P4 - Magnetism and magnetic fields Flashcards
Magnetic fields:
- Magnets have magnetic fields that go from the north pole to the south
- the strength of a magnetic field is called magnetic flux density measured in Tesla (T)
Poles on a magnet:
- Opposite poles attract (eg south and north)
- Like poles repel (eg north and north)
What are the two types of magnets?
Induced magnet: loses its magnetism after some time (occurs when magnet is brought next to a magnetic material - iron, nickel or cobalt- and induces a magnetism)
Permanent magnet: These magnets maintain magnetism
What is electromagnetism?
Current carrying wires have a magnetic field
Magnetic field strength decreases in strength the further from the wire.
How do you find the direction of magnetic field?
Use the right hand rule
Thumb represents wire direction and the four other fingers are the direction of magnetic field.
What is a solenoid and its magnetic field?
- Lots of coils joined together and the magnetic field in the solenoid is uniform
- ends of the solenoid act like poles (one side north and other south)
How do you increase magnetic field strength in solenoid?
1) Increase the current
2) Increase the number of turns in the coil
3) Decrease the length of solenoid but not the number of turns
4) Add a iron core at the centre of solenoid
Magnetic forces:
Current carrying wires in a magnetic field experience a force
- The more current the more force
- forces are perpendicular to magnetic field direction
How to find the direction of force when current carrying wire is in magnetic field?
Use Fleming's left hand rule Feed Me Chips Thumb --> Force Index --> magnetism Middle--> Current
Force on conductor carrying current equation:
Force (N) = Magnetic flux density (T) x length (m) x current (A)
F = BIL
What are motors?
Motors generate movement from electric current
How do motors work?
1) In direct current motor, there is current carrying wire between two magnets
2) current flows in different directions on each side of coil
3) Each side experiences a force in different directions. the coil rotates.
4) Split rings commutators- swap contacts each half turn to keep coil rotating in same direction
5) Movement is created
How do you speed up motor?
- Increase current
- increase magnetic field strength
- Add more turns to coil
How do loud speaker work?
1) Loudspeaker contains wire surrounded by permanent magnet.
2) Alternating current fed into coil which is wrapped around a cone
3) The coil becomes an electromagnet and the interaction between the permanent magnet’s field and electromagnet’s magnetic field generates a force pushing cone in one direction
4) Because it is alternating current the magnetic field constantly changes causing cones to be pushed back and forth
5) This generates sound waves
What is a generator?
A generator creates electric current from movement
How do alternators work?
Alternators create alternating current
1) A wire is rotated in a magnetic field which causes current to be induced as there is a changing magnetic field
2) As wire is constantly rotating the current direction continues to change direction –> Alternating current
What is a dynamo?
Dynamos produce direct current
1) Wire is rotated in a magnetic field and has a changing magnetic field within the wire due to rotation
2) The changing magnetic field induces current in the wire
3) The current constantly changes direction as wire is rotating generation alternating current
4) Split ring commutators are used to swap the contacts every half turn generate direct current.
What are transformer?
What are the types of transformer?
Transformers change p.d of alternating current
-We use transformers in order to transport current efficiently (Eg) if there is too much current flowing at once heat is lost into atmosphere via heat and it is wasteful)
- Step up transformer : p.d increased in secondary coil
- Step down transformer : p.d decreases in secondary coil
What do transformer look like?
- Coils at primary and secondary places around a iron core
How does transformer work?
1) When a.c current applied to primary coil it produces alternating magnetic field
2) Iron is a magnetic material so it magnetises, but because coil is alternating current the iron core magnetic field alternates
3) changing magnetic field induces p.d in secondary coil
How does power work in transformer?
How does current work in transformer?
Power is conserved in primary coil and secondary coil (Power the same at secondary and primary)
If voltage is increased than current is decreased and if voltage is decreased current is increased. (Due to equation Power = voltage x current) we know that power stays the same.
How do microphones work?
1) Sound waves cause diaphragm to move back and forth
2) As diaphragm moves, coil of wire moves in and out the permanent magnet inducing a potential difference.
3) Current is induced which varies due to sound waves
Transformers equation:
(Voltage in primary coil/ voltage in secondary coil) = (number of turns in primary coil/number of turns in secondary coil)
What are three ways to make a magnet?
1) Rub against a magnet in same direction
2) Hit with hammer in a magnetic field
3) Place in strong magnetic field