Magnetic Fields - Review Magnetic fields are produced by magnetised magnetic materials such as iron...

Magnetic Fields - Review

Magnetic fields are produced by

• magnetised magnetic materials such as iron

• electric currents

I


A magnetic material contains magnetised regions called domains

• if the magnetic domains are randomly oriented, the material is not magnetised

• if the magnetic domains become aligned, for example due to an external magnetic field, the material becomes magnetised


The magnitude of the magnetic field produced by an electric current depends on

• the magnitude of the current - the greater the current, the stronger the magnetic field

• the distance from the conductor - the greater the distance from the wire, the weaker the magnetic field

• the shape into which the conductor is formed - e.g. a coil


Magnetic fields are represented by magnetic lines of force

• direction of the field - indicated by an arrow pointing in the direction in which the north pole of a magnet points in the field

• magnitude of the field - indicated by the spacing of the field lines - closer spacing represents a stronger field


Magnetic fields are represented by

• crosses – representing a field into the page

• arrows - closer spacing stronger B field

• dots – representing a field out the page

X X X X

X X X X

X X X X


Bar magnet

Straight conductor

Solenoid


Remember the directions using the right hand grip rule - but NEVER quote this rule in the

exam - it is just a memory aid!

Straight conductor


Remember the directions using the right hand grip rule - but NEVER quote this rule in the

exam - it is just a memory aid!

Animations - seeRHRule.aviMagneticFieldWire.avi


The magnetic field “close to” the end of a bar magnet is uniform

X X X X X

X X X X X

X X X X X

X X X X X

X X X X X

B S N


The magnetic field inside, and “close to” the ends of a solenoid is uniform

X X X X X

X X X X X

X X X X X

X X X X X

X X X X X

B

Background - the cathode ray tube

A cathode ray tube (CRT) is a highly evacuate glass tube containing a source of electrons (cathode), and in which there is a strong electric field created by a high voltage between the cathode and a positive electrode (anode) at the opposite end of the tube.

Electrons travelling in straight lines through the vacuum, are accelerated from the cathode to the anode by an electric field, E

EE

e


A demonstration cathode ray tube contains a sloping phosphorescent screen, which emits light when electrons strike it, making the path of the electrons visible.

A beam of electrons is called a cathode ray. Cathode rays are not visible, since electrons neither reflect or emit light under these conditions.


Television receivers

Cathode ray tubes (CRTs) are a key component in devices including

Computer monitors

Cathode ray oscilloscopes (CROs)

Medical monitors and other scientific equipment based on the CRO

Moving charges experience a force

Electric charges moving in a magnetic field experience a force except when they move parallel to the magnetic field

The force is a maximum when the charge moves perpendicular to the magnetic field - in this case the magnetic field is into the page

The force is perpendicular to both the velocity direction and the magnetic field direction

X X X X X

X X X X X

X X X X X

X X X X X

X X X X X

B +v

F


The magnitude of the force depends on

• the magnitude of the charge

• the velocity of the charge

• the magnitude of the field

• the angle between the direction of the field and velocity


Electric charges moving in a magnetic field experience a force except when they move parallel to the magnetic field

This effect is called the motor effect

It is the principle underlying the operation of

• cathode ray tubes (used in TVs and computers)

• electric motors and generators

• loudspeakers

First-hand investigation of the motor effect

The motor effect can be demonstrated by placing a magnet near a cathode ray so that the field of the magnet is perpendicular to the velocity of the cathode rays

The observed result, which demonstrates the motor effect is the deflection of the cathode ray, in a direction perpendicular to the magnetic field and to the direction of the cathode ray velocity

X X X

X X X

X X X

First-hand investigation of the motor effect

The motor effect can be demonstrated by placing a magnet near a current carrying wire so that the magnetic field is perpendicular to the direction of the current flow in the wire

The observed result, which demonstrates the motor effect is the deflection of the wire, in a direction perpendicular to the magnetic field and to the direction of the current

X X X

X X X

X X X

e

deflection


The force on a charge moving in a magnetic field is at right angles to

• the velocity of the particle

• the magnetic field

A constant magnitude force, which is always perpendicular to the velocity of a particle, results in the particle travelling in a circular path

F = qvBThis equation is not in the syllabus

Motion of charges in the Van Allen belts

Charged cosmic rays encountering the magnetic field of the Earth experience a magnetic force, trapping them in regions called the Van Allen radiation belts

Van Allen radiation belts


The spiralling paths of the charged cosmic rays is a result of the particles having components of their motion parallel to the Earth’s magnetic field, which is unaffected by the field, and perpendicular to the field, which causes the particles to travel in circular paths. The combined effect is a spiralling path.


High energy charged particles interact with the Earth’s atmosphere at high latitudes (polar regions) to produce auroras.


A force is produced on a current carrying conductor in a magnetic field, except when the conductor is parallel to the field.

The magnitude of the force depends on and is directly proportional to

• the magnitude of the current, I• the magnitude of the magnetic field, B• the length of the conductor, l in the field• the sine of the angle between the field and the conductor

F = BIl

If the wire is at an angle to the field, the force is reduced by a factor of sin()

F = BIl sin() B

I


Calculate the maximum force on a conductor of length 5 cm in a magnetic field with an intensity of 2 x 10–4 T when the current in the wire is 200 milliamperes.

The maximum force is exerted when the conductor is perpendicular to the field, and is give by the expression

F = BIl

The force is perpendicular to the current and the field directions

Write the equation first!

