topic 13 - electromagnetic induction

Question 1

electromagnetic induction

Answer 1

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

Question 2

how can you induce a potential difference (2)

Answer 2

when an electrical conductor and a magnetic field move relative to each other
or when a magnetic field though an electrical conductor changes like in a transformer

Question 3

how can you move an electrical conductor and a magnetic field relative to each other to induce a p.d

Answer 3

rotate a magnet in a coil of wire or rotate a conductor in a magnetic field
if you rotate in the other direction OR turn the magnet around/reverse polaity the p.d /current will be reversed

Question 4

how can you move an electrical conductor and a magnetic field relative to each other to generate an alternating current

Answer 4

keep moving the magnet or coil backwards and forwards or rotating in the same direction

Question 5

how can you increase induced potential difference (3)

Answer 5

increase magnetic field strength
increase speed of movement/change of field
have more turns per unit of length on the wire coil

Question 6

why does the induced current always oppose the change that made it

Answer 6

a change in magnetic field can induce a current in a wire
when current flows through a wire, a second magnetic field is created around the wire
this field always acts against the change that made it

Question 7

how do generators work

Answer 7

they use electromagnetic induction to induce a current
generators supply a force to rotate a coil in a magnetic field
as the coil spins, the current is induced in the coil
the current changes direction every half turn

Question 8

what are the 2 types of generator

Answer 8

dynamo

alternator

Question 9

what kind of current does a dynamo generate and what enables it to do this

Answer 9

direct current

the split ring commutator swaps the connection every half turn to keep the current flowing in the same direction

Question 10

what kind of current does an alternator generate and what enables it to do this

Answer 10

alternating current

the alternator has slip rings and brushes so the contacts don’t swap every half turn and the current keeps alternating

Question 11

what do microphones do

Answer 11

microphones convert pressure variations of a sound wave into variations in current in an electrical circuit

Question 12

how do microphones work

Answer 12

sound waves hit flexible diaphragm attached to wire coil

the coil is surrounded by one pole and surrounds another

then the diaphragm moves, the coil moves within a magnetic field generating a current

the movement of the coil depends on soundwave properties (louder sound, stronger wave, more movement of diaphragm)

Question 13

how to loudspeakers work

Answer 13

the coil is wrapped around one pole of a magnet and it is surrounded by another

the a.c signal causes a force on the coil moving the cone

when the current is reversed, the force acts in the opposite direction

the movements of the coil back and forth cause the cone to vibrate which in turn causes the air to vibrate, creating variations in pressure and ultimately a sound wave

Question 14

transformers

Answer 14

use induction to change the p.d/voltage of an alternating current

Question 15

structure of a transformer

Answer 15

two coils of wire joined with an iron core

Question 16

how do transformers work

Answer 16

when an alternating p.d is applied across the primary coil, it produces an alternating magnetic field
the iron is easily magnetised and demagnetised
as the coil is producing an alternating magnetic field, the magnetisation of the coil alternates
changing magnetic field induces a p.d in the secondary coil

Question 17

step-up transformer

Answer 17

increases the p.d

more turns on the secondary coil than on the primary coil

Question 18

step-down transformer

Answer 18

decreases the p.d

fewer turns on the secondary coil that on the primary coil

Question 19

p.d and coils ratio equation

Answer 19

p.d primary coil / p.d secondary coil = turns primary coil/ turns secondary coil

Question 20

because transformers are almost 100% efficient (power in transformers equation)

Answer 20

voltage primary coil x current primary coil = voltage secondary coil x current secondary coil

Question 21

how do we generate electricity on a large scale (5)

Answer 21

1) burn fuels to heat water and convert it to steam
2) steam turns a turbine
3) turbine turns a generator - it turns a magnet inside the generator inducing a large p.d and a.c in the coils
4) coils are joined together in parallel to produce a single output

Question 22

national grid

Answer 22

a network of wires and transformers that connect UK power stations to consumers

Question 23

the national grid transfers energy at high power - why is current low and p.d high

Answer 23

high current is inefficient

it results in lots of energy being wasted to the thermal energy stored due to resistive heating

Question 24

how do we reduce losses of energy in the national grid

Answer 24

use of low resistance high p.d cable and transformers

Question 25

when do we use step-up transformers

Answer 25

at power stations to raise p.d and keep current low

Question 26

when do we use step-down transformers

Answer 26

at the consumer end, to lower p.d to safe usable levels