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7/29/2019 Physics: Motors & Generators PP
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Motors & Generators1. Motor Effect
2. Parallel Conductors
3. DC Motors
4. Torque
5. Magnetic Flux
6. Faradays Law
7. Lenzs Law & Back emf & Eddy
currents
8. Generators AC & DC
9. Transformers
10. Energy losses & Transmission Lines11. Westinghouse & Edison
12. AC Induction motors
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Uses of MagnetsEvery time you use a computer, you're using magnets. A hard drive relies on
magnets to
store data, and some monitors use magnets to create images on the screen. If your
homehas a doorbell, it probably uses an electromagnet to drive a noisemaker. Magnets
are also
vital components in:
CRT televisions
speakers microphones
generators
transformers
electric motors
burglar alarms compasses
car speedometers
In addition to their practical uses, magnets have numerous amazing properties.
they can induce current in wire
supply torque for electric motors.
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The Right Hand Palm Rule (RHPR)The right-hand palm rule can be used to determine the direction of the force
experienced on
a conductor. If the right thumb points in the direction of the conventional current in theconductor and the fingers of the right hand point in the direction of the external magnetic
field,
then the force on the conductor is directed outward from the palm of the right hand.
Fingers =
Thumb =
palm =
Magnetic fields surround a
magnetised object and are described
by field lines. Magnetic flux density isrepresented by the symbol B & has
the unit Tesla (T). Magnetic field lines
are directed out of the North pole &
into the South pole.
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Deduce the direction of the Force in each diagram
a) b)
c) d)B into
page (ip)
I down
page
(dp)
B out of page
(oop)
I
B to the
right ofpage
I down the
page
B down thepageI into
page
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Deduce the direction of the Force in each diagram
a) b)
c) d)
B into
page (ip)I down
page
(dp)
B out of page
(oop)
I
B to the
right ofpage
I down the
page
B down thepageI into
page
Force to the left
of page
Force out of
page
Force
to the
right
Force to the
right bottom
corner
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The Motor EffectThe motor effect is where a current carrying wire in a magnetic field experiences a force.
The direction of this force is perpendicular to both the direction of the current and the
direction of the magnetic field.
F =
B =I =
L =
=
Factors that affect the force acting on a current carrying conductor:
Strength of the magnetic field
Size of the current
Length of the conductor in the field
Angle between the conductor and the magnetic field.
F = BILsin
I coming
out of
page
(OOP)
B vertically up
page (UP)
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QuestionsQ1. A wire of length = 20cm is placed perpendicular to a magnetic field of intensity
B = 0.05T. What force does the wire experience if it carries a current I = 3A?
Q2. A conductor of length = 60cm is placed in a magnetic field B = 0.03T at an
angle of = 30. What is the force on the conductor if it carries a current I =
10A?
Solutions:
Do questions from text book: Motors & Generators p9 Q1,3,4
p12 Q1,2,4
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QuestionsQ1. A wire of length = 20cm is placed perpendicular to a magnetic field of intensity
B = 0.05T. What force does the wire experience if it carries a current I = 3A?
Q2. A conductor of length = 60cm is placed in a magnetic field B = 0.03T at an
angle of = 30. What is the force on the conductor if it carries a current I =
10A?
Solutions:
A1. F =BILsin A2. F = BILsin= 0.05 x 3 x 0.2 x sin 90 = 0.03 x 10 x 0.60 x sin 30
= 0.03N = 0.09N
Do questions from text book: Motors & Generators p9 Q1,3,4
p12 Q1,2,4
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Questions on the Motor Effect
Q1. The rigid wire XY can be moved about in the
space between the magnets as shown in the
diagram.Which of the following movements would
produce the greatest reading on the
galvanometer?
a) Downwards, quickly
b) Downwards, slowly
c) Sideways, quickly
d) Sideways, slowly
Q2. Which of the following changes would notaffect
the direction of the force on a wire carrying a
current in a magnetic field?a) Both the direction of the magnetic field and
the direction of the current is reversed.
b) The direction of the magnetic field is
reversed.
c) The magnetic field is removed.
d) The current is switched off.
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Magnetic Field near a ConductorThe magnetic field at a distance due to charges moving through a conductor
(a
current) is given by the formula:
B = KI
d
B =
K =I =
d =
The curved fingers show the direction of the
and thumb the
Do questions Motors & Generators Bk p6 Q1, 2, 3,
4, 5, 6
Note: This is not in the Year 12 Physics syllabus. But
is helpful in understanding future concept includingsolenoid current directions and magnetic pole
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Measuring Magnets Magnetic lines of force, orflux (), are measured in W. (Wb).
A field's strength, symbol .. , or the density of the flux, is measured inT (T).
Name Units Equation Description
Magnetic
Flux
Magnetic lines of force.
Is the product of magnetic f
density and the area under consideration.
Magnetic
Flux Density
(d of the flux lines) is a measure
of the number of l of force per unit
a...
Also called Magnetic Field Strength.
http://www.youtube.com/watch?v=pB7oZNBIqqc&NR=1
Surfing Bk: Motors & Generators p28,29 Q 1,2,3,4,5,6,8,11,14
http://www.youtube.com/watch?v=pB7oZNBIqqc&NR=1http://www.youtube.com/watch?v=pB7oZNBIqqc&NR=17/29/2019 Physics: Motors & Generators PP
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Fluxing Magnetic lines of force, orflux (), are measured in Webers (Wb).
Magnetic field's strength, symbol B, or the density of the flux, is measured inTesla (T).
Name Units Equation Description
Magnetic
Flux
Webers
(Wb)
= BA Magnetic lines of force.
Is the product of magnetic flux density
and the area under consideration.
Magnetic
Flux Density
Tesla (T) B =
A
(density of the flux lines) is a measure of
the number of lines of force per unit area
Also called Magnetic Field Strength.
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Magnetic Flux
Q1. In which arrangement is the flux the greatest? Explain.
Q2. In which arrangement is the flux the smallest? Explain.
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Amperes Law: Forces between Parallel
Conductors1 ampere is the amount of current flowing through two straight parallel
conductors 1 metre apart in a vacuum which produces a force of 2 X 10-7Newton
per metre of conductor.
Current carrying wires have a force between them.
The force is attractive if the currents travel in the same direction
The force is repulsive if the currents travel in the opposite direction
F =
K =
I1 =I2 =
=
d =
F = kI1 I2
d
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Activity: Complete the diagrams by drawing in direction of the magnetic fieldlines around each pair of conductors then determine the direction of the force
between each pair of conductors.
Two parallel conductors with
current flowing the same way
Two parallel conductors with current
flowing the opposite way
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Answers:
As you can see in the diagram, if two
parallel wires have currents traveling
in opposite directions, the magnetic
fields generated by those currents
between the wires will both point in thesame direction, in this case, into the
plane of the page. These wires would
repel each other.
Reason: The magnetic flux density
between wires is greater then the
density on the outside of each wire.
If two parallel wires have currents
traveling in the same direction, the
magnetic fields generated by those
currents between the wires willpoint in opposite directions
resulting in the wires attracting
each other.
Reason: magnetic flux density is
low between the wires since there
is a cancellation effect going on. So
the density is greater on the outside
of the wires.
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M i Fi ld d P ll l
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Magnetic Fields around Parallel
Wires (A) Current out of page for both
conductors.
Using the Right Hand Screw
Rule the direction of the
magnetic fields can be
deduced. (In this case
anticlockwise for both). Due to these two fields a
cancellation area exists
between them. i.e. flux is
zero.
Therefore the two wires have
an attractive force between
them.Activity: Write an explanation for what is happening in diagram (B).
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Two Conductors Into Page
Current into page for both conductors. Using the Right Hand Screw Rule the
direction of the magnetic fields can be deduced. (In this case clockwise for both)
Wire 1
Finding the force on wire 2 due to
wire 1 use vectors and the RHPR.
B
So if B is down the page and the I
is into page using RHPR the F is.................
Finding the force on wire 1 due to
wire 2 use vectors and the RHPR
Wire 2
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Two Conductors One Into Page the other Out of Page
Current going into and out of page. Using the Right Hand Screw Rule the
direction around one conductor is clockwise the other anticlockwise.