F = 2 x 10–4 x 200 x 10–3 x 5 x 10–2

F = 2 x 10–6 newtons


Note first


Therefore it must be either into or out of the page

Use whatever aid to memory you have decided to use…

The force is into the page

Since F = BIl, doubling either the current or the magnetic field strength would double the force on the wire.

What is the direction of the force on the wire?

How could the force be doubled without altering the length of the wire?

Force between current carrying conductors

Fl

=kI1I2d k = 2 x 10–7 NA–2

A force is produced between two parallel current carrying conductors

The force is a force of repulsion when the currents are in the

opposite directions

I

X X X

X X X

X X XBin

. . .

. . .

. . .

Bout

I

forceforce



The force is a force of attraction when the currents are in the same direction

Fl

=kI1I2d k = 2 x 10–7 NA–2

I

X X X

X X X

X X XBin

. . .

. . .

. . .

Bout

I

forceforce



The magnitude of the force between the conductors is

• proportional to the magnitude of the currents in each wire

• inversely proportional to the distance between the wires

• dependent on the magnetic properties of the medium between the wires

Fl

=kI1I2d

The medium between the wires determines the value of the constant k = 2 x 10–7 NA–2 in air or a vacuum



• The force is a force of attraction when the currents are in the same direction (b)

Fl

=kI1I2d

• The force is a force of repulsion when the currents are in the opposite directions (a)


Calculate the force between two straight conductors separated by a distance of 1.5 cm with a common length of 35 cm between them when the current in one wire is 200 milliamperes and the current in the other is in the opposite direction, with a magnitude of 5000 microamperes.


Write the equation first! Then substitute values… Fl

=kI1I2d

F0.35

=2x10–7x200x10–3x5000x10–6

1.5x10–2

F = 4.7 x 10–9 newtons


Calculate the force between the side of a square coil consisting of 20 turns carrying a current of 2 A and a straight conductor sharing a common length of 25 cm and carrying a current of 3 A if the distance between them is 2 cm.



=kI1I2d

F0.25

=2x10–7x2x3x20

2x10–2

F = 3 x 10–4 newtons


Q1. What is the force between two parallel conductors carrying currents in opposite directions one centimetre apart if the current in one is 10 amperes, in the other is 5 amperes and the common length is 2 m? (Ans. 0.002 N, repulsion)

Q2. If the distance between the wires was increased to 2 cm, what would be the new force between the wires? (use the fact that force and separation are inversely proportional)

The force is one of repulsion


=kI1I2d

F2

=2x10–7x10x5

1x10–2

F = 0.002 newtons

Torque

A torque is a force which acts to produce a rotational effect or a moment.

• magnitude of the force, F

τ=Fd• distance of the force from the point of rotation, d

The magnitude of a torque, depends on the

syllabus

The motor effect - force on a current-carrying wire

Consider a wire carrying a current across a magnetic field, B as shown

The net result is called the motor effect

The force on the moving charges in the wire is into the page [don’t say “down”]

This produces a resulting force on the wire that is also into the page

+ + + + + + + + + + + +

A wire carrying a current in a magnetic field experiences a force due to the movement of charges in the wire.

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Torque on a current-carrying loop

A rectangular loop of wire carrying a current can be placed in a magnetic field so that the force on opposite sides of the loop results in a turning force about an axis between the two sides.

τ=Fd

The net result is a pair of moments creating a torque which, given a suitable mechanical arrangement, may result in the loop’s rotation about the axis PQ

The force on side WZ is into the page

[never say “down” - it is ambiguous]

The force on side XY is out of the page

[never say “up” - it is ambiguous]

P Q

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Torque on a current-carrying loop

The force is measured in newtons

The distance is measured in metres

τ=Fd

The torque is therefore in …

newton metres (N.m)

P Q

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Features of a DC electric motor

The current in the loop produces a torque on the loop, causing it to rotate

A DC electric motor converts electrical energy to mechanical energy

A current-carrying coil

A magnetic field

A commutator

A DC current source

A coil on which a torque is produced by the interaction of a current and a magnetic field can be arranged, with other components, to produce an electric motor

A brush

The importance of the commutator

The coil in position (a) experiences a maximum torque.

Inertia carries the coil past the position shown in (b) and the commutator reverses the current direction so that the torque direction remains the same and the coil continues to rotate.

The torque causes rotation, clockwise viewed from above

At the position (b), no torque is produced

The importance of the commutatorFigure (a) viewed from above

The torque produced causes the coil to rotate clockwise

x. B

x

.

Bx

.B

Inertia causes the coil to rotate past the position (b) and the commutator

reverses the current in the coilIn position (b) there is no torque

because there is no current

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Principle of a DC electric motor

The commutator in an electric motor is the moving component of a motor, which provides an electrical contact between the external circuit supplying the energy to the motor and the rotating armature of the motor. Contact is achieved through brushes made of carbon, which make contact with the commutator via a smooth contoured surface matching the brushes to the commutator.

syllabus


CommutatorThis photograph shows the commutator of a motor made using many coils

The use of many coils, each in a different plane, results in a more uniform torque being produced.

The invention of the commutator was important for the development of electric motors, since it is the device that allows the current in the coil to be reversed every 180° so that the torque is always in the same direction.


A DC electric motor converts electrical energy to mechanical energy


• A DC motor’s operation is based on the principle that a current carrying conductor placed in, and at right angles to, a magnetic field tends to move in a direction perpendicular to the magnetic lines of force

• A rectangular coil of wire placed in a magnetic field such that two sides of the coil always carry a current perpendicular to the field will experience a torque due to the forces produced on the sides - the torque causes the coil to rotate

• A DC motor is similar in construction to a DC generator• A DC motor may be made to act as a DC generator by

mechanically turning the coil in the field (a DC generator is a DC motor operating “in reverse” - the energy transformation is reversed)


DC Motor

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Components of a DC electric motor

DC Motor

Making a DC electric motor

Practical Exercise - Constructing a DC Motor

Motor effect and loudspeakers

A loudspeaker converts electrical energy to mechanical energy. Alternating current in the coil produces a force that moves the speaker cone correspondingly. It uses the motor principle. Research

Motor effect and loudspeakers

Research

Because of the inertia of the speaker cone, speakers have to be designed differently to reproduce different frequency sounds.

