Magnetism + electromagnetism

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Cards (70)

Bar magnet 
A magnet with two ends called poles (North Pole and South Pole)
Magnets 
Magnetic forces are strongest at the poles
When two magnets are brought close together
They exert a force on each other
Forces between magnets 
Like poles repel
Unlike poles attract
Non-contact forces 
Magnets don't have to touch to experience the force
Permanent magnet 
Produces its own magnetic field
Induced magnet 
An object that becomes a magnet when placed in a magnetic field
Induced magnetism always causes a force of attraction
Induced magnets lose most or all of their magnetism quickly when the permanent magnet is removed
Magnetic field 
A region around a magnet where a force acts on another magnet or on a magnetic material
Plotting the magnetic field pattern of a magnet
1. Place compass near North Pole of bar magnet
2. Draw cross at North Pole of compass
3. Move compass so South Pole is on cross, draw cross at North Pole
4. Repeat, connecting dots to show field lines
5. Show direction from North to South Pole
The strength of the magnetic field depends on the distance from the magnet
The magnetic field is strongest at the poles of the magnet
A compass contains a small bar magnet
Compass placed near a bar magnet 
Needle points in the north-south direction
The Earth has its own magnetic field
The Earth's magnetic field is due to the Earth's core
Conventional current 
The direction of current flow as defined by the movement of positive charges
How a current flowing through a wire generates a magnetic field
1. Current flows through wire
2. Magnetic field produced around wire
3. Strength of magnetic field depends on current size
4. Magnetic field strongest closer to wire
5. Changing current direction changes magnetic field direction
Magnetic field around wire
Can be detected using a compass
Deflects compass needle
Increasing current 
Increases strength of magnetic field
Moving further from wire
Decreases strength of magnetic field
Right hand grip rule
Used to determine direction of magnetic field produced by a wire carrying current
Solenoid 
Coiled wire
Produces strong and uniform magnetic field inside
Solenoid 
Magnetic field shape similar to bar magnet
Right hand rule for solenoid
Curl fingers in direction of conventional current, thumb points to north pole
Ways to increase strength of magnetic field in a solenoid
Increase current size
Increase number of turns in coil
Place iron core inside
Electromagnet 
Solenoid containing an iron core
Electromagnets are useful as the magnetic field strength can be changed by changing the current, and they can be turned on and off
How does a motor work?
Motor effect:
Sides of the coil experience an opposite force (downwards and upwards)
The force causes moments that act in the same direction
The moments cause the coil to rotate
Each half turn the two halves of the commutator swap from one brush to the other
The commutator reverses the current keeping it spinning in the same direction
What is the motor effect? 
- a current-carrying wire is put between magnetic poles
- the magnetic field around the wire interacts with the magnetic field it has been placed in. - This causes the magnet & the conductor to exert a force on each other
-this can cause the wire to move
Factors affecting a motor (uses d.c)
- speed the coil rotates (increase the current, use a stringer magnet)
-direction of the coil (swap the poles of the magnet, reverse the direction of current)
-force supplied to the motor (increase by increasing the current, strength of magnetic field, adding more turns to the coil)
How does an alternator work?
Generator effect:
- coil forces to move through a magnetic field
-the coil cuts through the magnetic field
- so there is a potential difference across the coil
-the commutator rings provide a complete circuit
-so current is induced in the coil
-every half turn the p.d reverse so the current changes direction ( is alternating )
How does a dynamo work?
Generator effect:
Coil rotating which cuts the field lines
This indices a p.d between the end of the coil
The split ring commutator changes connections between coils and brushes every half turn in order the keep the current on the same direction
This happens each time the coil is perpendicular to the magnetic field
How does the generator effect work ?
-A coil of wire is moved in a magnetic field or a magnetic is moved into the coil of a wire
-this changes the magnetic field (as the wire is constantly moved) so a p.d is induced in the wire
-this therefore induces a current
-the current is alternating as the wire is constantly moved
How does a moving coil loudspeaker work ?
Motor effect:
(Electrical —> sound )
-current generates a magnetic field
-magnetic field interacts with the magnetic field of the permanent magnets magnetic field
-this causes a resultant force causing the cone to move
-when the current switches the direction of the force changes
- causing the cone to move in and out
-generating sound waves
How does a microphone work?
Generator effect:
-sound waves cause the diaphragm to vibrate
-the diaphragm causes the coil/ wire to vibrate
- the coil/ wire moves through the magnetic field
-causing a potential difference to be induced (across the ends of the wire)
How does a transformer work?
- Alternating current generated in primary coil
- Produces a changing magnetic field in the iron core - And then in the secondary coil
- Generates alternating potential difference in the secondary coil
- And hence alternating current in secondary coil
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Transformer 
Device used to change the potential difference of an electricity supply
Transformer 
Consists of two coils of wire
Primary coil and secondary coil are completely separate
Coils are wound around an iron core
Iron core is easily magnetized
Transformer only works with alternating current