Wire 1 Wire 2
Finding the force on wire 2 due to
wire 1 use vectors and the RHPR
Finding the force on wire 1 due to
wire 2 use vectors and the RHPR
Q estions
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QuestionsQ1. Two long straight wires carry currents of 2A & 4A respectively due North. Calculate
the force, both size and direction, acting on a 1m length of the wire carrying 2A(which lies to the west of the other wire) if the wires are 1cm apart in air.
Q2. Two long parallel wires separated by a distance d carry currents ofI1 and I2respectively. They exert a force of F on each metre of wire. What is the new force ifthe currents are both doubled and the separation is halved?
Q3. Two parallel wires 12cm apart each carry currents of 8A in the same direction.What is the force acting on a 1 metre length of one of the wires?
Solution:
A1. F = KI1I2 x L A2. F = K I1I2 xL A3. F = KI1I2 x L
d d d
= 2 x 10 7 x 2 x 4 x 1 New force is given by: F = 2x10 7 x 8x8 x1
0.01 F = K 2I1 x 2I2 x L 0.12
= 1.6 x 10 N/m d/2 = 1.067 x10-4N
towards
F = K x 8 xI1 xI2 x L the other wire
d
= 8F
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Question on Two parallel Conductors
Q1.
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Questions
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TorqueTorque is the turning effect of a force. This turning force (Torque) is delivered
by a
motor. It is the product of the force and the perpendiculardistance from theaxis
to the line of action of the force. Its units are (Nm).
Starting Torque: The torque or twisting force delivered by a motor at theinstant it is switched on.
= Fd
F
Perpendiculardistance (d)
rod
axis
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Torque on a Coil in a Magnetic Field
T =
n =
B =
I =
A =
The torque on a current carrying coil in a magnetic field is dependent on: The n................. of coils
The s................. of the magnetic field
The a.................. of the coil
The a.................. between the coil and the magnetic field
The c.. in the coil
T = ......................
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Torque on a Coil in a Magnetic Field
T = torque (Nm)
n = number of coils
B = magnetic field (T)
I = current (A)
A = area (m)
The torque on a current carrying coil in a magnetic field is dependent on:
The number of coils
The strength of the magnetic field
The area of the coil
The angle between the coil and the magnetic field
The current in the coil
T = nBIA cos
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Torque v Position of Coil
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Torque v Position of CoilThe magnetic field is directed d..
the page.
In this position torque is a mbecause the long ends of the coil (that are
at right angles to the magnetic field) have
a maximum p..
distance from the a...
In this position torque is z
because the long ends of the coil ( at
r.. angles to the magnetic
field) are directly over the a.
and therefore perpendicular distance is
z...
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Torque v Position of CoilThe magnetic field is directed downthe page.
In this position torque is a maximumbecause the long ends of the coil (thatare at right angles to the magnetic
field) have a maximum perpendiculardistance from the axle.
In this position torque is zero becausethe long ends of the coil ( at rightangles to the magnetic field) are directly
over the axle and thereforeperpendicular distance is zero.
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How the Torque Direction & Equation is Derived
A rectangular coil lying parallel to a magnetic field has sidesAB and CD which carry currents at right angles to themagnetic field. They experience the force due to the
motor effect.But sides BC and AD do not as the current travelling
through those sides is parallel to the magnetic field.
Using the RHPR the force on side AB is up and force onside CD is down. Therefore it rotates in a clockwisedirection (looking from the commutator end).
If F = BIL for sides AB and CD
And d= width /2
The Torque on these sides is T = 2 (Fd)
= 2 (BIL w/2)
= BILw
Now since L x w = Area of the rectangular coil
T = BIA
And of course if there is more than one coil in the armatureT = nBIA
And if the coil is inclined at an angle to the magnetic field
T = nBIAcos
B C
DA
If the coil is a circular shape
then the area is the area of a
circle i.e. A = r
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Torque Diagram of a Motor
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QuestionsQ1. A rectangular coil of 25 turns and with an area A=0.04m is placed in a
magnetic field with B = 0.10T. If the current I = 4.0A what is the torque
when the coil is:a) Parallel to the field?
b) Inclined at 45 to the field?
c) Perpendicular to the field?
Q2. A rectangular coil consisting of 500 turns and with a width of 10cm and adepth of 20cm is placed in a radial magnetic field of intensity 10T. What
torque does the coil experience if it carries a current of 2A?
Solution:
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Questions
Q1. A rectangular coil of 25 turns and with an area A=0.04m is placed in a magnetic fieldwith B = 0.10T. If the current I = 4.0A what is the torque when the coil is:
a) Parallel to the field?
b) Inclined at 45 to the field?c) Perpendicular to the field?
Q2. A rectangular coil consisting of 500 turns and with a width of 10cm and a depth of20cm is placed in a radial magnetic field of intensity 10T. What torque does the coilexperience if it carries a current of 2A?
Solution:A1. a) T = nBIAcos A2. T = nBIAcos
= 25 x 0.10 x 4 x cos 0 = 500 x 10 x 2 x 0.10 x 0.20 x cos0
= 0.4 Nm (the side cutting the magnetic = 200 Nm
field is furthest from the axis)
b) T = 25 x 0.10 x 4 x cos 45= 0.28 Nm
c) T = 25 x 0.10 x 4 x cos 90
= 0 Nm (the perpendicular distance from the axis is zero)
Do questions from Motors & Generators BK p20 all questions
htt // t b / t h? Xi7 8 MPI0E&f t
http://www.youtube.com/watch?v=Xi7o8cMPI0E&feature=relatedhttp://www.youtube.com/watch?v=Xi7o8cMPI0E&feature=related7/29/2019 Physics: Motors & Generators PP
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Electric MotorsElectric motors are everywhere! In your house, almost every mechanical
movement that you see around you is caused by an AC or DC.. motor.
An electric motor is all about magnets and magnetism: A motor usesmagnets to create .. So if you have two bar magnets withtheir ends marked "north" and "south," then the north end of one magnetwill attract the end of the other. On the other hand, the northend of one magnet will the north end of the other (andsimilarly, south will repel south). Inside an electric motor, these attracting
and repelling forces create .. motion.
http://www.youtube.com/watch?v=Xi7o8cMPI0E&feat
ure=related
Electric motors convert .. energy into
energy.
They operate on . or...Motors can further be classified as:
a) .. motorsb) motors
Word Bank:AC, DC, commutator,electric, electrical, induction, mechanical,
motion, repel, rotational, south
http://www.youtube.com/watch?v=Xi7o8cMPI0E&feature=relatedhttp://www.youtube.com/watch?v=Xi7o8cMPI0E&feature=relatedhttp://www.youtube.com/watch?v=Xi7o8cMPI0E&feature=relatedhttp://www.youtube.com/watch?v=Xi7o8cMPI0E&feature=related7/29/2019 Physics: Motors & Generators PP
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Electric MotorsElectric motors are everywhere! In your house, almost every mechanical
movement that you see around you is caused by an AC or DC electric motor.
An electric motor is all about magnets and magnetism: A motor uses magnets tocreate motion. So if you have two bar magnets with their ends marked "north"and "south," then the north end of one magnet will attract the south end of theother. On the other hand, the north end of one magnet will repel the north endof the other (and similarly, south will repel south). Inside an electric motor,these attracting and repelling forces create rotational motion.
Electric motors convert electrical energy into mechanical energy.
They operate on DC orAC.
