magnetism and electromagnetism (paper 2)

Question 1

give examples of magnetic materials

Answer 1

iron

steel

nickel

cobalt

Question 2

what is a magnetic field

Answer 2

a region where other magnets of magnetic materials experience a force

Question 3

is a magnetic force a contact or non contact force

Answer 3

non contact force

Question 4

which way do magnetic field lines always go from

Answer 4

north to south

Question 5

what does the distance between the magnetic field lines mean

Answer 5

the closer the lines are the stronger the magnetic field is

the further away from a magnet you get, the weaker the field is

Question 6

which part of the magnet is where the magnetic field is the strongest

Answer 6

the poles

Question 7

what are permanent magnets

Answer 7

produce their own magnetic field

Question 8

what are induced magnets

Answer 8

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

Question 9

what is the force between an induced magnet and a permanent magnet

Answer 9

always attractive

Question 10

what does changing the direction of the current do

Answer 10

changes the direction of the magnetic field

Question 11

how do you use the right thumb rule

Answer 11

using your right hand , point your thumb in the direction of the current and curl your fingers

the direction of your fingers is the direction of the field

Question 12

how can you increase the strength of the magnetic field that a wire produces

Answer 12

by wrapping the wire into a coil called a solenoid

Question 13

what is the magnetic field like inside of a solenoid

Answer 13

strong and uniform

it has the same strength and direction at every point in that region

Question 14

what is the magnetic field like outside of the solenoid

Answer 14

just like the one around a bar magnet

Question 15

how can you increase the field strength of a solenoid

Answer 15

putting a block of iron in the centre of the coil

the iron core becomes an induced magnet whenever current is flowing

if you stop the current, the magnetic field disappears

Question 16

what is a solenoid with an iron core called

Answer 16

an electromagnet - a magnet whose magnetic field can be turned on and off with an electric current

Question 17

what are some uses of electromagnets

Answer 17

cranes - to attract and pick up things made from magnetic materials e.g. in scrap yards

within other circuits to act as switches

Question 18

what causes the motor effect

Answer 18

when a current carrying wire is put between magnetic poles, the magnetic field around the wire interacts with the magnetic field that it has been placed in , this causes the magnet and the conductor to exert a force on each other

this is called the motor effect and can cause the wire to move

Question 19

how can you find the direction of a force using Fleming’s left hand rule

Answer 19

1. using your left hand , point your first finger in the direction of the field

2 point your second finger in the direction of the current

3. your thumb will then point in the direction of the force

Question 20

how can the direction of a motor be reversed

Answer 20

swapping the polarity of the dc supply

swapping the magnetic poles over

Question 21

what is the generator effect

Answer 21

the induction of a potential difference (and a current if there is a complete circuit) in a wire which is experiencing a change in magnetic field

Question 22

how can rotation cause the generator effect

Answer 22

as you turn the magnet, the magnetic field through the coil changes, this change in the magnetic field induces a potential difference, which can make a current flow in the wire

every time the magnet moves half a turn, the direction of the magnetic field through the coil reverses. when this happens, the potential difference reverses, so the current flows in the opposite direction around the coil of wire

if you keep turning the magnet in the same direction then the potential difference will keep on reversing every half turn so you will get an alternating current

Question 23

how can induced potential difference be increased

Answer 23

increasing the sped of the movement - cutting more magnetic field lines in a given time

increasing the strength of the magnetic field (so there are more field lines that can be cut)

Question 24

what does an alternator do

Answer 24

rotate a coil in a magnetic field and generate alternating currents

Question 25

how does an alternator work

Answer 25

their construction is pretty much lie a motor

as the coil or magnet spins , a current is induced in the coil , the current changes direction every half turn

instead of a split ring commutator , alternators have slip rings and brushes so the contacts do not swap every turn

this means they produce an alternating potential difference

Question 25

how do dynamos work

Answer 25

work in the same way as alternators BUT have a split ring communicator instead of slip rings

this swaps the connection every half turn to keep the current flowing in the same direction

Question 25

what do dynamos do

Answer 25

generate direct current

Question 26

what do oscilloscopes show

Answer 26

how the potential difference generated in the coil changes over time

Question 27

how do loud speakers work because of the motor effect

Answer 27

1. an alternating current is sent through a coil of wire attached to the base of a paper cone
2. the 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 coil to move)
3. when the current reverses the force acts in the opposite direction , which causes the cone to move in to opposite direction too
4. so variations in the current make the cone vibrate , which makes the air around the cone vibrate and creates the variations in pressure that cause a sound wave
5. the frequency of the sound wave is the same as the frequency of the ac, so by controlling the frequency of the ac you can alter the sound wave produced

Question 28

how do microphones generate current from sound waves

Answer 28

1. microphones are basically loudspeakers in reverse
2. sound waves hit a flexible diaphragm that is attached to a coil of wire, wrapped around a magnet
3. this causes the coil of wire to move in the magnetic field , which generates a current
4. the movement of the coil depends on the properties of the sound wave (louder sounds make the diaphragm move further)
5. this is how microphones can convert the pressure variations of a sound wave into variations in current in an electric circuit

Question 29

what do transformers do

Answer 29

change the size of the potential difference of an alternating current

Question 30

what does a transformer look like

Answer 30

they all have two coils of wire, the primary and secondary joined with an iron core

Question 31

what do step up transformers do

Answer 31

step the potential difference up, they have more turns on the secondary coil than the primary coil

Question 32

what does a step down transformer do

Answer 32

steps down the potential difference, they have more turns on the primary coil than the secondary coil