P4 - Magnetism and magnetic fields

Question 1

Magnetic fields:

Answer 1

• 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)

Question 2

Poles on a magnet:

Answer 2

• Opposite poles attract (eg south and north)

- Like poles repel (eg north and north)

Question 3

What are the two types of magnets?

Answer 3

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

Question 4

What is electromagnetism?

Answer 4

Current carrying wires have a magnetic field

Magnetic field strength decreases in strength the further from the wire.

Question 5

How do you find the direction of magnetic field?

Answer 5

Use the right hand rule

Thumb represents wire direction and the four other fingers are the direction of magnetic field.

Question 6

What is a solenoid and its magnetic field?

Answer 6

• 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)

Question 7

How do you increase magnetic field strength in solenoid?

Answer 7

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

Question 8

Magnetic forces:

Answer 8

Current carrying wires in a magnetic field experience a force

• The more current the more force
• forces are perpendicular to magnetic field direction

Question 9

How to find the direction of force when current carrying wire is in magnetic field?

Answer 9

Use Fleming's left hand rule
Feed Me Chips
Thumb --> Force 
Index --> magnetism
Middle--> Current

Question 10

Force on conductor carrying current equation:

Answer 10

Force (N) = Magnetic flux density (T) x length (m) x current (A)

F = BIL

Question 11

What are motors?

Answer 11

Motors generate movement from electric current

Question 12

How do motors work?

Answer 12

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

Question 13

How do you speed up motor?

Answer 13

• Increase current
• increase magnetic field strength
• Add more turns to coil

Question 14

How do loud speaker work?

Answer 14

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

Question 15

What is a generator?

Answer 15

A generator creates electric current from movement

Question 16

How do alternators work?

Answer 16

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

Question 17

What is a dynamo?

Answer 17

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.

Question 18

What are transformer?

What are the types of transformer?

Answer 18

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

Question 19

What do transformer look like?

Answer 19

• Coils at primary and secondary places around a iron core

Question 20

How does transformer work?

Answer 20

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

Question 21

How does power work in transformer?

How does current work in transformer?

Answer 21

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.

Question 22

How do microphones work?

Answer 22

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

Question 23

Transformers equation:

Answer 23

(Voltage in primary coil/ voltage in secondary coil) = (number of turns in primary coil/number of turns in secondary coil)

Question 24

What are three ways to make a magnet?

Answer 24

1) Rub against a magnet in same direction
2) Hit with hammer in a magnetic field
3) Place in strong magnetic field

Question 25

What are the three magnetic materials?

Answer 25

Iron , Nickel, Cobalt, any alloy with these

Question 26

What happens when you break a magnet?

Answer 26

When breaking a magnet to small pieces, the two pieces will become smaller magnets (have a north and south pole)

Question 27

What are hard and soft magnets?

Answer 27

Hard magnets: Takes more effort to make but last for longer

Soft magnets: Takes less effort to make but last for shorter

Question 28

How does magnetic fields work with Earth?

Answer 28

The south pole on Earth is the geographical north

The north pole on Earth is the geographical south

Question 29

How to find magnitude and direction of force when wire is in a magnetic field?

Answer 29

Magnitude: Use F = BIL
Direction: Use left hand rule

Question 30

How does magnetic induction work?

Answer 30

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.

Question 31

How does relay circuits work?

Answer 31

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.

Question 32

How do doorbells work?

Answer 32

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

Question 33

How to generate current in a wire?

Answer 33

You need a changing magnetic field

• Move magnet in and out of coil
• Rotate coil

Question 34

What happens when two magnetic fields interact?

Answer 34

They experience a force

eg: when you bring to magnets together, the magnetic fields interact and they either repel or attract.