Motors can further be classified as: a) commutator motors
b) induction motors
http://www.youtube.com/watch?v=d_aTC0iKO68&feature=endscreen&NR=1
http://www.youtube.com/watch?v=Ue6S8L4On-Y&feature=related\
http://www.youtube.com/watch?v=d_aTC0iKO68&feature=endscreen&NR=1http://www.youtube.com/watch?v=d_aTC0iKO68&feature=endscreen&NR=1http://www.youtube.com/watch?v=d_aTC0iKO68&feature=endscreen&NR=1http://www.youtube.com/watch?v=d_aTC0iKO68&feature=endscreen&NR=1http://www.youtube.com/watch?v=Ue6S8L4On-Y&feature=related/http://www.youtube.com/watch?v=Ue6S8L4On-Y&feature=related/http://www.youtube.com/watch?v=Ue6S8L4On-Y&feature=related/http://www.youtube.com/watch?v=Ue6S8L4On-Y&feature=related/http://www.youtube.com/watch?v=Ue6S8L4On-Y&feature=related/7/29/2019 Physics: Motors & Generators PP
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Electric motors are an important application of the motor effect. An electric motor
consists of:
a permanent external field magnet ()
and a coiled conducting armature (.) which is free to rotate within the
field of the magnet.
.
and a .. (designed differently if A.C. or D.C.) connects the
armature to an external voltage source.
The speed of rotation of a motor depends on: the .. of current flowing through it,
The .. of coils on the armature,
the of the field magnet,
The of the armature,
and the mechanical .connectedto the shaft.
Word Bank: amount, brushes, commutator,
load, number, permeability,
rotor, stator, strength,
p y
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Electric motors are an important application of the motor effect. An electric motor consists
of:
a permanent external field magnet (stator)
and a coiled conducting armature (rotor) which is free to rotate within the field of the
magnet.
Brushes
and a commutator (designed differently if A.C. or D.C.) connects the armature to anexternal voltage source.
The speed of rotation of a motor depends on: the amount of current flowing through it,
the number of coils on the armature,
the strength of the field magnet,
the permeability of the armature,
and the mechanical load connected to the shaft.
Do questions from Motors & Generators BK p
Stator
http://www.youtube.com/watch?v=0ajvcdfC
65w&NR=1&feature=endscreen
Motors Everywhere!
http://www.youtube.com/watch?v=0ajvcdfC65w&NR=1&feature=endscreenhttp://www.youtube.com/watch?v=0ajvcdfC65w&NR=1&feature=endscreenhttp://www.youtube.com/watch?v=0ajvcdfC65w&NR=1&feature=endscreenhttp://www.youtube.com/watch?v=0ajvcdfC65w&NR=1&feature=endscreenhttp://www.youtube.com/watch?v=0ajvcdfC65w&NR=1&feature=endscreenhttp://www.youtube.com/watch?v=0ajvcdfC65w&NR=1&feature=endscreen7/29/2019 Physics: Motors & Generators PP
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Motors Everywhere!
The fan over the stove Tape player in the answering machine
In the microwave oven Clothes washer
dispose-all under the sink Dryer
The blender Vacuum cleaner
Electric can opener Electric drill
Refrigerator FanPower windows Hair dryer
Electric toothbrush Electric razor
Fans for the heater and the radiator The starter motor in a car
Windshield wipers Electric radio antennasSeveral in the VCR Several in a CD player or tape deck
Many in a computer (each disk drive has
two or three, plus there's a fan or two)
Most toys that move have at least one
motor
Electric clocks The garage door opener
Questions on Electric Motor
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Questions on Electric Motor
Q1. Which of the following procedures will notgenerate e.m.f?
Holding a magnet stationary inside a coil.
Rotating a coil in a magnetic field.
Rotating a magnet around a stationary coil.
Moving a bar magnet across a flat piece ofmetal.
Q2. For which one of the following is analternating current essential in its operation?
An electromagnet A galvanometer
A transformer
An electric lamp
Q3. Which of the following must be made from
a material which maintains its magnetism? The commutator for a d.c. motor
The magnet in a moving coil meter
The core of a transformer
The slip rings of an a.c. generator
Q4. A coil of copper wire wrappedaround a core could be used as anelectromagnet. Which of the followingcombinations would produce thestrongest electromagnet?
Number of turns - few, core - soft-iron Number of turns - few, core - steel Number of turns - many, core -copper Number of turns - many, core - soft-iron
Q5. An electric motor could be usedas a battery. capacitor. dynamo. transformer.
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Parts of a DC Electric Motor
.Part Description / Function
Rotor (armature)
Stator
The axle
Terminals
Split ring
Commutator
Brushes
Core
Permanent external field magnet. This doesnt usually move.
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The portion of the structure of a DC motor that rotates.
g y
Holds the armature and the commutator
The ends of each wire (one wire for each pole)
A cylindrical ring mounted on the armature shaft
consisting of a number of copper segments arranged
around the shaft . Reverses the current direction each180 rotation.
The motor brushes electrically connect the armature coils to
the power source as the commutator rotates. Usually made
of Carbon.
The iron portion of the rotor. This is usually made
up of cylindrical laminated steel or iron plates. Therotor core is to enable the rotor to turn within the
stator.
P t f DC El t i M t
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Parts of a DC Electric Motor
.
Part Description / Function
The portion of the structure of a motor that rotates.
Permanent external field magnet. This doesnt usually move.Holds the armature and the commutator
The ends of each wire (one wire for each pole)
A cylindrical ring mounted on the armature shaft consisting of a
number of copper segments arranged around the shaft.
Reverses the current direction each 180 rotation.The motor brushes electrically connect the armature coils to the
power source as the commutator rotates. Usually made of
Carbon.
The iron portion of the rotor. This is usually made up of
cylindrical laminated steel or iron plates. The rotor core is to
enable the rotor to turn within the stator.
Word Bank: The axle, Brushes, Core, Rotor (armature), Split ring Commutator,
Stator, Terminals
A P t f DC El t i M t
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Answers: Parts of a DC Electric Motor
.Part Description / Function
Rotor (armature) The portion of the motor that rotates.
Stator Permanent external field magnet. This doesnt usually move.
The axle Holds the armature and the commutator.
Terminals The ends of each wire (one wire for each pole).
Split ring
Commutator
A cylindrical ring mounted on the armature shaft consisting of a
number of copper segments arranged around the shaft.Reverses the current direction each 180 rotation.
Brushes The motor brushes electrically connect the armature coils to the
power source as the commutator rotates. Usually made of
Carbon.
Core The iron portion of the rotor. This is usually made up ofcylindrical laminated steel or iron plates. The rotor core is to
enable the rotor to turn within the stator.
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Questions on DC MotorsQ1. Which part of a simple d.c. motor reverses the direction of
current through the coil every half-cycle?
The armature
The split rings The brushes
The slip rings
Q2. Which of the following best describes the effects of doublingthe speed of rotation of a d.c. motor?
Maximum output voltage halved and frequency halved
Maximum output voltage doubles and frequency halved Maximum output voltage halved and frequency doubles
Maximum output voltage doubles and frequency doubles
Q3. What is the purpose of the carbon brushes in a d.c. motor?
Reverse the current through the coil every half-turn.
Supply current to the motor.
Provide sliding contact with rotating split rings.
Ensure that magnetic field lines are being cut.
Q4. Why is a commutator used in a d.c. motor?
It allows the coil to rotate by preventing the
wiresfrom being tangled.
It allows the coil to rotate by reversing the
current
through the coil every half-turn.
It produces greater turning effect by becoming
magnetically induced.
It produces a greater turning effect byincreasing the
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AC Motors As in the DC motor case, a current is passed through the coil, generating a
.. on the coil.
Since the current is alternating, the motor will run smoothly only at the frequencyof the sine wave. It is called a . motor.
More common is the .. motor, where electric current is
induced in the rotating coils rather than supplied to them directly.
One of the drawbacks of this kind of AC motor is the high current which must
flow through the rotating contacts. . and. at those contacts can waste energy and shorten the
lifetime of the motor.
Word bank: heating, induction,
Sparking, synchronous, torque
Question: From the point of view
of the commutator which way
is this armature rotating.
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AC Motors As in the DC motor case, a current is passed through the coil, generating a
torque on the coil.
Since the current is alternating, the motor will run smoothly only at the frequencyof the sine wave. It is called a synchronous motor.
More common is the induction motor, where electric current is induced in therotating coils rather than supplied to them directly.
One of the drawbacks of this kind of AC motor is the high current which must
flow through the rotating contacts. Sparking and heating at those contacts canwaste energy and shorten the lifetime of the motor.
