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Topic 7 - Magnetism and Electromagents I - Coggle Diagram
Topic 7 - Magnetism and Electromagents I
Permanent and Induced magents
Magnetic fields
All magnets have two poles, north/ south seeking
All magents produce a magnetic field - a region where other magnets or magnetic materials experience a force
Field diagrams
Lines always go from north to south
Closer together lines are stronger than further apart lines
Further away from a magnet you get, the weaker the field is
Magentic field is strongest at the poles of the magnet
Two poles put near eachother will exert a force on eachother
Magnet type
Permanent magnets produce their own magnetic field
Induced magnets are magnetic materials that turn into a magnet when they're put into a magnetic field
Force between a permanent and induced magnet is always attractive
Induced magnets quickly lose their magnetism when the field i taken away
Electromagnetism
Moving charge
When a current flows through a wire, a magnetic field is created around the wire
Field is made from concentric circles perpendicular to the wire at the centre
Changing the direction of the current changes the direction of the magnetic field
Larger currents produce stronger fields, and the lines are closer together
Solenoids
You can increase the strength of the magnetic field that a wire produces by wrapping the wire into a coil called a solenoid
Field lines around each loop of wire lines up with eachother resulting in lots of field lines pointing in the same direction which are very close together
Solenoid's magentic field is strong and uniform, has the same strength and direction at every point in the region
Outside the coil, the magnetic field is just like the one round a bar magnet
You can increase the field strength of the solenoid by putting a block of iron in the centre, which becomes an induced magnet whenever current is flowing
If you stop the current, the magnetic field disappears
Solenoid with an iron core is called an electromagnet
Uses
Used in cranes to attract and pick up things made from magnetic materials
Used in motors
Doorbells
Transformers
Transformers change the size of the pd of an alternating current
They all have two coils of wire, the primary and the secondary, joined with an iron core
When an alternating pd is applied across the primary cooil, the iron core magnetises and demagnetises quickly
The change in magnetic field induces and alternating pd in the secondary coil
If the second coil is part of a complete circuit, this causes a current to be induced
Ratio between the primary and secondary pd is the same as the ratio between the number of turns on the primary and secondary coils
Step up transformers -have more turns on the primary coil that the secondary coil
Step down transformers - they have more turns on the pimary coil than the secondary
Equation
Ratio between the input/ output pd is the same as the ratio between the number of turns on the primary/ secondary coil
Input power is equal to the output power
Low current means that less energy is waster heating the wires and the curroundainfs, making the national grid an efficient way of transmititng power
A high pd is needed for a low current
V1I1 = V2I2
Alternating current in primary coil causes continuously changing magnetic fluc in the core
Why they won't work for dc
A direct current would create a stationary magnetic field
There wouldn't be any cutting of magneitc field lines
An alternating EMF wouldn't be induced in the secondary coil
No pd is induced and no current will flow in the secondary coil