The more rapidly the speaker cone must vibrate, the lower the mass must be so that the force produced by the motor effect on the coil can change the motion of the cone very rapidly.

The centre-reading galvanometer

A centre-reading galvanometer is a very sensitive DC ammeter.

The zero marker on the scale is in the centre, and current readings can be either positive or negative.

Such meters are built using the convention that if the meter produces a positive reading (pointer deflects to the right), then the current is flowing into the positive terminal of the meter.

+ I

Motor effect and galvanometers

A galvanometer operates on the same principle as an electric motorA current in the coil in a magnetic field produces a torque on the coil, which rotatesEquilibrium is achieved by having a spiral spring producing an opposing torque

Research

spiral spring


The torque on the current carrying coil is proportional to the current in the coil

Research

spiral spring


Research

The torque on the current carrying coil is proportional to the current in the coil

The torque turns the coil to which the pointer is attached

Equilibrium is reached, and the measurement can be recorded, when the opposing torque of the spiral spring equals that of the electromagnetically produced torque on the coil

Faraday’s discovery of the generation of electric current

Background

In 1820, Oersted discovered that an electric current in a wire produced a force on a compass needle placed near the wire… thus establishing a connection between electricity and magnetism.

Michael Faraday discovered that a force was exerted on a current flowing in a conductor in a magnetic field [1821].

This is the principle behind the operation of all electric motors.


Michael FaradayMichael FaradayThe credit for generating electric current on a practical scale goes to the famous English scientist, Michael Faraday.Faraday was greatly interested in the invention of the electromagnet, but his brilliant mind took earlier experiments still further.If electricity could produce magnetism, why couldn't magnetism produce electricity?


While carrying out investigations on how a current in one coil could produce a current in another coil, Faraday discovered that a permanent magnet moved in and out of a coil produced an electric current in the coil.

The current flows in one direction as the magnet is pushed into the coil, and in the opposite direction when the magnet is pulled out of the coil.

syllabus

Magnetic field strength and flux density

Seeing the pattern made visible by sprinkling iron filings around the magnet...

Faraday developed the model that magnets were surrounded by lines of force.

Faraday deduced that the current produced by a magnet inserted into a coil was greater if there was a greater number of lines of force were cut by the coil.

Cutting the lines of force with a coil produced a current.


The total number of lines of force is proportional to a quantity called the magnetic flux.

The amount of flux passing through the square between the magnetic poles depends on the

• area of the square

• the angle it makes to the field

• the strength of the field


The amount of flux, or number of lines of force per square metre is called the flux density.

Flux density is another term for magnetic field strength

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Magnetic flux, flux density and area

The flux density of this magnetic field…

exceeds the flux density of…

this magnetic field

The number of flux lines through the square is less in the bottom diagram

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Generation of potential difference by changing flux

Faraday established that the magnitude of the current produced was dependent upon the number of lines of force cut by the conductor in unit time.

Investigate how the current can be changed.

What factors does the number of lines of flux cut by the coil depend upon? first

Generation of electric current using a coil and magnet

Three factors determine the number of lines of flux cut by the coil in a given time, and hence the magnitude of the current produced when the magnet is moved

• the number of turns on the coil

• the strength of the magnet

• the speed at which the magnet is moved

Increasing any of these variables increases the current produced.

first

The generated potential difference is proportional to the rate of change of flux through a circuit.

Generation of potential difference

The flux through a circuit can be changed in many ways including..

moving a conducting ring or loop such that the flux through the loop varies, depending on the position of the loop.

first

At which point/s in the movement of the coil from left to right does the flux change?

X X X X X

X X X X X

X X X X X

X X X X X

v

conducing loop

B


first

There is no flux change as the coil moves through positions W, Y or Z

The flux only changes as the loop passes through these positions. As the loop moves through these positions, a potential difference is generated in the loop


The flux through a circuit can be changed by…

moving a magnet in and out of a solenoid

changing the current in one coil,the magnetic field of which passes through another coil [see transformers]

ac

Flux changes in this coil

A potential difference is generated across the ends of the coil through which the flux changes


The flux through a circuit can be changed by rotating a conducting loop in a magnetic field

As the angle the loop makes to the magnetic field, changes, the flux changes accordingly.

northsouth

coil

loop

loop

While the rod is moving, the flux enclosed by the current loop changes, inducing a potential difference causing a current to flow clockwise around the loop


A conducting rod sliding along a conducting loop generates a potential difference in the circuit - the loop plus the conducting rod.

B

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X X X X X

X X X X X

X X X X X

X X X X X

X X X X X

X X X X X

Review - the field produced by a solenoid

The magnetic field produced by a current in a solenoid (coil) is similar in shape to the magnetic field surrounding a bar magnet

A current flowing in a completed circuit through the coil in the direction indicated results in the right hand end of the coil being a north pole.NSUse the right hand grip rule as a memory aid … but never quote this rule in the exam… it is not a physical law!