Answer: clockwise
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Commutators
Split Ring Slip Ring
The main purpose of the commutator is
Draw the two types of commutators
C
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Commutators
Split Ring Slip Ring
The main . of the commutator is to make sure the . into
the armature maintains the correct . so the armature rotates in the
same . direction. DC and AC commutators are designed
. to ensure this occurs.
Draw the two types of commutators
C t t
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Commutators
Split Ring Slip Ring
The main purpose of the commutator is to make sure the current into thearmature maintains the correct direction so the armature rotates in the samecontinuous direction. DC and AC commutators are designed differently to
ensure this occurs.
Draw the two types of commutators
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GalvanometersThe D'Arsonval galvanometer is a moving coil
ammeter. It uses magnetic deflection, where current
passing through a coil causes the coil to move in amagnetic field.The voltage drop across the coil is kept to a
minimum to minimize resistance across the
ammeter in any circuit into which it is inserted.
The modern form of this instrument was
developed by Edward Weston, and uses two spiralsprings to provide the restoring force. Bymaintaining a uniform air gap between the iron coreof the instrument and the poles of its permanent
magnet, the instrument has good linearity and
accuracy.
Basic meter movements can have full-scaledeflection for currents from about 25 microamperes
to 10 millamperes and have linear scales.
Activity: Do Questions from Motors & GeneratorsBK p 45? Q1 to 5
Analog electric meters (i.e.,galvanometer, ammeter,
voltmeter) operate on the
motor principle.
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LoudspeakersThe fluctuating magnetic field in the coil is produced due to the fluctuating
current
which causes the coil and cone to vibrate in response to these current
fluctuations
and produce sound.
Activity: Create a step by step guide on how a loudspeaker works usingthe paragraph above.
Steps:
1. Coil attached to a AC supply sets up a fluctuating magnetic field.
2. The permanent magnet produces a magnetic field.3. These two interacting fields cause the coil to vibrate towards and away
from the permanent magnet.
4. This vibrating coil attached to a cone makes the cone also vibrate. i.e.
sound.
Question: Does the louds eaker utilise the motor effect? Ex lain.
Loudspeakers/ Galvanometer
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Loudspeakers/ GalvanometerActivity: Create a step by step guide on how a loudspeaker or
Galvanometer work and draw a labelled diagram.
Steps:
1.
2.
3.
4.
5.
6.
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Faradays Law of Induction
Did you know ?Newton was born the year Galileo died, Maxwell was born in 1831, the year Faradaydiscovered EM induction and died in 1879 the year of Einsteins Birth.
Since an electric current can produce
a magnetic field.
Faraday asked:
Can a magnet create a electric
current?
Answer: Yes as long as the magnet
moves inside a coil. This is the way
we produce electricity. Achanging magnetic field will induce a
changing electric current. This is
called Faradays law of induction. = - d dt
where is themagnetic flux
Questions on Faradays Law
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Questions on Faraday s LawQ1. The diagram shows a permanent magnet moving up
and down on the end of a spring. The movement of themagnet induces an e.m.f. in the coil. Which factor, onits own, would decrease the maximum value of the
induced e.m.f.?a) Increasing the number of turns on the coil
b) Increasing the strength of the magnet
c) Raising the coil
d) Raising the support of the spring
Q2. If a bar magnet is pushed into a solenoid, an e.m.f. willbe induced across the ends of the solenoid. This e.m.f.can be made larger by
a) using a bar of soft iron instead of a magnet.
b) using a solenoid made of low resistance wire.
c) moving the magnet more quickly.
d) connecting a voltmeter across the solenoid.
Electromagnetic Induction
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Electromagnetic InductionAn electric current is induced in a conductor as it moves relative to a magnetic field.
The
term induction refers to a temporary condition in the circuit, and the induced current
disappears when the event causing induction stops.
Induction of a current in a coil of wireWhenever there is a change in the number of magnetic field lines passing through aloop of wire a voltage (or emf) is generated (or induced) in the loop of wire. This ishow an electric generator works. The phenomenon is known as electromagneticinduction and is explained by Faraday's law of induction.
Lenzs Law
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Lenz s Law Lenzs Law was formulated by the German physicist Heinrich Lenz in 1834, over a
decade after Faraday and Henry had discovered electromagnetic induction. Lenzs
Law gives the direction of the induced electromotive force and current resulting
from electromagnetic induction. He discovered that the magnetic field of aninduced current always opposes the change in magnetic field that is causing the
induced current.
An example of this is when the north pole of a magnet approaches a helix, the
induced current forms a second north pole which repels the first. If the north pole
of the magnet is moved away from the helix, then the induced current sets up asouth pole to attract the bar magnet and again opposes the motion of the magnet.
The above scenario is only one example of electromagnetic induction. Lenz's Law
ensures that all induced currents have magnetic fields that oppose the change that
induces them. Lenzs law is a direct application of the principle of conservation of
energy. The induced current must have received energy in order to begin to flow.
Lenzs Lawhttp://www.youtube.com/watch?v=kU6NSh7hr7Q&feature=r
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Lenz s LawLenzs Law states: that the .. of the induced . is such that the
current it
produces creates a . field .. the change that produced this
emf.
This is equivalent to an emf in the opposite direction to the applied emf (hence .
emf).
Q1. Describe what is happening in
diagram 1.
Q2. Describe what is happening in
diagram 2.
N
elated
Word Bank: back, emf, direction, magnetic,
opposing,
Lenzs Law
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Lenz s LawLenzs Law states: that the direction of the induced emf is such that the current it
produces creates a magnetic field opposing the change that produced this emf.
This is equivalent to an emf in the opposite direction to the applied emf (hence backemf).
Q1. Describe what is happening in
diagram 1.
As the north pole of the magnetapproaches the coil, a current is
induced in the coil that makes the end
closest to the magnet a North pole, so as
to oppose the entry of the bar magnet.
(Use right hand grip rule). The needle in
the galvanometer moves to the leftindicating the flow of current.
Q2. Describe what is happening in
diagram 2.
N
http://www.youtube.com/watch?v=kU6NSh7hr7Q&feature=related
Q ti
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Questions:Work out the direction of the current in each of the solenoids (draw coils and
current direction), and the north and south poles of the solenoids due the
bar magnet movement. (All coils are attached to a galvanometer).
1. 2.
3. 4.
N SS N
S N N
What happens to the
coil on the trolley?
A
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Answers:
1. 2.
3. 4.
N S
S N
S N N
The trolley moves away from the
bar magnet.
N
S
N
S
Trying to attract magnet Trying to repel magnet
Trying to attract magnet
Trying to repel magnet.
E l F d L
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Example on Faradays Law A neutral, straight, conducting wire contains equal
amounts of positive and negative charges.
However, the electrons are free to move insidethe wire, while the positive nuclei are not.
If a straight conducting wire is placed in a plane
perpendicular to a uniform magnetic field, and is
moving in a direction perpendicular to the field,
then each charge q in the wire experiences a
magnetic force of magnitude F = qvB. The
negatively charged electrons will accelerate in
response to this force. Since they cannot leave
the wire, negative charge will accumulate on one
end of the wire, while positive charge will be left
behind on the other end. The separated chargesproduce an electric field, which exerts a force on
the other charges in the wire. This electric force
opposes the magnetic force. Once the electric
force is strong enough to cancel the magnetic
force, electrons will no longer accelerate, and
their net motion will stop due to the resistance ofthe wire. We then have vB = E. The electric
Questions:
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Questions:Determine the direction of the induced current in each of the questions below. Use the RHPR.
a) b)
c) d)
Red line represents the conductor.Green arrow the applied force direction.
Brown dots and orange crosses the direction on the magnetic field.
Questions:
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Questions:Determine the direction of the induced current in each of the questions below. Use the RHPR.
a) b)
c) d)
Purple arrow represents the direction of the induced forceRed arrow represent the direction of the induced current
Questions on Lenzs Law
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Questions on Lenz s LawQ1. When a magnet was pushed
towards a solenoid, the sensitive
meter connected to the solenoiddeflected to the right. When the
same magnet was pulled away
from the solenoid at the same
speed, what was the deflection
on the meter?
