Physics: Motors & Generators PP

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=1


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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.................


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





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=related


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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=related


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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/


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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
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/http://www.youtube.com/watch?v=Ue6S8L4On-Y&feature=related/


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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=endscreen


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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

http://10.28.168.30:9053/Player.htm?358
http://10.28.168.30:9053/Player.htm?358http://10.28.168.30:9053/Player.htm?358


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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


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.


C t t


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Commutators


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.



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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.
http://en.wikipedia.org/wiki/File:Galvanometer_diagram.svg


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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
http://www.youtube.com/watch?v=kU6NSh7hr7Q&feature=relatedhttp://www.youtube.com/watch?v=kU6NSh7hr7Q&feature=relatedhttp://www.youtube.com/watch?v=kU6NSh7hr7Q&feature=relatedhttp://www.youtube.com/watch?v=kU6NSh7hr7Q&feature=related


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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.


diagram 2.

N

elated

Word Bank: back, emf, direction, magnetic,

opposing,

Lenzs Law
http://www.youtube.com/watch?v=kU6NSh7hr7Q&feature=relatedhttp://www.youtube.com/watch?v=kU6NSh7hr7Q&feature=relatedhttp://www.youtube.com/watch?v=kU6NSh7hr7Q&feature=related


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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).


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.


diagram 2.

N

http://www.youtube.com/watch?v=kU6NSh7hr7Q&feature=related

Q ti
http://www.youtube.com/watch?v=kU6NSh7hr7Q&feature=relatedhttp://www.youtube.com/watch?v=kU6NSh7hr7Q&feature=related


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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 à'.

C) fig2: Loop moving South, no

induced current.

D) fig1: Loop moving West, induced

current à'.

E) fig2: Loop moving East, induced

current `b'.

F) fig1: Magnet moving East,

induced current à'.

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.



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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)
http://images.wikia.com/engineering/images/3/3c/3phase-rmf-320x240-180fc.gifhttp://images.wikia.com/engineering/images/3/3c/3phase-rmf-320x240-180fc.gif


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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
http://www.youtube.com/watch?v=gK9LD1G6fX8&feature=relatedhttp://www.youtube.com/watch?v=gK9LD1G6fX8&feature=related


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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=related


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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=related


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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
http://www.phasemotorparts.com/photo/864994d4126e71f0015f2ae2e85863c7/synchronous-motor-laminations.jpg


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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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stopped.

Impact of Electrical Generation on Society



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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.

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