Lenz’s Law

When a flux change induces a current in a conductor, the induced current produces a magnetic field that opposes the change in flux that caused the current

If the magnet is moved in the direction indicated, v…

then by Lenz’s law, the left hand end of the solenoid must become a south pole, opposing the motion of the magnet - the movement of the south end towards the solenoid.

v S

The direction of the current in the solenoid can be deduced from the magnetic polarity and the direction of the windings of the solenoid.

syllabus

Lenz’s Law


Reversing the direction of movement of the magnet reverses the direction of the induced current in the coil.

Note that the polarity of the solenoid opposes the change that is inducing the current - not simply the polarity of the moving magnet.

Lenz’s Law

What is the direction of the current induced in the loop by the moving magnet?

The current induced must produce a magnetic field that opposes the movement of the bar magnet.

The current produces a north pole to the left of the coil and a south pole to the right.

N S

The current must flow anticlockwise, viewed from the side of the approaching magnet, to produce a field, which opposes the change of flux produced by the movement of the magnet.

Lenz’s Law

As the magnet approaches the ring the magnetic flux increases

Bind

Changing flux induces current

magnetic fieldB OPPOSES flux change that created it

The system behaves such that it tries to keep the flux constant

induced B field to the left offsets increased B due to magnet

Lenz’s LawConsider a metal conducting loop in a magnetic field

A conducting rod is then slid along the metal conducting loop

Lenz’s LawAs the conducting rod moves to the right…

The flux increases through the area enclosed by the circuit

Lenz’s LawThe increasing flux induces a current

The current must produce a field that opposes the changing flux

Lenz’s LawA current must flow anticlockwise around the loop to produce a

field out of the page, opposing the increasing flux

II

Lenz’s LawA current flowing towards the top of the page in the conducing rod

results in a force on the rod towards the left, opposing its motion

IForce

The induced current in the loop must be clockwise to create a magnetic field into the page. This is consistent with Lenz’s law, since the flux of the field B through the current loop is decreasing, the field created by the induced current must counteract this decrease.

Lenz’s LawWhile the rod is moving to the right, the flux enclosed by the current loop decreases. The induced current in the loop must create a current, the field produced by which opposes the change in the flux. Hence it is into the page.

B

X X X X X

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X X X X X

X X X X X

X X X X X

X X X X X

X X X X X

X X X X X

I

I


Using a coil, magnet and a centre-reading galvanometer (a very sensitive ammeter), an electric current can be generated.

When the magnet is pushed into the coil, a current flows in one direction - indicated by the deflection of the meter pointer in one direction.When the magnet is moved in the opposite direction in the coil, a current flows in the opposite direction - indicated by the deflection of the meter pointer in the opposite direction.

first


Quiz

Predict the behaviour of the meter if the magnet is passed through the coil in the direction indicated. The polarity of the meter connections to the coil is shown.

+ –A


Answer

When the magnet begins to move into the coil, the left hand end of the coil becomes a north pole.

The current must flow away from the positive meter terminal for this to happen. The needle therefore deflects to the left (negative).

As the magnet passes through the centre of the coil the current reverses direction.

As it leaves…

+ –A

Lenz’s Law and the conservation of energy


Imagine that the motion of the magnet as shown resulted in the end of the solenoid becoming a north pole.

This would produce a force on the magnet, causing it to accelerate towards the solenoid.

v N

This would increase the rate of flux change through the solenoid, producing a larger current and solenoid field, causing the magnet to accelerate more.This feedback cycle would result in an increase in the kinetic energy of the magnet and the current in the solenoid that required no further input of energy. This violates the law of conservation of energy. It is therefore impossible. Lenz’s law is a special example of the law of energy conservation.

Lenz’s Law and back emfReview

Outline the principle of operation of an electric generator.• when a flux change is produced through an electric circuit or

conductor, an emf is produced, resulting in an electric current if there is a complete circuit

• this is the principle on which an electric generator operates

• As the coil rotates and the flux changes, an emf is induced

• When the plane of the coil is perpendicular to the field, the flux is a maximum

Review question

Explain the principle of the electric motor

Identify the energy transformations occurring in an electric motor

Current flowing perpendicular to a magnetic field in a coil of wire produces a force on two sides of the coil which results in a torque on the coil. The torque produces rotation of the armature about the axle on which the coil is situated.

The purpose of an electric motor is to convert electrical energy to mechanical energy. Motors are less than 100% efficient and some of the electrical energy is converted to heat energy.

Lenz’s law and back emf

An emf is produced in any coil of wire rotated in a magnetic field because of the change of flux taking place through the coil.

Question Do you think the emf produced as the rotor coil spins in an electric motor opposes or aids the rotation of the motor?Explain your answer.

The induced emf in an electric motor opposes the rotation of the motor. If it aided the rotation, it would cause the motor to spin ever faster with no further input of energy - a violation of the law of conservation of energy.

Hence an emf is produced as the coil of an electric motor as the motor rotates. The faster the rotation, the greater the emf.

back emf

Lenz’s law and back emf

As an electric motor turns, an emf opposing the applied voltage is produced in the rotor coil of the motor.

Motors sometimes have resistor built in, which limits the current that can flow in the rotor coil until the motor is spinning fast enough for the back emf to prevent the motor from burning out due to excessive current in the rotor coil.

This induced emf opposing the rotation of the motor is called back emf.As the speed of a motor increases, the back emf increases until the effects opposing the rotation of the motor - the load, friction and back emf - result in an equilibrium being reached causing the motor to spin at a constant speed.