The same and to the right
The same but to the left
Greater but to the right
Greater but to the left
Q2. A bar magnet is rotated on ashaft
near to a coil as shown. A cathode-
rayoscilloscope connected to the coilindicates the induced e.m.f.
Which change does not increase thesize of the induced e.m.f.?
Moving the magnet closer tothe coil . Turning the magnet in the
opposite direction at a greaterspeed.
Turning the magnet in theopposite direction at the samespeed.
Using a coil with more turns.
Example on Faradays Law
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Example on Faraday s Law A magnet is moved
quickly towards a wire
loop as shown.
The flux through the wire
loop (red) is increasing. A
current starts flowing in
the loop. The magneticfield produced by this
current opposes the flux
changes that produce it.
Induced currents in coils caused by changes in
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Each magnet is moved in a certain direction and induces a current in the
loop of wire that will cause a magnetic field to be induced so as to hindered
the movement of the magnet.
Induced currents in coils caused by changes inmagnetic flux
Lenzs Law and Loops
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Lenz s Law and Loops
Lenz's law describes the tendency of nature to resist any change inmagnetic flux passing through a loop of wire.
Changes in flux can be canceled by inducing a magnetic field in theappropriate direction.
Question: Lenzs Law and Loops
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Question: Lenz s Law and Loops The problem statement, all variables and given/known data
A loop rests in the xy plane. The z axis is normal to the plane and positive
upward. The direction of the changing flux is indicated by the arrow on the z axis.
The diagram that correctly shows the direction of the resultant induced current in
the loop is:-
Correct one isFigure 4. The magnetic flux in this one is increasing inthe -z-direction. From Lenz's Law, there is an opposition to this change,
so the induced current will be counter-clockwise, which corresponds to
the +z-direction?
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1. The problem:The figures show two different
situations where a current may be
induced in a loop according to
Faraday's Law, with the directiongiven by Lenz' Law. The magnetic
field is shown by the x's in Fig. 2.
Select ALL correct answers for the
current in the loop.
A) fig2: Loop moving North, induced
current `b'.
B) fig1: Magnet moving West,
induced current `a'.
C) fig2: Loop moving South, no
induced current.
D) fig1: Loop moving West, induced
current `a'.
E) fig2: Loop moving East, induced
current `b'.
F) fig1: Magnet moving East,
induced current `a'.
CDEF
Slidewire generator
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The induced current (I)
causes a force F =B I L to be
exerted on the purple bar.
This force is in the direction
opposite (left), to the original
velocity v (right).
Slidewire generatorBecause the magnetic flux () through the loop is changing there is an emf
induced in the loop in accordance with Faraday's law.
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If we place the wire on a conducting rail, a current will start to flow in thecircuit formed by the rail and the wire.
Electric Generators
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ec c Ge e a o sElectric Generators (alternator (AC), ordynamo (DC)) are a group of devices
used
to convert mechanical energy into electrical energy by electromagnetic means.The
armature, which is the structure supporting the conductors cuts the magnetic fieldand
carries the induced current in a generator. Below shows one complete rotation of a
rectangular coil.DC Generator (Dynamo)
Voltage Curve
AC Generator (Alternator)
Voltage Curve
TimeTimeVolta
ge
Voltag
e
This is a sine curve. In positions 1, 3,
5 the coil does not cut the flux lines
and therefore the current is zero.
The result is that current flows in
one direction only and the voltage
is always above the zero line.
1 2 3 4 51 2 3 4 5
DC Generator (Dynamo)
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DC Generator (Dynamo)
In a DC generator the direction of the induced current in the loops must bechanged one-half turn of the generator shaft. This is donewith rings on the rotating armature contacting the stationary." which are in turn connected to the wires coming out of thegenerator.
DC Generator
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DC Generator
In a DC generator the direction of the induced current in the loops must bechanged every one-half turn of the generator shaft. This is done with splitrings on the rotating armature contacting the stationary "brushes" whichare in turn connected to the wires coming out of the generator.
AC Generator
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AC Generator The ac generator uses Faraday's laws of induction, it consists of a
coil of wire rotating a magnetic field. As the coil rotates it cuts the
magnetic flux generating an EMF, the EMF produced is given by
Faraday's law.
The electrons flow first in one direction and then, in the other. The
generator produces an alternating current. One advantage that AC
has over DC is that it can easily be "stepped up" or "stepped down"
with a transformer.
AC Generator (Alternator)
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AC Generator (Alternator)
In an AC electric generator, or , the flux through the loops of
wire (wound on the armature) ., and therefore according
to Faraday's law there will be an emf induced in the loops of wire. This
induced emf causes a current to flow in the loops.
AC Generator
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AC Generator
In an AC electric generator, or alternator, the flux through the loops of wire
(wound on the armature) changes, and therefore according to Faraday's
law there will be an emf induced in the loops of wire. This induced emf
causes a current to flow in the loops.
Question: AC Generator Voltage
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A simple a.c. generator produces a voltage which varies with time as shownin the
diagram above. Which graph below shows how the voltage varies with time
whenthe speed of rotation is halved?
Polyphase Motors/ Generators
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yp By having 3 sets of armature coils separated by 120 three emfs (and
currents) can be produced during each revolution. Such 3 phase
generators or motors are more efficient than single phase ones.
These motors are limited primarily to industrial applications (for higher-
power) although they may be used in air conditioning units around the
home.
The starting & reversing torque characteristics of these motors are
exceptionally good.
List the advantages of Polyphase motors and
Generators over single phase ones:
emf (electromotive force)
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emf (electromotive force)A synonym for voltage that is usually restricted to generated voltage. i.e. what the power pack
produces in the school science lab. If the term electromotive force is used, it is not aforce, but energy per unit charge.
Explanation: When a conductor is connected across the terminals of a battery or electriccell, a flow of charge(I) results. As the charges flow through the energy source they gainenergy. The emf of a source of electrical energy is defined as the energy supplied to eachunit of charge that passes through the source, i.e.
=emf = energy supplied
charge
Although potential difference and emf have the same unit and we think of them the same theyare actually different.
Explanation:
When a charge q moves through a potential difference of V volts across a conductor, itdoes work given by W = qV. This work is equal to the energy transformed to heat in theconductor. Potential difference measures the energy released by the electric charge perunit of charge, i.e.
p.d = potential difference = energy released
charge
Back emf
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The emf that a motor generates is called the back emf. The back emf increases with the speed, because of Faraday's law. So, if the motor has no load, it turns very quickly and speeds up until
the back emf, plus the voltage drop due to losses, equal the supplyvoltage.
The back emf can be thought of as a 'regulator': it stops the motorturning too quickly.
Question:
Q1. Why does a motor generate a back emf?
By Lenz's law, the emf generated by the motor coil will oppose thechange that created it. Therefore an emf in the opposite directionwill be generated (induced).
Eddy Currents
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yDefinition: Localised currents induced in an iron or steel core by
alternating magnetic flux.
Explanation: When a metal sheet is present in the magnetic field,electromagnetic induction causes an eddy current perpendicular to the
magnetic flux lines to flow on the surface of the sheet.
Results: These currents translate into energy in the form of heat. Theminimization of this heat is an important factor in lamination design inboth motors and generators. In the case of solid conductors like metal
sheets these currents are circular.
Uses: These currents are used in:-
1. Electromagnetic braking
2. Induction cooking
How to work out Direction of Eddy Currents in Sheets of
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Metal Pick a point on the metal surface in the magnetic field but close to where
the field ends. Since the eddy current must oppose the motion of the metalsheet (by Lenzs Law), the eddy current will cause a force on the sheet
in the opposite direction to the motion of the sheet. So, the force isback towards the left, the field is into the page & therefore by the RHPR, theeddy current must move up towards the top of the page. Since the eddycurrent forms at the boundary of the magnetic field (ie where the magneticflux changes from a particular value to zero), the eddy current will form in aclockwise direction in this case, as shown. In other words, the circle mustcome out of the field, not go further back into the field.
