P15 - Motor/ Generator effects

Question 1

why does an electric motor work

Answer 1

a force acts on a wire in a mg field as a current passes through

Question 2

size of the force can be increased by

Answer 2

• increasing current
• using a stronger magnet

Question 3

force is greatest

Answer 3

when wire is perpendicular to mg field

Question 4

force is 0 when

Answer 4

wire is parallel to mg field

Question 5

direction of force is always at a right angle to

Answer 5

wire and mg field lines

Question 6

direction of force is reversed if

Answer 6

either current or mg field lines are reversed

Question 7

Fleming’s left hand rule

Answer 7

Forefinger = field
-second finger = current
thumb = movement of force

Question 8

magnetic flux density - symbol and unit

Answer 8

measure of the strength of the mg field
-symbol B
unit tesla T

Question 9

force =

Answer 9

BIL

Question 10

electric motor uses

Answer 10

motor effect

Question 11

electric motor changes speed by

Answer 11

changing current

Question 12

Motor effect process

Answer 12

• coil of insulated wire is forced to rotate
• coil is connected to battery by 2 graphite brushes
• brushes press onto metal split ring commutator fixed to coil

Question 13

why graphite brushes

Answer 13

conducts electricity
- causes little friction with rotating commutator

Question 14

as the current passes through - coil spins because:

Answer 14

-force acts on each side of coil (motor effect)
- force on one side is in the opposite direction to the force on the other side

Question 15

split ring commutator and direction of current

Answer 15

reverses current every half turn
- as sides swap over each half turn the coil is pushed in the same direction every half turn

Question 16

generator

Answer 16

• contains coils of wire hat spin in a mg field
  -pd is induced

Question 17

why is pd induced in a generator

Answer 17

crosses the mg field lines

Question 18

electromagnetic induction

Answer 18

happens when any conductor crosses mg field

Question 19

if conductor is a part of a complete circuit

Answer 19

• induced pd induces current pass through
  -aka generator effect

Question 20

generator test

Answer 20

coil of insulated wire
-connected to centre reading ammeter
- when end of bar magnet is pushed into coil
= ammeter pointer deflects

Question 21

Generator effect happens due to

Answer 21

-movement of bar magnet = induces pd
- induces pd = current as apart of circuit

Question 22

induced magnetic field always opposes

Answer 22

original change
-so work done by person moving the magnet

Question 23

in generator effect stronger magnet =

Answer 23

pd and current increases

Question 24

faster a conductor crosses mg field

Answer 24

bigger pd induced

Question 25

the electricity generated is a result of

Answer 25

work done by a person moving the magnet

Question 26

when a dc EM is used

Answer 26

must be switched on/off to induce pd

Question 27

direction of induced current always

Answer 27

opposes og change that caused it

Question 28

alternator / ac generator

Answer 28

rectangular coil is forced to spin in a uniform mg field -attached to centre reading meter by metal brushes -that press onto 2 metal slip rings

Question 29

ac generator purpose of having brushes press onto slip rings

Answer 29

provides a constant connection between coil/meter

Question 30

alternator meter shows

Answer 30

coil turns in one direction = meter deflects one way then the opposite

Question 31

ac generator current always changes

Answer 31

-as pd changes - both alternate as they change direction

Question 32

alternator - pd is greatest when

Answer 32

plane of coil is parallel to direction of mg field -coil crosses directly

Question 33

alternator - pd is 0 when

Answer 33

perpendicular as sides of coil moves parallel to mg field

Question 34

the faster the coil rotates in an alternator

Answer 34

bigger frequency of AC - each full cycle of alternating pd = smae time as one full rotation of coil

Question 35

alternator - bigger peak value

Answer 35

crosses field at a faster rate

Question 36

to increase peak value of ac generator

Answer 36

-use a stronger magnet - coil with a bigger area - more turns in coil

Question 37

AC generator waveform on oscilloscope of PD

Answer 37

- peaks when side of coil crosses - rotated faster = more and taller waves

Question 38

DC dynamo/ dc generator diff

Answer 38

split ring commutator instead of 2 slip rings

Question 39

dc process - as coil spins

Answer 39

- split ring reconnects to coil the opposite way around every half turn - so every time coil is perpendicular - induced pd doesn't reverse

Question 40

dc values vary from

Answer 40

0 to max value twice each cycle

Question 41

moving coil sound devices

Answer 41

-microphone generates alternating PD - as sound waves make coil vibrate

Question 42

microphone process

Answer 42

- coil is attached to small diaphragm + between poles - pressure variations of sound waves on diaphragm make it vibrate so coil vibrates in mg field - alternating PD induced in coil

Question 43

- alternating PD induced in coil has the same frequency as

Answer 43

sound waves

Question 44

Loudspeaker

Answer 44

creates sound waves when alternating PD is applied to coil

Question 45

loudspeaker process

Answer 45

- current in coil causes a force due to motor effect -current alternates so force reverses -making coil/diaphragm vibrate -creates sound waves - same frequency as alternating PD