DC motor

Back emf - its relation to the supply voltage

The back emf produced in an electric motor opposes the applied supply voltage used to operate the motor

The motor reaches an equilibrium at its operating speed, with the torque generated by the motor being in equilibrium due to the combined effects of the torque produced by the current in the rotor coil, the load the motor is driving, the frictional effects within the motor and the effect of the back emf

back emf– +As the speed of the motor increases, the back emf also increases, opposing the effect of the applied emf and hence limiting the speed of the motor.

syllabus

Back emf - its relation to the supply voltageDemonstration

Connect a small DC electric motor to a battery with an ammeter in series

When the motor is held so that it cannot turn, the current increases dramatically - 5 to 10 times greater.

Explanation: When the motor is prevented from turning, there is no back emf and so the effective potential is greater than when the motor is running, so the current is greater.

When the motor is first turned on, there is a surge of current.

The current quickly stabilises at a lower value.

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Lenz’s law and eddy currents

Lenz’s Law Applies

A flux change through a conductor induces a potential difference

The induced potential difference causes a current to flow

The current produces a magnetic field

This magnetic field opposes the original change in flux

Eddy currents are produced in any conductor through which there is a changing flux

Definition

Eddy currents are circulating currents, or current loops, within a conducting material, produced as a result of potential differences induced by a changing magnetic flux through the conductor.

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Induction cook topsInduction cook tops induce an electric current in a special metal cooking vessel using the principle of Faraday’s law of induction. The induced currents are referred to as “eddy currents”The current in the cooking vessel directly heats the cooking vessel, because of its low resistance.

Reference: http://stuweb.ee.mtu.edu/~mtromble/induction/cooktops.html

There is no direct electrical contact between the cooking vessel and the cook top. A changing magnetic field, produced by alternating current in coils below the cooking surface induces voltages in the cooking vessel causing eddy currents AC current in coils

Changing B field

induced voltage

current heat

Induction cook tops

Components of a CookTek Induction-Efficient Pan*

(1) 18/10 stainless steel is non-reactive to food and easy to clean.(2,3,4) 1145 aluminium with a layer of 3004 aluminium between for even heat

distribution. (5) 18/10 stainless steel for superior bond. (6) Magnetic stainless steel for efficient induction. (7) 18/10 stainless steel resists pitting and rusting.


* from CookTek Online

A changing magnetic field produced by coils in the cook top induce eddy currents in the metal cooking vessel

Induction cook topsSummary of operation


1. AC current in coils

2. Produces changing B field

3. Induces voltage in metal vessel

4. Currents produces heat

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Eddy currents and switching devices

A ground fault circuit breaker uses a sensing coil to detect magnetic flux changes produced by eddy currents in an iron ring to switch off the current if there is an earth related fault.

xx

A sudden change in the current to the earth (ground) connection induces changing eddy currents in the iron ring.A current is induced in the sensing coil.This current activates the circuit breaker, isolating the device from the electricity supply.

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Eddy currents and magnetic braking

Induced eddy currents produce magnetic fields opposing the change in flux that causes the potential difference.

These opposing magnetic effects can be used to produce a braking effect on the object, the movement of which resulted in the changing flux that induced the eddy currents.

Applications include

• slowing of maglev trains

• slowing the “space probe 7” ride at Australia’s Wonderland

syllabus

Investigating eddy currents and magnetic braking

1. Using a neodymium magnet, hold the magnet just clear (1 mm) of a long non-magnetic metal surface (such as aluminium) and quickly move the magnet quickly across the surface.

Describe your observations

Explanation: The relative movement between the magnet and the metal conductor causes a change in flux through the conductor. Thus a current is induced in the conductor, which produces a magnetic field that opposes the changing flux produced by the movement - hence opposing the movement. The induced current, and hence the magnetic braking stops when the movement stops.

Eddy currents

When a metal disk moves in a magnetic field the induced emf results in currents known as “eddy currents”

Eddy currents

Eddy currents result in heating of the metal plate

Eddy currents can be reduced by cutting slots in the metal plate.

The slots act like open switches and prevent the flow of eddy currents and hence the loss of heat energy.

This principle is important in reducing eddy current heat losses in motors and transformers.


1. Drop a neodymium magnet through a copper pipe with a diameter just a little larger than the magnet.

Describe your observations

Explanation: As the neodymium magnet falls through the copper tube (a non-magnetic material), the changing flux induces eddy currents in the copper.

The eddy currents produce magnetic fields that oppose the cause of the flux change - the falling magnet, thus opposing the movement and causing the magnet to fall slowly through the tube.

I


The south end of the falling magnet is pointing upward and north end is facing downwards.

To oppose this motion (the cause of the flux change through the copper tube), by producing an upward force, the induced eddy currents must produce a north pole above the eddy current as shown.

S

I

The eddy currents produce magnetic fields that oppose the movement, causing the magnet to fall slowly through the tube.

Eddy currents above the falling magnet will flow in a direction that results in the south pole being attracted upwards - as per Lenz’s law.

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Eddy currents and electricity meters

Eddy currents are induced in a metal disc in the electricity meter.

These cause the disc to rotate.

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Identifying the main generator components

In addition to the identified components of the generator, there must be some method of mechanically turning the coil in the magnetic field.

This is may be done using a turbine driven by water or steam, or a belt, as in the case of the alternator in a car.

magnetic fieldbrushes

slip ringscoil

magnets

syllabus

Generation of alternating current

Three methods can be used to generate AC in the lab• Move a magnet in and out of a solenoid in a circuit• Move a long wire back and forth across the Earth’s field lines• Rotate a coil, connected to a pair of slip rings, in a B field

first

Which method would be most appropriate in a commercial power station?

Three things are essential to generate a potential difference• A conductor• A magnet• Relative motion

Generation of alternating current

first

Note!

