P4 - electricity & magnetism (electromagnetic forces & its effects, motors & generators, transformer)

Question 1

https://www.youtube.com/watch?v=79_SF5AZtzo
https://www.youtube.com/watch?v=ltpPhpi-CC4
https://www.youtube.com/watch?v=evWpDrRAyCc
https://www.youtube.com/watch?v=pkzY7QfTowM
https://www.youtube.com/watch?v=7RtBUEZbKmI
https://www.youtube.com/watch?v=IxqUjM8cOcU
https://www.youtube.com/watch?v=7RtBUEZbKmI
https://www.youtube.com/watch?v=IxqUjM8cOcU&t=11s

Question 2

note:
-take I as conventional current direction, ( + ) to ( - )
-take field lines from N to S

Question 3

diff btw coil & solenoid

Answer 3

coil = 1 turn
solenoid = multiple turns

Question 4

define EMF

Answer 4

-Electro Motive Force eg, batter, generator
-electrical work done by source to moving an unit charge through the circuit
-max pd btw 2 point in a circuit when no current flows
-measure in V
-when cell is connected to circuit, pd drops because of energy wastage in cell

Question 5

define induced emf

Answer 5

when electromotive force is induced in any conductors when there is relative movement btw conductor & mag field

Question 6

2 ways to induce emf in conductor

Answer 6

-conductor moves, mag field stationary (wire & mag)
-conductor stationary, mag field changes (coil/ solenoid & mag)

Question 7

what does induced emf do when connected to a complete circuit

Answer 7

-makes delocalized electrons move
-flow of electrons produce induced I
*has to be connected to a complete circuit

Question 8

how does emf work when
-conductor moves, mag field stationary (wire & mag)

Answer 8

-conductor (wire) cuts field lines & induced emf in conductor (wire)

Question 9

how does emf work when
-conductor stationary, mag field changes (coil/ solenoid & mag)

Answer 9

-as mag moves through conductor (solenoid), field lines cut through turns on conductor (each individual wire)
-emf induced in coil

Question 10

ways to measure size of induced emf

Answer 10

-sensitive voltameter;
needle will deflect when mag is pushed in/ out of coil, not when stationery
-ammeter if conductor is connected to a complete circuit

Question 11

state faraday’s law of electromagnetic induction

Answer 11

emf induced in a conductor is proportional to the rate of mag field lines cut by the conductor

Question 12

state Lenz’s law for coil/ solenoid & mag

Answer 12

-the direction of an induced emf always opposes the change causing it
= any mag field created by induced emf will try to stop the wire/ magnet from moving in/out the solenoid/ coil

Question 13

how does Lenz’s law work (solenoid & mag)
(how does moving mag through coil/ solenoid work)

Answer 13

1)Mag in
-when N of mag is pushed in coil
-changing mag field induced emf
-induced I
-generates mag field in coil
-end of coil near to mag will be N to oppose mag being pushes in; repel
2) Mag out
-when mag is pushed out of coil
-changing mag field induced emf
-induced I
-generates mag field in coil
-end of coil near to mag will be S to oppose mag being pushes out; attraction

IN: repel OUT: attract
mag: coil: mag: coil:
S N N S S N S N
-constant attraction & repulsion

Question 14

where are electromagnetic induction used in

Answer 14

-electrical generators that convert mechanical E to electrical E
-transformers used in electrical power transmission

Question 15

2 ways to do induced emf

Answer 15

1)wire & mag
2)coil/ solenoid & mag

Question 16

movement of needle in voltmeter when measuring size of induced emf for solenoid & mag

Answer 16

-when mag is stationary (in & out of coil), no induced emf, no movement of needle
-when mag moves in, induced emf & I, needle moved to 1 side
-when mag moves out, induced emf & I direction changes, needle moves to opposite side

Question 17

factors that increase the induced emf for coil/ solenoid & mag

Answer 17

-add more turns to coil
-move mag faster in & out
-increase strength of permanent mag = more dense field lines = more emf induced

Question 18

how does moving wire through mag work (wire & mag)

Answer 18

-wire is moves up & down btw U shapes mag
-field liens are cut & induced emf

Question 19

direction of needle in voltmeter when measuring size of induced emf for wire & mag

