P7-electromagnetism

Question 1

What is produced around a wire when current flows through a conducting wire?

Answer 1

a magnetic field

Question 2

what two things affect the strength of a magnetic field?

Answer 2

the current through the wire and the distance from the wire

Question 3

what is an electromagnet?

Answer 3

a magnet which can be switched on and off

Question 4

Give an example of an electromagnet:

Answer 4

a solenoid with an iron core

Question 5

What is a solenoid and why are they used?

Answer 5

it is a wire wrapped into a coil. it increases the strength of the magnetic field

Question 6

what is the magnetic field inside a solenoid like?

Answer 6

it is strong and uniform, has same strength and direction at every point.

Question 7

what is the magnetic field outside a solenoid like?

Answer 7

like the one around a bar magnet

Question 8

how can you increase the strength of a solenoid even more?

Answer 8

by adding a block of iron in the centre (iron becomes an induced magnet when current is flowing)

Question 9

state two uses of electromagnets:

Answer 9

1-for cranes to pick up and drop materials

2-act as switches in circuits

Question 10

Name the effect: When a potential difference is induced in a wire experiencing a change in magnetic field

Answer 10

the generator effect

Question 11

what is induced when a coil is moved through a magnetic field?

Answer 11

a potential difference

Question 12

what happens when you stop moving the wire between the two magnets?

Answer 12

the potential difference disappears. this is because the pd is created by the change in magnetic field

Question 13

what happens to the PD each time the direction of the wire changes?

Answer 13

the direction of the PD changes

Question 14

what changes when a complete circuit is used instead of a wire?

Answer 14

both potential difference AND current are produced

Question 15

what happens if you move the bar magnet alongside the magnetic field lines?

Answer 15

no potential difference is created.

Question 16

name three things which can increase the size of the PD:

Answer 16

1-increase magnet strength
2-move wire faster
3-shape wire into coil, and the more turns in the coil the higher the PD

Question 17

name two ways of decreasing the PD:

Answer 17

1-decrease magnet strength

2-move wire slower

Question 18

what is another way of inducing potential difference?

Answer 18

moving a magnet in and out of a coil

Question 19

what happens to the current when the direction of movement of the magnet changes?

Answer 19

current direction changes

Question 20

what does the induced current create?

Answer 20

its own magnetic field

Question 21

the magnetic field _______ the movement of the magnet.

meaning we are doing ____ to move the magnet.

Answer 21

opposes, work. when N pole in coil, that end of coil becomes N so repels magnet. When N pole pulled out, that end becomes S pole, attracting magnet.

Question 22

explain how alternators work:

Answer 22

generators rotate coil in magnetic field, coil spins and current is induced. every half turn, current swaps direction. Slip rings and brushes used so contacts don’t swap every half turn. AC comes out here

Question 23

In an alternator, when is generated PD highest?

Answer 23

when the coil is horizontal

Question 24

what happens to the PD when the coil is vertical?

Answer 24

the PD goes to zero

Question 25

why does the PD reverse direction?

Answer 25

because the two sides of the coil are moving in a different direction to before

Question 26

what’s different about dynamos compared to alternators?

Answer 26

dynamos have split ring commutators. this swaps the connection every turn to keep current flowing in same direction.

Question 27

describe 4 ways to increase induced current:

Answer 27

1-faster rotation
2-stronger magnetic field
3-more turns on coil
4-coil has a larger area

Question 28

how do microphones work?

Answer 28

sound wave hits diaphragm, coil of wire moves in a magnetic field (generating current) . Movement of coil depends on sound wave’s properties. Microphones convert pressure variations of sound wave into variations in current in an electrical current

Question 29

what does a basic transformer consist of?

Answer 29

a primary coil and a secondary coil wound around an iron core

Question 30

why is iron used for transformers?

Answer 30

it is easily magnetised

Question 31

why are transformers used?

Answer 31

to increase or decrease the size of alternating PD

Question 32

what is the difference between a step-up transformer and a step-down transformer?

Answer 32

step up- increases PD

step down- decreases PD

Question 33

why do transformers only work with a.c?

Answer 33

because a changing magnetic field is required to induce a.c in the secondary coil.

Question 34

why is a low current used in the national grid?

Answer 34

so less energy is wasted heating wires and national grid is more efficient.