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
What are the three magnetic materials?
Iron , Nickel, Cobalt, any alloy with these
What happens when you break a magnet?
When breaking a magnet to small pieces, the two pieces will become smaller magnets (have a north and south pole)
What are hard and soft magnets?
Hard magnets: Takes more effort to make but last for longer
Soft magnets: Takes less effort to make but last for shorter
How does magnetic fields work with Earth?
The south pole on Earth is the geographical north
The north pole on Earth is the geographical south
How to find magnitude and direction of force when wire is in a magnetic field?
Magnitude: Use F = BIL
Direction: Use left hand rule
How does magnetic induction work?
When magnetic field changes, current is induced in wire. When magnetic field increases current is induced to cancel magnetic field out.
When magnetic field increases, current flows in direction to cancel it out and same occurs when magnetic field decreases.
How does relay circuits work?
1) When switch is pressed a current flows through circuit and this creates an electromagnetic (the solenoid).
2) This solenoid attracts the iron core and closes second circuit
3) This allows current to flow through second circuit and turn on.
How do doorbells work?
1) When buzzer pressed the switch closes allowing current to flow.
2) This current flow turns the coil of wire into an electromagnet
3) This then attracts the iron core which hits the clapper on the doorbell
How to generate current in a wire?
You need a changing magnetic field
- Move magnet in and out of coil
- Rotate coil
What happens when two magnetic fields interact?
They experience a force
eg: when you bring to magnets together, the magnetic fields interact and they either repel or attract.