Answer 19

-when wire is stationary ( in & out of coil), no induced emf, no movement of needle
-when wire moves in, induced emf & I, needle moved to 1 side
-when wire moves out, induced emf & I direction changes, needle moves to opposite side

Question 20

factors that increase the induced emf for wire & mag

Answer 20

-increase length of wire
-move wire btw mag faster
-increase strength of mag

Question 21

factors affecting magnitude of induced emf

Answer 21

-speed of movement of wire/ mag; more speed, more emf
-no. turns on coil; more turns cut field lines, more emf (solenoid)
-size of coil; more area of coil, more wire cuts field lines, more emf
-strength of mag field; more strength, more field lines in an area, more lines cut, more emf

Question 22

factors affecting direction of induced emf

Answer 22

-orientation of poles; switching poles of mag = emf induced in opposite direction
-direction of movement of wire/ mag; reversing direction of conductor/ mag= emf induced in opposite direction

Question 23

difference between motors & generators

Answer 23

motor - convert electrical E to mechanical E (Fleming left hand rule)
uses motor effect, I -carrying conductor in mag field experiences a force, causing it to move
generator - convert mechanical E to electrical E (Fleming right hand rule)
uses generator effect/ electromagnetic induction, conductor moves through mag field, induces emf & I

Question 24

how to use Flemings’s left hand rule (motors)
I carrying conductor

Answer 24

-thumb - direction of the force or motion
-Index finger - direction of mag. field (N to S)
-middle finger - direction of I ( + ) ( - ) (conventional current flow).
-all are perpendicular to each other
FBI

Question 25

how to use Flemings's right hand rule (generators) normal conductor

Answer 25

Thumb - direction of conductor's Motion Index - direction of mag field (N to S) Middle - direction of induced I -all are perpendicular to each other Mother, Father, Child

Question 26

describe motor effect

Answer 26

-I carrying wire in between a mag = 2 mag. fields will interact -causes force on wire to push it out -wire has to be perpendicular to mag field to get full force, if wire is same direction = no force -use Fleming's left hand rule (I carrying wire experiences a force)

Question 27

how electric motors work for dc

Answer 27

-I carrying coil has its own mag field which interacts with the external mag field -forces act on opposite directions on each side of coil = turning effect -I travels coil in opposite direction -coil will continue spinning in same direction -use a split ring commutators = swap ( + ) ( - ) connections every half turn (reverse I direction) = force acting on coil will always be same direction = coil rotate in same direction (to connect coil to carbon brushes) -every 360 rotation, coil will go back to org position 1 turn = 360 -more force on coil, faster it turns

Question 28

force on I carrying conductor in a mag field

Answer 28

-I carrying conductor has its own mag field & experiences a force when interacting with external mag field -to experience a forces, I has to be perpendicular to external mag field -coil will move because of force

Question 29

2 ways to reverse direction of force for dc motor

Answer 29

-reversing the direction of the current -reversing the direction of the magnetic field

Question 30

how to increase motor effect

Answer 30

-increase I -add more turns to coil (stack them) -use stronger magnets

Question 31

how to increase generator effect

Answer 31

-move wire/ mag more quickly/ increase frequency of rotation of coil -add more turns to coil -use stronger magnets -put soft iron core in coil

Question 32

describe generator effect/ electromagnetic induction

Answer 32

-complete circuit where coil movement is perpendicular to the mag field & either one moves = field lines cut = induced emf = induced I

Question 33

how generators work for ac

Answer 33

-coil will cut the magnetic field lines -emf & I is induced in the coil -coil will spin because of force -I travels coil in opposite direction -coil will spin back to original position -use a slip ring commutators = force acting on coil will alternate directions (to connect coil to carbon brushes) -every 180 rotation, coil will go back to org position -1 turn = 180

Question 34

diff btw ac & dc

Answer 34

dc - unidirectional I, split ring, less efficient for long distance ac - bidirectional I, slip ring, more efficient for long distance

Question 35

use of components in motor/ generators: -permanent magnet -rotating coil -slip rings -split ring -carbon brushes

Answer 35

permanent magnet - to provide a uniform magnetic field rotating coil - to cut the magnetic field as it rotates and allow an induced current to flow slip rings - to allow the alternating current to flow between the coil & external circuit split ring - to allow reverse I direction to flow between the coil & external circuit carbon brushes - to provide a good electrical connection between the coil and the external circuit

Question 36

when is emf induced highest in ac generator & why

Answer 36

-position of coil is horizontal -motion of coil is perpendicular to the field -greater no. lines are cut

Question 37

when is emf induced lowest in ac generator & why

Answer 37

-position of coil is vertical -motion of coil is parallel to the field -no lines are cut

Question 38

how does a ac generator graph look

Answer 38

-emf induced against time -starting position horizontal = sine graph (emf at max) -starting position vertical = cosine graph (emf at min.)

