the magnetic field depends on the distance
Electromagnets
When a current flows through a wire, a magnetic field is produced and this can be proved using a compass
use right hand grip rule to work out the direction of the magnetic field
Ways to increase magnetic field with a solenoid:
1 - increase current
2 - increase number of coils
3 - iron core (electromagnet)
Electromagnetic devices
a solenoid with an iron core
A relay (used to turn circuits on and off safely):
the low voltage circuit contains an electromagnet and is safe to be switched on and off
in the high voltage circuit, the switch is replaced with 2 metal contacts and one is connected to a spring to keep them apart
*When the low voltage circuit is switched on, a current flows so there is a magnetic field. this attracts the iron block and causes the contacts to close and switch on the high voltage circuit.
The electric motor
When a wire is placed in a magnetic field, a force is experienced on both sides : left side is upwards, right side is downwards
There is a moment on both sides, so the loop rotates clockwise but when it is at 90* it will stop rotating and this is fixed by switching the direction of the current
A split - ring commutator is used and it is connected to conducting brushes to allow the current to pass onto the ring. the current produces a turning force on the motor and makes it rotate clockwise instead.
The motor effect
When a magnetic field interacts with another magnetic field, causing the wire to experience a force in one direction
force = magnetic flux density x current x length
Flemings left hand rule is used to work out the direction of the force
If the conductor is parallel to the magnetic field, it WILL NOT experience a force
When the potential difference changes direction as it moves in and out of the magnetic field, but if we have a complete circuit then the current is induced
An induced current and potential is given when the wire is kept still but the magnetic field is moved
Loudspeakers and headphones
cone has a coil of wire wrapped around one end and is connected to an alternating current
a permanent magnet goes in the coil of wire
The MF from the coil interacts with the MF of the magnet and they attract/repel
This produces a resultant force which causes the cone to move
When the current switches direction, the direction of the force on the cone switches direction
This causes the cone to move in and out, generating soundwaves
*Changing the frequency of the AC supply, changes the frequency that the cone vibrates
high frequency = high pitch