• The conductor - the coil connected to a completed circuit• The magnet• Relative motion - the magnet or coil must be moved

When the magnet is pushed in and out of the coil, an alternating current is induced in the completed circuit through the microammeter.

The Earth’s Magnetic Field

The Earth’s field is similar in form to that of a bar magnet.

The angle the Earth’s field makes to the the surface of the Earth depends on latitude.

first

Near the poles the field is almost vertical, while at the equator, the field is parallel to the ground.

The Earth’s Magnetic Field at Different Locations

first

At the South Pole At the EquatorSydney

Around Sydney, the angle of the field is about 30° to the ground, pointing in a northerly direction.

A conductor perpendicular to the field, moved back and forth perpendicular to the field will have an alternating current produced in it if it is connected to a circuit.

Rotation of a coil in a magnetic field

first

syllabus

Comparison of generator and motor structure

Generators and motors have the same key components. A motor can be run as a generator, converting mechanical

to electrical energy - although not very efficiently.

Comparison of generator and motor structure

Motor structureKey components

MagnetsCoil

Commutator or slip ringsArmature , stator and rotor

Generator structureKey componentsMagnetsCoilCommutator or slip ringsArmature, stator and rotor

Generators and motors have the same key components. A motor can be run as a generator,

converting mechanical to electrical energy

Comparison of generator and motor function

Demonstrations

• A loudspeaker (in principle the same as an AC motor) can be operated as a microphone (in principle the same as an AC generator)

• Connect the inputs of two moving coil galvanometers to each other and pick up one of the meters and gently rock it so that the needle moves back and forth.

The needle on the other meter moves correspondingly, because the meter being rocked is acting as an AC generator, and the other meter is acting as a motor. (moving coil meters and motors work on the same principle - called the motor principle)

syllabus

Describing the operation of an AC generatorAn AC generator consists of one or more coils in a magnetic field. When the coil is turned, usually by a turbine driven by water or steam, the change in magnetic flux through the coil induces a potential difference across the ends of the coil.

In a coal fired power station, the generators are turned turbines operated by steam produced from a boiler.

Describing the operation of an AC generator

In a simple AC generator consisting of a single coil having many turns, the output of the coil is connected by brushes to an external circuit by a pair of slip rings.

The AC potential difference produced has a frequency equal to the frequency of rotation of the coil.

slip rings


In the AC generator below, in which positition/s of the armature is there a potential difference between the slip rings?Identify the positive terminal.

[ B and D ]

+

+


Two Types of AC Generators

• Revolving armature– rotor is an armature which is rotating inside a stationary

electromagnetic field– seldom used since output power must be transmitted through slip-

rings and brushes• Revolving field

– dc current is supplied to the rotor which makes a rotating electromagnetic field inside the stator

– more practical since the current required to supply a field is much smaller than the output current of the armature


Revolving Armature

Describing the operation of an AC generatorRevolving Field


• The AC generator converts mechanical energy to electrical energy in the form of an alternating current

• Mechanical energy is used to turn a coil in a magnetic field

• The magnetic flux change through the coil induces a potential difference across the ends of the coil

• The circuit with the rotating coil is completed by slip rings that connect the rotating coil to the stationary external circuit

• Alternating current flows with a frequency equal to the frequency of rotation of the generator

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Generation of AC electricity for commercial distributionSee excellent website - http://www.tampaelectric.com/eduction/TEEDElecGen.html

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Transmission line energy lossesEnergy losses occur from transmission lines because they have a small resistance and they carry a current. E = P x t = I2R x t

The energy loss is proportional to the square of the current, and so high voltages are used on the transmission line to minimise the current, requiring the use of step-up and step-down transformers.

Describing the operation of a DC generatorA DC generator converts mechanical to electrical energy.

The rotation of the coil in a magnetic field produces a flux change through the coil, which induces a potential difference across the coil.

Mechanical energy is used to rotate the armature / coil assembly of a DC generator e.g. using a steam or water turbine.

The ends of the coil connect to the external circuit via a split ring commutator.

The DC generator produces a varying DC voltage, as shown in the graph.

Describing the operation of a DC generatorTo convert the output of a DC generator to a constant voltage, a smoothing circuit must be used.

This typically contains a capacitor placed across the varying DC output.

This capacitor stores energy while the voltage is high.

When the voltage drops, the stored energy in the capacitor is released, maintaining the voltage at a constant level.

capacitor

Describing the operation of a DC generatorIdentify the generator in the diagram below.What component makes this identification possible?

This is a DC generator since the coil is connected to the external circuit using a split ring commutator.

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A model generator - this one has a split ring commutator

What kind of generator is this?

In what way would an AC generator differ from this generator?

An AC generator has slip rings instead of a split ring commutator.

This is a DC generator

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Producing an electric current with a voltaic cellA voltaic cell produces an electrical potential by exploiting the different attraction which atoms have for their electrons. Chemists call this the reduction potential of the element.

A simple voltaic cell uses two different metal electrodes in a conducting solution called an electrolyte. Two electrolyte solutions are use in the cell shown here. (zinc and copper sulfate)

firstNote

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Producing an electric current with a voltaic cellCarry out an investigation to measure the potential difference produced by two different pairs of metals used in a simple electrolytic cell as shown below. Record which metal electrode was the positive electrode in each case.

first

Cu(+)/Zn __ volts

Fe(_)/Zn __ volts

Cu(_)/Fe __ volts

Ni(_)/Zn __ volts

Tabulate your results, and those of other groups in your class.