Question 39

where do mag field lines happen in when I flows/ 3 diff ways to use right hand grip rule

Answer 39

-straight wires -solenoids -circular coils

Question 40

mag field in straight wire

Answer 40

-made up of concentric circles at the center of the wire -circular field pattern = wire has no poles -mag field strongest near the wire, weakest away from wire

Question 41

right hand grip rule for straight wires find direction of I/ mag field

Answer 41

thumb: direction of I curled fingers: direction of mag field line (arrow) (clockwise/ anti- clockwise) -reversing direction of I = reversing direction of field lines

Question 42

mag field in solenoid

Answer 42

-turns in solenoid increase strength of mag field because more wire in smaller space -field lines are similar to bar mag: emerge from N, return to S -the side that I enters is N pole ( + ) terminal -center of solenoid = straight field lines (S runs to N) -outside of solenoid = (N runs to S)

Question 43

right hand grip rule for solenoids finds N pole of mag field in solenoid/ direction of I flow

Answer 43

-wrap solenoid with fingers of right hand thumb: points to N pole of mag. field (direction of mag field) curled fingers: direction of I flow in solenoid

Question 44

mag field in circular coil

Answer 44

circular coil = 1 turn in solenoid -field lines: emerge from N, returns to S -mag field lines depend on direction of I

Question 45

right hand grip rule for circular coils finds N pole of mag field in coil/ direction of I flow

Answer 45

thumb - points to N pole of mag. field (direction of mag field) curled fingers = direction of I & curl fingers according to it

Question 46

how to increase strength & change direction of mag field in straight wires

Answer 46

-increase I flow -reverse I flow direction

Question 47

how to increase strength & change direction of mag field in solenoid

Answer 47

-increase I flow -add more turns -add a soft iron core, will be an electromag with mag field = stronger mag field -reverse I flow direction

Question 48

define transformer

Answer 48

device used to change the size of an alternating voltage or current

Question 49

what does a basic transformer consist of

Answer 49

-primary coil (input coil) -secondary coil (output coil) -soft iron core = used because it is easily magnetized

Question 50

types of transformers & what they do

Answer 50

step up - increase V, Primary coil< Secondary coil step down - decrease V, Primary coil> Secondary coil (change voltage)

Question 51

formula for transformer calculations

Answer 51

Np Vp Is ------ = -------- = ----- Ns Vs Ip n = no. turns p = primary s = secondary -do cross multiplication to find a missing on

Question 52

formula for ideal transformer

Answer 52

Power in primary = Power in secondary P = IV

Question 53

formula for efficiency

Answer 53

output ----------- x 100% input

Question 54

how are ideal transformers diff from real one

Answer 54

ideal - 100% efficient, no heat loss real - not 100% efficient, heat loss

Question 55

why do transformers have a hole in the middle of the soft iron core

Answer 55

-more surface area, heat up less easily -air can absorb heat

Question 56

relationship between heat loss, power & efficiency in transformers

Answer 56

less heat loss, less power loss, more efficiency

Question 57

different formulas for Power

Answer 57

P = IV P = Change in E/ t P = Work done/ t P = I^2V (V = IR, P = IV)

Question 58

how is electricity transmitted through power cables

Answer 58

-when I flows in wire, there is heating = E wasted -lower the I, more efficient the energy transfer -electricity in cables is low I, high V -V can be changed by transformer before it reaches homes because high V is dangerous at home

Question 59

how does a transmission of electricity in power cables look like

Answer 59

power plant, step transformer, cables, step down transformer, homes step up = high V, low I step down = low V, low I

Question 60

why does power cables that transfer electricity have high V, low I

Answer 60

high V = ensure same power transfer with same I low I = less heat loss in cables, increase efficiency