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Phy > Chap 6 - electromagnetic forces & its effects, motors & generators, transformer > Flashcards

Chap 6 - electromagnetic forces & its effects, motors & generators, transformer Flashcards

1
Q

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

A
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2
Q

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

A
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3
Q

diff btw coil & solenoid

A

coil = 1 turn
solenoid = multiple turns

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4
Q

define EMF

A

-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

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5
Q

define induced emf

A

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

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6
Q

2 ways to induce emf in conductor

A

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

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7
Q

what does induced emf do when connected to a complete circuit

A

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

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8
Q

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

A

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

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9
Q

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

A

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

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10
Q

ways to measure size of induced emf

A

-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

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11
Q

state faraday’s law of electromagnetic induction

A

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

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12
Q

state Lenz’s law for coil/ solenoid & mag

A

-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

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13
Q

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

A

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

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14
Q

where are electromagnetic induction used in

A

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

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15
Q

2 ways to do induced emf

A

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

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16
Q

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

A

-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

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17
Q

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

A

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

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18
Q

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

A

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

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19
Q

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

A

-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

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20
Q

factors that increase the induced emf for wire & mag

A

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

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21
Q

factors affecting magnitude of induced emf

A

-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 more wire cuts field lines, more emf
-strength of mag field; more strength, more field lines in an area, more lines cut, more emf

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22
Q

factors affecting direction of induced emf

A

-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

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23
Q

difference between motors & generators

A

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

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24
Q

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

A

-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

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25
Q

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

A

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

26
Q

describe motor effect

A

-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)

27
Q

how electric motors work for dc

A

-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

28
Q

force on I carrying conductor in a mag field

A

-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

29
Q

2 ways to reverse direction of force for dc motor

A

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

30
Q

how to increase motor effect

A

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

31
Q

how to increase generator effect

A

-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

32
Q

describe generator effect/ electromagnetic induction

A

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

33
Q

how generators work for ac

A

-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

34
Q

diff btw ac & dc

A

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

35
Q

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

A

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

36
Q

when is emf induced highest in ac generator & why

A

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

37
Q

when is emf induced lowest in ac generator & why

A

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

38
Q

how does a ac generator graph look

A

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

39
Q

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

A

-straight wires
-solenoids
-circular coils

40
Q

mag field in straight wire

A

-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

41
Q

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

A

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

42
Q

mag field in solenoid

A

-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)

43
Q

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

A

-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

44
Q

mag field in circular coil

A

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

45
Q

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

A

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

46
Q

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

A

-increase I flow
-reverse I flow direction

47
Q

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

A

-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

48
Q

define transformer

A

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

49
Q

what does a basic transformer consist of

A

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

50
Q

types of transformers & what they do

A

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

51
Q

formula for transformer calculations

A

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

52
Q

formula for ideal transformer

A

Power in primary = Power in secondary
P = IV

53
Q

formula for efficiency

A

output
———– x 100%
input

54
Q

how are ideal transformers diff from real one

A

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

55
Q

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

A

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

56
Q

relationship between heat loss, power & efficiency in transformers

A

less heat loss, less power loss, more efficiency

57
Q

different formulas for Power

A

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

58
Q

how is electricity transmitted through power cables

A

-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

59
Q

how does a transmission of electricity in power cables look like

A

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

60
Q

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

A

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

Phy (11 decks)

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