Example:
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Here we examine a different method ofchanging the magnetic fieldinside a loop of wire. The yellow is a region ofconstant magnetic field(into the screen). When we move the loop across this region, since thearea affected by the magnetic field changes, the loop senses a changing
magnetic field, and a current is induced in the loop. Notice here that when the loop is completely outside or inside this region
of constant magnetic field, there is no current.
Graph the changing emf occurring as the loop enters then passes through the
magnetic field and out the other side. (x-axis = time; y-axis = emf strength)
Uses of Induction
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Electromagnetic Breaking eddy currents
Induction cooking eddy currents
Transformers AC induction motor - Squirrel cage
- Synchronous motors
Electromagnetic Braking
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g g
Electromagnetic braking relies on the opposing effect of the induced
currents to
create a retarding force on a conductor, such as:- metal wheels on trains
sheet metal on certain fun park rides
In Science triple beam balances.
The Advantages:Since the effect is greatest when the wheels of a train are moving fastest, it
follows that as the wheels slow down the braking decreases resulting in
smooth braking.
Reducing Eddy Currents:The eddy currents can be reduced though by creating slits in the metal
wheels or the sheet metal. What this does is reduce the size of the eddy
currents that can form.http://www.youtube.com/watch?v=gK9LD1G6fX8&feature=related
Induction Cooking
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gInduction cookers use coils placed beneath a glass ceramic cook top to
generate heat for cooking. Steps:
1. Alternating current in the coil sets up an oscillating magnetic field2. Which induces eddy currents in metal pans, placed in the vicinity of
the varying magnetic field.
3. These currents cause the metal to get hot and therefore heat itscontents.
Advantages: Almost all the heat goes into heating
the pan and its contents and not the
element. Therefore it is cheaper tooperate i.e. More efficient.
The glass ceramic cook top is easy toclean as it is flat with no depressions.
Q1. Why do the metal pans have
induced eddy currents and not the egg?
A1. Free electrons available to move.
Transformers
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Transformers are devices for transferring electrical energy from one circuit to
another while changing the size of an AC voltage.
They are composed of:-
Two coils of wire (primary and secondary)
Iron core
Changing AC supply connected to the primary coil
The changing AC voltage in the primary coil sets up a changing magnetic field
(flux) in the iron core. And by mutual induction this changing magnetic fieldinduces a voltage in the secondary coil.
Question : The diagram shows what type of transformer? (Step up or stepdown)
Answer: Step up
Transformers
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Transformers allow 240V to be stepped down to convenient levels for
digital electronics (only a few volts) or for other low power applications
(typically 12V). List 2 devices that use lower than 240V in your house and
2 devices that use higher than 240V in your house or factories
Lower than 240V Higher than 240V
The core has high magnetic permeability, i.e a material that forms a
magnetic field much more easily than free space does, due to the
orientation of atomic dipoles.
In the diagram, the core is laminated soft iron. The result is that the field is
concentrated inside the core, and almost no field lines leave the core. Itfollows that the magnetic fluxes through the primary and secondary coils
are approximately equal.
Transformers
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Transformers allow 240V to be stepped down to convenient levels for digital
electronics (only a few volts) or for other low power applications (typically
12V). List 2 devices that use lower than 240V and 2 devices that use higher
than 240V.
Lower than 240V Higher than 240V
Laptop TV
School Power Packs
The core (shaded) has high magnetic permeability, i.e a material that forms a
magnetic field much more easily than free space does, due to the orientation
of atomic dipoles.
In the diagram the core is laminated soft iron. The result is that the field is
concentrated inside the core, and almost no field lines leave the core. It
follows that the magnetic fluxes through the primary and secondary coils are
approximately equal.
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Step Up / Step Down
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S ep Up / S ep oTransformers Voltage Purpose
Step Up Higher in secondary coil Power Transmission over distances
Step Down Lower in secondary coil Safe distribution to residences
Vp = Is = Np
Vs Ip Ns
From the conservation of energy
Power in = power outVI (primary coil) = VI (secondary coil)
VpIp = VsIs,Vp/Vs = Is/Ip
So you don't get something for nothing: if you increase the voltage, youdecrease the current by the same factor. In some cases, decreasing the current isthe aim of the exercise. In power transmission lines, for example, the power lost inheating the wires due to their resistance is proportional to the square of the current.P = IR . So it saves a lot of energy to transmit the electrical power from powerstation to city at very high voltages so that the currents are only modest.
Questions:Q1. A transformer has a primary of 5000 turns and a secondary of 250 turns. If the primary
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Q1. A transformer has a primary of 5000 turns and a secondary of 250 turns. If the primaryvoltage is 240V what is the secondary voltage?
Q2. A 12V 24W globe is connected to the secondary in the previous example. If it operatesat its correct power rating:
a) What power is used in the primary?
b) What current is in the primary?
Q3. What is the turns ratio in the school transformers when it produces 12V?
Solutions:A1. Vp = Np A2. a) Assuming a perfect transformer A3. Vp =240Vs = Ns power in = power out Vs =12
VpIp = 24W
240 = 5000 r = Np /Ns = Vp/Vs
Vs 250 b) VpIp = 24W = 240 / 12
Ip = 24 / 240
Vs = 12V Ip = 0.1A Np : Ns
20 : 1
Do questions: Motors & Generators p47
Questions on TransformersQ1. A transformer consists of a coil of 1200 turns
d th il ith t t l f 120 t hi h
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and another coil, with total of 120 turns, whichcan be tapped at various places. Which pair ofterminals would you connect to a 12 V, 24 Wlamp for it to be lit normally?
RT RV
SU
TV
Q2. A step-up transformer has a turns ratio of 1:100.An alternating supply of 20 V is connected acrossthe primary coil. What is the secondary voltage?
0.2 V
5 V
100 V
2000 V
Q3. A transformer is needed to convert a mains 240V supply to a 12 V supply. If there are 2000 turnson the primary coil, how many turns should therebe on the secondary coil?
100
200
24 000
90 000
Q4. What is the purpose of astep-down transformer?
It makes the output currentlower than the input current.
It makes the output currentequal to the input current.
It makes the output voltagehigher than the input voltage.
It makes the output voltagelower than the input voltage.
Transformers
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AC vs DC
Transformers only work on AC, which is one of the great advantages of AC.
Without transformers, the waste of electric power in distribution networks,
already high, would be enormous. It is possible to convert voltages in DC,but
more complicated than with AC. Further, such conversions are ofteninefficient
and/or expensive.
AC has the further advantage that it can be used on AC motors, which are
usuallypreferable to DC motors for high power applications.
AC DC
Transformers function
using
Complicated conversion
EfficiencyExpensive
Transformers
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AC vs DC
Transformers only work on AC, which is one of the great advantages of AC.
Without transformers, the waste of electric power in distribution networks,
already high, would be enormous. It is possible to convert voltages in DC,but
more complicated than with AC. Further, such conversions are ofteninefficient
and/or expensive.
AC has the further advantage that it can be used on AC motors, which are
usuallypreferable to DC motors for high power applications.AC DC
Transformers function using Y N
Complicated conversion N Y
Efficiency Good Poor
Expensive Low High
Efficiency of transformers
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yIn practice, real transformers are less than 100% efficient.
1. Resistive losses in the coils
(losing powerI.r) For a given material, the resistance of the coils can bereduced
by making their cross section large. The resistivity can also be made low byusing
high purity copper.
2. Eddy current losses in the core.
These can be reduced by laminating the core. Laminations reduce the areafor eddy
currents to form, and therefore the energy thust lost.How Describe
Resistive losses incoils
Losing power.Equation I.r
Making the coil cross sectionlarge reduces resistivity
Resistive losses incoils
Type of material Using high purity copper
reduces resistivity
Eddy currentlosses in core
Reduce the area for eddycurrents to form in thecore.
Laminating the core reduces
eddy currents
Laminations
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Early designers of AC motors encountered problems traced to lossesunique to alternating current magnetics.
Both rotor and stator cores of AC motors are composed of a stack of
insulated laminations. The laminations are coated with insulatingvarnish before stacking and bolting into the final form. Eddy currentsareminimized by breaking the potential conductive loop into smallersegments. The thin isolated laminations break these loops. Also, thesilicon (a semiconductor) added to the alloy used in the laminationsincreases electrical resistance which decreases the magnitude of eddycurrents.