Producing an electric current with a voltaic cellCarry out an investigation to measure the potential difference produced by two different pairs of metals used in a simple electrolytic cell as shown below. Record which metal electrode was the positive electrode in each case.

first

Typical results

Cu(+)/Zn 0.9 volts

Fe(+)/Zn 0.5 volts

Cu(+)/Fe 0.4 volts

Ni(+)/Zn 0.8 volts

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QuizThree different metal electrodes, A, B and C were used in pairs to make three electrochemical cells.

C is positive relative to AA is positive relative to BTherefore C must be positive relative to BThe voltage produced will be 0.7 volts

A B

C

A C

B

Two of the results obtained are as follows.

A and B 0.5 volts A positiveA and C 0.2 volts C positive

Predict the voltage that was produced using electrodes B and C.

Quiz

Predict, at the instant shown in the diagram, the potential difference across the ends of the loop.

Justify your prediction.

The potential difference is zero.

Since the conductor at this instant is moving parallel to the magnetic field lines, there is no force produced on the charges in it, so there is no PD.

Hint: Look carefully at the direction of motion of the conductor in the magnetic field.

Comparison of benefits of AC and DC generators

• AC generators produce a voltage that can be readily transformed• AC generators are simpler and more reliable (they have fewer parts)• AC can be used for operating motors suitable for a range of applications

Reference: http://www.phys.unsw.edu.au/~jw/HSCmotors.html#links

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Comparison of benefits of AC and DC generatorsThomas Edison pioneered DC and George Westinghouse and Nikola Tesla championed AC.AC is easily transformed permitting transmission over long distances with less energy loss, but AC is more deadly than DC. Reference:

“The war of the currents, or let’s Westinghouse him” by Ira Flatow Edison’s electric chair, using AC, was used in the 1892 execution of Charles MacElvaine. It was part of Edison’s public relations campaign to portray AC power as a menace to public safety.

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More

The effects of generators on the environment• Generators need an energy input to operate, usually either

• Water turbine

• Steam turbine

The effects of generators on the environment

Water turbines

• Usually require the building of large dams, which may arguably have a very significant effect on the environment

• e.g. Tasmania, Snowy Mountains, China, Brazil

Large areas covered by water when dams are built inevitably involve the destruction of habitat and species.

Rotting vegetable matter in dams contributes further to carbon dioxide and methane production - both greenhouse gases.

Dams may interfere with the movement of animals, affecting breeding and food supplies.


The Three Gorges Dam in China…

When completed in 2009, this will be the largest dam in the world.


The Three Gorges Dam in China…

Environmental issues• Inundation• Flood concerns• Increased earthquakes• Water pollution• Sedimentation• Species affected• Human resettlement


Brazil … Tucurui Dam

Hydroelectric generators produce over 90% of Brazil’s electrical energy

hydroelectric energy production

Hydroelectric power stationGenerators at a large Canadian hydroelectric power station


Steam turbines

• Steam turbines can use existing bodies of water, without the need for dams, however the water needs to be heated requiring either…

• The burning of fossil fuels, coal (in NSW), gas or oil, resulting in the production of large quantities of carbon dioxide, as well as other pollutants. The carbon dioxide may be linked to global warming - it is a greenhouse gas.

• The operation of nuclear power plants (which are used to heat water to steam to operate steam turbines) resulting in radioactive waste products, which must be safely stored for long periods.


Cooling towers at a nuclear power plant

Thermal pollution and gaseous emissions


Location of nuclear power stations


Steam turbines

Steam turbines produce thermal pollution.

Not all the heat produced can be used efficiently.

Hot waste water must be cooled before it can be recycled into the environment. Despite cooling, water enters Lake Macquarie in NSW from adjacent power stations at a higher temperature than the lake itself.

This promotes growth of algae, especially when it is in combination with nitrogen and phosphorous based chemicals entering the system from homes and farms, acting as fertiliser for the algae.

syllabus

The insulation of high voltage transmission lines

Glass or ceramic insulators are used on both high voltage and lower voltage transmission lines. These can be seen on most electricity poles. The higher the voltage, the more insulation is required.

detail

High voltage transmission lines

High voltage transmission lines are the backbone of the national electricity grid in Australia.

Voltages are stepped up at the power station (from 23 kV typically produced by the generators to 330 kV or 500 kV) for transmission across the electricity grid.

The electricity grid is a network of interconnected transmission lines and power stations.

Finally, the voltage is stepped down progressively at points in the grid, ultimately to 240 V for domestic use.

High voltage transmission lines

The insulation of high voltage transmission lines

The higher the voltage, the more insulation is required

ceramic insulators

ceramic insulators

conducting bypass

powerlines

Detail of the ceramic insulators on this high voltage transmission line tower are shown on the left

The insulation of high voltage transmission linesReference: ElectricitySaskpower.pdf

The insulation of high voltage transmission linesCeramic insulators on transmission lines are very varied in their design

Generally the greater the voltage, the greater the separation must be the separation between the current carrying wires and the supporting structures.

Identify the supporting structures, insulators and conducting wires in these photographs

Additional reference: ElectricitySaskpower.pdf

Lightning protection of high voltage transmission lines

High voltage transmission lines are excellent lightning targets because

High voltage transmission lines are protected by earthed conductors connecting the highest points of the supporting towers to each other

• they are metal conductors• they are usually the tallest object in

the vicinityTransmission lines must be protected from lightning strikes because• strikes produce voltage surges that

damage both the supply system and connected appliances

• they are expensive to repair• damage interrupts energy supply

earthed conductors

high voltage transmission lines

syllabus

Human health and high voltage power lines• Some studies appear to show a weak association between exposure to

power-frequency magnetic fields and the incidence of cancer however… • Epidemiological studies done in recent years show little evidence that

power lines are associated with an increase in cancer• A connection between power line fields and cancer remains biophysically

implausible• "The scientific evidence suggesting that [power-frequency

electromagnetic field] exposures pose any health risk is weak.”(A 1999 review by the U.S. National Institutes of Health )

• "Laboratory experiments have provided no good evidence that extremely low frequency electromagnetic fields are capable of producing cancer, nor do human epidemiological studies suggest that they cause cancer in general.”(A 2001 review by the U.K. National Radiation Protection Board (NRPB))

Reference: Power Lines and Cancer.pdf taken from http://www.mcw.edu/gcrc/cop/powerlines-cancer-FAQ/toc.html

Human health and high voltage power linesDo power lines produce electromagnetic radiation?

