Magnetism and Electromagnetism

Question 1

ShabWhat two poles do magnets have?

Answer 1

north pole and south pole

Question 2

What is a magnetic field?

Answer 2

region where other magnets or magnetic materials experience a force

Question 3

In a magnetic field diagram, what direction do the lines go?

Answer 3

lines always go north to south

Question 4

What do the lines in a magnetic field diagram show?

Answer 4

show which way a force would act on a north pole if it was put at that point in the field

Question 5

What does how close the lines are mean?

Answer 5

closer the lines are - stronger the magnetic field

Question 6

What is inside a compass?

Answer 6

• a tiny bar magnet
• when they arent near a magnet - compasses always point north
• this is because the earth generates its own magnetic field

Question 7

What are the two types of magnet?

Answer 7

permanent and induced

Question 8

What are permanent magnets?

Answer 8

produce their own magnetic fields

Question 9

What are induced magnets?

Answer 9

magnetic materials that turn into a magnet when put into a magnetic field

Question 10

What happens when a current flows through a wire?

Answer 10

when a curruent flows through a wire - magnetic field is created around the wire

Question 11

What is the magnetic field made up of in a wire?

Answer 11

field is made up of concentric circles perpendicular to the wire with the wire in the centre

Question 12

How to strengthen the magnetic field in a wire?

Answer 12

increase current or get closer to the wire

Question 13

What is a solenoid?

Answer 13

a coil of wire

Question 14

What is the purpose of a solenoid?

Answer 14

you can increase strength of magnetic field that a wire produces by wrapping the wire into a coil called a solenoid

Question 15

Why does a solenoid icnrease the strength of a magnetic field?

Answer 15

• results in lots of field lines pointing in same direction that are very close together
• the individual magnetic fields generated by each coil add together resulting in stronger overall field

Question 16

How to further increase the field strength of a solenoid?

Answer 16

• put block of iron in centre of the coil
• iron core becomes an induced magnet whenever current is flowing

Question 17

Why is a solenoid with an iron core an electromagnet?

Answer 17

• if you stop current - magnetic field disappears
• magnetic field can be turned on and off

Question 18

What is the motor effect?

Answer 18

• when a current -carrying wire is put between magnetic poles - magnetic field around wire interacts with magnetic field it has been placed in
• causes the magnet and the conductor to exert a force on each other
• is called the motor effect and can cause the wire to move

Question 19

How does the wire experience full force?

Answer 19

• wire has to be at 90° to magnetic field to feel full force
• force always acts at right angles to magnetic field os magnets and direciton of currrent in wire

Question 20

What is the magnetic flux density?

Answer 20

• how many field (flux) lines there are in a region
• shows the strength of the magnetic fie

Question 21

What three factors changes the force acting on conductor in a magnetic field?

Answer 21

• magnetic flux denisity
• size of current
• length of cunductor

Question 22

What hand is the motor effect?

Answer 22

left hand

Question 23

How does a loudspeaker work?

Answer 23

• alternating current flows through coil of wire attached to the base of a paper cone
• coil surrounds one pole of a permanent magnet, and is surrounded by the other pole, so the current causes a force on the coil which causes the cone to move
• when current reverses - force acts in opposite direction - causes cone to move in the opposite direction
• variations in current make the cone vibrate - makes the air around the coil vibrate and creates variations in pressure that causes sound waves
• frequency of the sound wave is the same as the frequency of the ac - by controlling the frequency of the ac you can alter the sound wave produced

Question 24

How does an electric motor work?

Answer 24

• sides of the coil parallel to the magnet experience a force in opposite directions
• forces cause moments that act in the same direction, causing the coil to rotate
• split-ring commutator swaps the positive and negative connections every half term
• each half-revolution the commutator reverses the current in the coil, keeping the forces in the same direction

Question 25

What is the generator effect?

Answer 25

- the induction of a potential difference (and current if there is a complete circuit) in a wire which is moving relative to a magnetic field, or experiencing a change in magnetic field. - when a wire experiences a changing magnetic field - a potential difference will be induced across the wire - if the ends of the wire are connected to make a closed circuit - a current will flow around the circuit

Question 26

How does the generator effect work?

Answer 26

- if take a piece of wire and bend into coil shape - move it through magnetic field - it will induce a pd in the wire - if you move the conductor in the oppositge direction - potential difference is reversed - if you keep coil moving backwards and forwards - you produce a pd that keeps swapping direction - an alternating pd

Question 27

What do alternaters generate?

Answer 27

An alternating current

Question 28

How to increase the size of the potential difference generated in electromagnetic induction?

Answer 28

- move the coil through the magnetic field faster - add more turns to the coil - increase magnetic field strength

Question 29

What do dynamos generate?

Answer 29

direct current

Question 30

Differences between structure of dynamo and alternator?

Answer 30

alternator has slip rings and brushes whereas dynamo has a split-ring commutator

Question 31

How does an alternator work?

Answer 31

- coil rotates between the poles of a magnet - causes it to cut through magnetic field lines - causes a potential difference to be induced across the coil due to electromagnetic induction - there is a complete circuit, so a current is induced in the coil - slip rings and brushes cause the potential difference to reverse direction every half turn - every half turn the current changes direction, producing AC

Question 32

How does a dynamo work?

Answer 32

- coil rotates between the poles of a magnet - causes it to cut through magnetic field lines - causes a potential difference to be induced across the coil due to electromagnetic induction - there is a complete circuit, so a current is induced in the coil - split-ring commutator causes connections to swap every half turn so the PD flows in same direction - the current flows in the same direction, producing DC

Question 33

How do microphones work?

Answer 33

- sound waves cause the diaphragm to vibrate - causes the coil to vibrate - the coil / wire moves through the magnetic field - a potential difference is induced across the ends of the coil / wire - as the coil is part of complete circuit -> the induced voltage causes a current to flow that matches the soundwave pattern

Question 34

What do transformers do?

Answer 34

transformers change the size of the potential difference of an alternating current

Question 35

How does a transformer work?

Answer 35

- when an alternating pd is applied across the primary coil - iron core magnetises and demagnetises quickly - changing magnetic field induces an alternating pd in the secondary coil - if the second coil is part of a complete circuit - causes a current to be induced - ratio between P coil and S coil = ratio between number of turns on the P and S coils