Induction Motors
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Is an alternating current motor where the primary winding on one member (usually the
stator) is connected to the power source. A secondary winding on the other member
(usually the rotor) carries the induced current. There is no physical connection to thesecondary winding; its current is induced. The AC induction motor was invented in
1888 by
Tesla. Prior to this only DC motors were known.
Working Model of Teslas
Induction Motor
Induction Motors in your Home
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Believe it or not, nearly everyone you know has induction motors within every home.
These motors are often called "squirrel cage motors" and are in washingmachines, dryers, water pumps and many other devices.
Besides being numerous and cheap they use no brushes and do not produce anyRFI (Radio Frequency Interference.
OK, what is so great about it? There is nothing complicated about the conversion,no weird rewiring, no complicated math! There are no brushes to wear out.
They can not be overloaded; if too
much of a load is applied, it simply
quits. Removing the load will usually
cause the motor to start again.
Typical electric squirrel cage
motors
Squirrel Cage Induction Motors:
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The single-phase AC induction motor is
the most common AC motor in use
today.A changing magnetic field in the stator
induces
an AC current in the rotor. The current inthe
rotor produces its own magnetic field, which
then interacts with the magnetic field of thestator, causing the rotor to turn.
Clearly, the name induction motor comesfrom
the fact that no current is fed directly to the
rotor from the mains supply. Current is
induced in the rotor by the changingmagnetic
field of the stator.
Activity: Create a flow diagram of how the
Squirrel cage induction motor works from
the
http://www.youtube.com/watch?v=HWrNzU
Cjbkk&feature=related
Flow Diagram of How an AC Induction Motor
works
http://www.youtube.com/watch?v=HWrNzUCjbkk&feature=relatedhttp://www.youtube.com/watch?v=HWrNzUCjbkk&feature=relatedhttp://www.youtube.com/watch?v=HWrNzUCjbkk&feature=relatedhttp://www.youtube.com/watch?v=HWrNzUCjbkk&feature=related7/29/2019 Physics: Motors & Generators PP
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works
Flow Diagram of How an AC Induction Motor
works
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worksA changing magnetic field in the stator
Induces an AC current in the rotor
The current in the rotor produces its ownmagnetic field
Which then interacts with the magnetic field ofthe stator, causing the rotor to turn.
The rotor turns in the same direction as thestators magnetic field
The Rotorhttp://www.youtube.com/watch?v=7tEsJ-
xAoEQ&feature=related
http://www.youtube.com/watch?v=7tEsJ-xAoEQ&feature=relatedhttp://www.youtube.com/watch?v=7tEsJ-xAoEQ&feature=relatedhttp://www.youtube.com/watch?v=7tEsJ-xAoEQ&feature=relatedhttp://www.youtube.com/watch?v=7tEsJ-xAoEQ&feature=relatedhttp://www.youtube.com/watch?v=7tEsJ-xAoEQ&feature=related7/29/2019 Physics: Motors & Generators PP
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The r..of an induction motor consists of a c..arrangement ofc. ora. conductingb attached to two end r.. at either end of the
bars. These end rings short circuit the bars and allow current to flowfrom one side of the cylinder to the o. This type of rotor isusually referred to as a s. c owing to itsresemblance to the cage or wheel that people use to exercise petsquirrels or mice.
The squirrel cage fitsinto a laminated iron
core or armature, which
is mounted on the shaft
of the motor.
Word Bank: aluminium, bars, cage, copper, cylindrical, other, rings,rotor, squirrel
The Rotor
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The rotorof an induction motor consists of a cylindricalarrangement ofcopperoraluminum conducting bars attached to two end ringsateither end of the bars. These end rings short circuit the bars and allow
current to flow from one side of the cylinder to the other. This type ofrotor is usually referred to as a squirrel cage, owing to its resemblanceto the cage or wheel that people use to exercise pet squirrels or mice.
The squirrel cage fitsinto a laminated iron
core or armature, which
is mounted on the shaftof the motor.
Th t t r i t f b f il f i d l i t d i
The Stator
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The stator consists of a number of coils of wire wrapped on laminated ironcores. The stators the rotor. Single-phase alternating
current flowing through the stator coils produces a c....
magnetic field that threads through the rotor. This changing magnetic fieldi... an alternating current in the rotor, which in turn setsup its own changing m field. The changing magnetic
field produced by the stator actually rotates and drags the magnetic field of
the rotor around with it. Thus, the rotor rotates in the s..direction as the rotating field of the stator.
Questions:Q1. The Squirrel cage is the
(rotor/stator).
Q2. Which one moves, the rotor or
stator?
Q3. Is there any contact between the
rotor and stator?
Q4. Which (rotor/ stator) is on the
inside?
Q4. How is an induction motor
different from a normal AC motor?
The statorconsists of a number of coils of wire wrapped on laminated iron
The Stator
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pp
cores. The statorsurrounds the rotor. Single-phase alternating currentflowing through the stator coils produces a changing magnetic field thatthreads through the rotor. This changing magnetic field induces an
alternating current in the rotor, which in turn sets up its own changingmagnetic field. The changing magnetic field produced by the stator actuallyrotates and drags the magnetic field of the rotor around with it. Thus, the
rotor rotates in the samedirection as the rotating field of the stator.
Questions:Q1. The Squirrel cage is the
(rotor/stator).Q2. Which one moves, the rotor or
stator? RotorQ3. Is there any contact between the
rotor and stator? NoQ4. Which (rotor/ stator) is on the
inside? RotorQ4. How is an induction motor different
from a normal AC motor? No Brushesor connection between stator and
rotor.
Diagram of Induction Motor
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Label the diagram below.
Answer: Diagram of Induction Motor
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stator
Question: Induction Motors
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Q1. Discuss the reason why induction based electric motors are the most
common motors in use today. Make sure you list the advantages of
induction motors over commutator and brush type motors.
Q2. List some common devices that use induction motors.
A1.
There is no physical connection to the secondary winding; its currentis induced. Therefore less wear and tear as no commutator orbrushes are involved. This leads to less maintenance and is morecost efficient.
They can not be overloaded; if too much of a load is applied to themotor, it simply quits. Removing the load will usually cause themotor to start again.
Do not produce any RFI (Radio Frequency Interference).
A2. Air conditioners, vacuum cleaners, washers, dryers, fans, gardenleaf blowers, most motorised kitchen appliances that run on ACsupply.
Induction Motor
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This is a cut-away of a
industrial three phase AC
motor.
Synchronous Motor
A synchronous motor is composed of the following parts:
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A synchronous motor is composed of the following parts:
A three-phase medium voltage stator similar to that of an induction motor.
A wound rotor (rotating field) which has the same number of poles as the
stator Is supplied by an external source of direct current (DC).
Large machines may include additional parts for cooling the machine,supporting the rotor, lubricating and cooling the bearings, and variousprotection and measurement devices.
How a Synchronous Motor Works
The operation of a synchronous motor is simple to imagine. The armature
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The operation of a synchronous motor is simple to imagine. The armaturewinding, when excited by a poly-phase (usually 3-phase) supply, creates arotating magnetic field inside the motor. The field winding, which acts as apermanent magnet, simply locks in with the rotating magnetic field and rotates
along with it. During operation, as the field locks in with the rotating magnetic field,the motor is said to be in synchronization.
Once the motor is in operation, the speed of the motor is dependent only on thesupply frequency. When the motor load is increased beyond the break down load,the motor falls out of synchronization i.e., the applied load is large enough to pullout the field winding from following the rotating magnetic field. The motorimmediately stalls after it falls out of synchronization.
Explaining How the Synchronous Motor Works in the Animation
First stare at the pole of the top-most (orange) winding. Observe that it pulses
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p p ( g ) g palternately red and blue, indicating that the current in the winding generatesalternately a north and south magnetic pole here. This is, of course, due to thecurrent in the winding passing first in one direction and then the other.
Now look at all the other orange windings. Their poles pulse in the same patternas the top one and at the same time. The orange winding are all connectedtogether and carry the same current.
