P4 - electricity & magnetism (electromagnetic forces & its effects, motors & generators, transformer) Flashcards
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
note:
-take I as conventional current direction, ( + ) to ( - )
-take field lines from N to S
diff btw coil & solenoid
coil = 1 turn
solenoid = multiple turns
define EMF
-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
define induced emf
when electromotive force is induced in any conductors when there is relative movement btw conductor & mag field
2 ways to induce emf in conductor
-conductor moves, mag field stationary (wire & mag)
-conductor stationary, mag field changes (coil/ solenoid & mag)
what does induced emf do when connected to a complete circuit
-makes delocalized electrons move
-flow of electrons produce induced I
*has to be connected to a complete circuit
how does emf work when
-conductor moves, mag field stationary (wire & mag)
-conductor (wire) cuts field lines & induced emf in conductor (wire)
how does emf work when
-conductor stationary, mag field changes (coil/ solenoid & mag)
-as mag moves through conductor (solenoid), field lines cut through turns on conductor (each individual wire)
-emf induced in coil
ways to measure size of induced emf
-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
state faraday’s law of electromagnetic induction
emf induced in a conductor is proportional to the rate of mag field lines cut by the conductor
state Lenz’s law for coil/ solenoid & mag
-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
how does Lenz’s law work (solenoid & mag)
(how does moving mag through coil/ solenoid work)
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
where are electromagnetic induction used in
-electrical generators that convert mechanical E to electrical E
-transformers used in electrical power transmission
2 ways to do induced emf
1)wire & mag
2)coil/ solenoid & mag
movement of needle in voltmeter when measuring size of induced emf for solenoid & mag
-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
factors that increase the induced emf for coil/ solenoid & mag
-add more turns to coil
-move mag faster in & out
-increase strength of permanent mag = more dense field lines = more emf induced
how does moving wire through mag work (wire & mag)
-wire is moves up & down btw U shapes mag
-field liens are cut & induced emf
direction of needle in voltmeter when measuring size of induced emf for wire & mag
-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
factors that increase the induced emf for wire & mag
-increase length of wire
-move wire btw mag faster
-increase strength of mag
factors affecting magnitude of induced emf
-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
factors affecting direction of induced emf
-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
difference between motors & generators
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
how to use Flemings’s left hand rule (motors)
I carrying conductor
-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
how to use Flemings’s right hand rule (generators)
normal conductor
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
describe motor effect
-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)
how electric motors work for dc
-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
force on I carrying conductor in a mag field
-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
2 ways to reverse direction of force for dc motor
-reversing the direction of the current
-reversing the direction of the magnetic field
how to increase motor effect
-increase I
-add more turns to coil (stack them)
-use stronger magnets
how to increase generator effect
-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
describe generator effect/ electromagnetic induction
-complete circuit where coil movement is perpendicular to the mag field & either one moves = field lines cut = induced emf = induced I
how generators work for ac
-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
diff btw ac & dc
dc - unidirectional I, split ring, less efficient for long distance
ac - bidirectional I, slip ring, more efficient for long distance
use of components in motor/ generators:
-permanent magnet
-rotating coil
-slip rings
-split ring
-carbon brushes
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
when is emf induced highest in ac generator & why
-position of coil is horizontal
-motion of coil is perpendicular to the field
-greater no. lines are cut
when is emf induced lowest in ac generator & why
-position of coil is vertical
-motion of coil is parallel to the field
-no lines are cut
how does a ac generator graph look
-emf induced against time
-starting position horizontal = sine graph (emf at max)
-starting position vertical = cosine graph (emf at min.)
where do mag field lines happen in when I flows/ 3 diff ways to use right hand grip rule
-straight wires
-solenoids
-circular coils
mag field in straight wire
-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
right hand grip rule for straight wires
find direction of I/ mag field
thumb: direction of I
curled fingers: direction of mag field line (arrow) (clockwise/ anti- clockwise)
-reversing direction of I = reversing direction of field lines
mag field in solenoid
-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)
right hand grip rule for solenoids
finds N pole of mag field in solenoid/ direction of I flow
-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
mag field in circular coil
circular coil = 1 turn in solenoid
-field lines: emerge from N, returns to S
-mag field lines depend on direction of I
right hand grip rule for circular coils
finds N pole of mag field in coil/ direction of I flow
thumb - points to N pole of mag. field (direction of mag field)
curled fingers = direction of I & curl fingers according to it
how to increase strength & change direction of mag field in straight wires
-increase I flow
-reverse I flow direction
how to increase strength & change direction of mag field in solenoid
-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
define transformer
device used to change the size of an alternating voltage or current
what does a basic transformer consist of
-primary coil (input coil)
-secondary coil (output coil)
-soft iron core = used because it is easily magnetized
types of transformers & what they do
step up - increase V, Primary coil< Secondary coil
step down - decrease V, Primary coil> Secondary coil
(change voltage)
formula for transformer calculations
Np Vp Is
—— = ——– = —–
Ns Vs Ip
n = no. turns
p = primary
s = secondary
-do cross multiplication to find a missing on
formula for ideal transformer
Power in primary = Power in secondary
P = IV
formula for efficiency
output
———– x 100%
input
how are ideal transformers diff from real one
ideal - 100% efficient, no heat loss
real - not 100% efficient, heat loss
why do transformers have a hole in the middle of the soft iron core
-more surface area, heat up less easily
-air can absorb heat
relationship between heat loss, power & efficiency in transformers
less heat loss, less power loss, more efficiency
different formulas for Power
P = IV
P = Change in E/ t
P = Work done/ t
P = I^2V (V = IR, P = IV)
how is electricity transmitted through power cables
-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
how does a transmission of electricity in power cables look like
power plant, step transformer, cables, step down transformer, homes
step up = high V, low I
step down = low V, low I
why does power cables that transfer electricity have high V, low I
high V = ensure same power transfer with same I
low I = less heat loss in cables, increase efficiency