Next, look at the green windings. Their poles pulse in the same way, but not at thesame time.
This is also true of the magenta windings.
To see how the different windings generate magnetic poles moving in a circular
manner, wait for a red (or blue, if you prefer) pulse at one pole and then move youreyes to the next counter clockwise winding and wait for your color there. It shouldtake about a second. With a little practice, you will be able to follow a pulse aroundthe stator.
Now for the really tricky part. As you follow a red pulse around the stator poles,glance at the rotor and note the positions of the magnetic poles there. You will seea red rotor pole being "pushed" around in front of the red pulse you're following bymagnetic repulsion. You will also see a blue rotor pole being "pulled" by magnetic
attraction behind the red pulse you're following. That's it. That's how it works.
Advantages of Synchronous Motors
The initial cost of a synchronous motor is more than that of a conventional ACinduction motor due to the expense of the wound rotor and synchronizing circuitry
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induction motor due to the expense of the wound rotor and synchronizing circuitry.These initial costs are often off-set by:
Precise speed regulation makes the synchronous motor an ideal choice for certainindustrial processes and as a prime mover for generators.
Are available in small sizes for applications requiring precise timing such as timekeeping, (clocks) and tape players.
Synchronous motors have speed / torque characteristics which are ideally suited fordirect drive of large horsepower, low-rpm loads such as reciprocating compressors.
AC Synchronous Motor Synchronous motors have the characteristics of constant speed
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Synchronous motors have the characteristics ofconstant speedbetween no load and full load.
They are often used to drive DC generators.
Synchronous motors are designed in sizes up to thousands ofhorsepower.
They may be designed as eithersingle-phase ormultiphase machines.
In the diagrams above, we have a squirrel cage AC induction motor, and a
synchronous motor. The inventor of the three-phase AC motor was NikolaTesla.
Induction v Synchronous Motors
Induction Synchronous
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Induction Synchronous
The changing magnetic field produced by
the stator actually rotates and drags the
magnetic field of the rotor around with it.
Is an AC motor distinguished by a rotor spinning
with coils passing magnets at the same rate as
the alternating current thus resulting in a rotatingmagnetic field which drives it.
Must slip in order to produce torque. Does not rely on slip under usual operatingconditions and as a result, produces torque atsynchronous speed. In other words they operate
synchronously with line frequency.Speed is determined by the number of
pairs of poles and the line frequency.Speed is determined by the number of pairs of
poles and the line frequency.
Synchronous motors are available in sub-
fractional sizes to high-horsepower direct-current
industrial sizes.
In the fractional horsepower range, synchronous
motors are used where precise constant speed
is required.
In high-horsepower industrial sizes, the
synchronous motor provides a highly efficient
means of converting AC energy to work.
Power Transmission
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In a power station a turbine drives the alternator. This is achieved either
by:
Using the force of moving water (hydro-electricity) Using the pressure of steam produced by burning coal, oil, or natural
gas
Using the heat energy released from nuclear reactions
Each alternator produces 3 phase electricity with voltages as high as25000V (25KV) and currents as large as 20000A. Large step up
transformers further boost the voltage to as high as 500 000V (500KV)
for distribution over the power lines which form part of the electricity
grid.
Additional transformers between the power stations and the consumer
reduce energy losses and gradually lower the voltage so by the time itgets to household users it is 240V.
Question:
Power Transmission The transformer's ability to step AC voltage up or down with ease gives AC an
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y p g p g
advantage unmatched by DC in the realm of power distribution in figure below.
When transmitting electrical power over long distances, it is far more efficient to do
so with stepped-up voltages and stepped-down currents (smaller-diameter wire with
less resistive power losses), then step the voltage back down and the current backup for industry, business, or consumer use.
Energy Loses during TransmissionEven good electrical conductors like copper used to supply electricity, to cities and
towns generate substantial resistances (Resistance is proportional to the length
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towns generate substantial resistances. (Resistance is proportional to the lengthof the conductor).
To minimise energy losses in wires the current needs to be kept low and the voltage
high.heat loses I
Question:
Q1. Why are there energy losses that occur as energy is fed through transmission linesfrom the generator to the consumer?
Q2. A transmission cable has a resistance of 5. If 10KW of power is fed into the cable,
calculate the power wasted in the cable if it is transmitted at:a) 1000V
b) 100 000V
Solution:
A1. Resistance due to length of line and type of wire.
A2.a) P = VI b) P = VI
10 000 = 1000 x I 10 000 = 100 000 x I
I = 10A current in cable I = 0.1A
Hence power dissipated in the cable is: Hence power loss in cable is:
P = I R = 10 x 5 = 500W P = I R = 0.1 x 5 = 0.05W
Safety with Power Transmission
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Obviously high voltage transmission lines (500KV) need to be insulated from the
tall towers that support them. If a tower was to become live it would kill any
person who came in contact with it. Look at the photos on the next slide to helpyou.Protection Description How it works
Ground
Wires
Shield Wire
Porcelain
Distance
Tall Towers
Features of Power Transmission Lines
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Answers: To Safety Features of PowerTransmission
Protection Description How it works
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Ground Wires Typically, one or two ground wires
are placed on top and extend to the
ground via the pole.
Intercept lightning and harmlessly divert it to
ground to earth it, thus limiting damage to
current carrying lines.
Shield Wire A protective wire strung above the
conducting cables protects the
transmission lines
It is designed to intercept a lightning strike
and divert it to earth reducing excessive
current passing through lines to houses.
Porcelain Insulators of glass or porcelain discs
or composite insulators using
silicone rubber or EPDM rubbermaterial assembled in strings or long
rods whose lengths are dependent
on the line voltage and
environmental conditions.
Used as a material to prevent electricity
short circuiting and finding the path of least
resistance (into the ground via the tower).
Distance
BetweenTowers
Can be from 30m to 150m
depending on the voltage in thelines. The more voltage the greater
the distance.
To prevent one tower receiving a lightening
strike passing the excessive current to thenext tower.
Tall Towers The height of the towers is
proportional to the amount of voltage
in the power lines
Distance from the ground prevents sparking
from lines reaching the ground.
Impact of Electrical Generation on Society
List 10 things that you would not be able to do if all electrical power suddenly
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List 10 things that you would not be able to do if all electrical power suddenly
stopped.
Impact of Electrical Generation on Society
List 10 things that you would not be able to do if all electrical power suddenly
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List 10 things that you would not be able to do if all electrical power suddenly
stopped.
Watch TV
Use Computers
Fill your car with petrol
Watch a movie in a theatre
Refrigerate foodWork at night ( no lights)
Shop ( cash register use electricity)
Bank
Cook hot food.
Listen to a radio
Social ImplicationsThe advent of the Industrial Revolution saw textile looms powered by steam engines
effectively wipe out the textile cottage industry The demand for labour in the new factories
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effectively wipe out the textile cottage industry. The demand for labour in the new factories
led to mass migration of people from rural communities to the larger towns and cities. Poor
working conditions, overcrowding, inadequate sanitation...... led to formation of slums with
their associated social problems. Many of these problems were exacerbated with theintroduction of electric lighting enabling workers to work even longer hours.
Industrial revolution
Change of rural to urban societies increase in population density in cities
Working hours increase night time shifts (as light globes now available to light factories)
Electrical equipment and devices increase in homes and the work place electrical supplydemand increases - Leisure activities using electronics increases
Society no longer functions without electricityshops cant sell (cash registers etc) -
ATMs electronic cash.
Environmental Implications:
Today most electrical energy is produced in power stations that burn fossil fuels (coal, oil
and natural gas) with their associated environmental costs of global warming and acid rain.
Burning of fossil fuel (coal in coal power stations)
Greenhouse gases increase (CO2)
Global warming increases
Climate change - melting of ice caps - El nino floods etc
History of Generators and MotorsHistory
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Two related physical principles underlie the operation of generators and motors.
First observed by the French physicist Andr Marie Ampre in 1820. If a current ispassed through a conductor located in a magnetic field, the field exerts a mechanicalforce on it. This principle is that ofelectromagnetic reaction.