To be an effective radiation source an antenna must have a length comparable to its wavelength. Power-frequency sources are clearly too short compared to their wavelength (5,000 km) to be effective radiation sources. Calculations show that the typical maximum power radiated by a power line would be less than 0.0001 microwatts/cm^2, compared to the 0.2 microwatts/cm^2 that a full moon delivers to the Earth's surface on a clear night.

How do radiofrequency radiation and microwaves cause biological effects?

A principal mechanism by which radiofrequency radiation and microwaves cause biological effects is by heating (thermal effects). This heating can kill cells. If enough cells are killed, burns and other forms of long-term, and possibly permanent tissue damage can occur. Cells which are not killed by heating gradually return to normal after the heating ceases; permanent non-lethal cellular damage is not known to occur. At the whole-animal level, tissue injury and other thermally-induced effects can be expected when the amount of power absorbed by the animal is similar to or exceeds the amount of heat generated by normal body processes. Some of these thermal effects are very subtle, and do not represent biological hazards.

Reference: Power Lines and Cancer.pdf taken from http://www.mcw.edu/gcrc/cop/powerlines-cancer-FAQ/toc.html

syllabus

The purpose and principle of a transformer

• The primary purpose of a transformer is to change a voltage from one value to another, using magnetic induction

• A secondary purpose of a transformer is to isolate one part of a circuit from another physically and electrically, while allowing the transfer of energy from one part to the other

A transformer is an electromagnetic device consisting of two conducting coils, isolated electrically from each other, but linked magnetically.

The purpose and principle of a transformer• A transformer is a device that

transfers energy by electromagnetic induction

• Primary and secondary windings (insulated from each other electrically) are wound onto a ferromagnetic core

• Used to raise voltage (“step-up transformer”) or lower voltage (“step-down transformer”)

• Voltage is raised when the primary winding has fewer turns than the secondary winding, and voltage is lowered when the primary winding has more turns than the secondary winding


Transformers operate on the principle of Faraday’s law

1. A changing current in the primary coil, produces a changing magnetic flux

2. A The changing flux from the primary coil, induces a potential difference across the secondary coil


1. The changing current in the primary coil, is usually achieved by applying an alternating voltage, resulting in an alternating current (AC)

2. As the alternating current changes magnitude and direction, a magnetic field is produced, which changes in a corresponding manner

AC input

AC output

3. The field from the primary coil is intensified and concentrated (also referred to as increasing the flux linkage) through the secondary coil by an iron core

4.The changing flux through the secondary coil, induces a potential difference across the secondary coil

syllabus

Investigation - examining a transformer

Transformers frequently have more than two coil windings.

Examine a transformer and record the input and output information using a diagram.

first

What type of primary input is required to produce a secondary output on a transformer?

Quiz

Would connecting a battery to the primary coil of a transformer produce an output voltage?

Predict

Ouch!!

Investigation - modelling a transformer

Construct a simple transformer by winding a primary coil of 20 turns and a secondary coil of 100 turns onto a soft iron bar

Apply a low voltage (2-4 volts) AC to the 20 turn coil

Measure and record the input and output voltages

Apply a low voltage (2-4 volts) AC to the 100 turn coil

Measure and record the input and output voltages

Investigation - modelling a transformer

Typical results

20 turn coil input voltage (primary) __ volts

100 turn coil output voltage (secondary) __ volts

100 turn coil input voltage (primary) __ volts

20 turn coil output voltage (secondary) __ volts

Faraday’s original transformer (left)

Faraday’s Observations of Induction

Quiz

Predict what will happen when the switch is closed.

Explain the processes underlying your prediction.

Model of Faraday’s experiment demonstrating induction


Three levels of answers:

Elementary: The compass needle will be deflected when the switch is closed.

Better: The compass needle will deflect momentarily, tending to align parallel to the axis of the coil in which it is located. It will then return to the position shown in the diagram.

Best: The compass needle will deflect momentarily, with the north end, tending to align parallel to the axis of the coil in which it is located, and pointing to the right. It will then return to the position shown in the diagram.


Explanation

Closing the switch causes a current to flow in the coil connected to the battery.

As the current changes from zero to a steady value, the magnetic field intensity increases.

The iron torus links the changing flux from the primary coil connected to the battery to the secondary coil.

The flux change through the secondary coil induces a current, because the circuit is closed, producing a magnetic field parallel to the axis of the secondary coil, causing the compass to deflect in that direction.

Faraday’s Observations of InductionDetailed Explanation

The current flows clockwise around the primary circuit when the battery is connected.

NN

IIThis produces the magnetic polarity shown, with the magnetic field intensity increasing.The current induced in the secondary, must flow in the direction shown, producing a magnetic field with a flux opposing the increasing flux of the primary.The magnetic polarity and the current of the secondary is therefore as shown.The magnetic field inside the secondary coil increases, and its direction is to the right, deflecting the north end of the compass in that direction. When the current in the primary reaches a constant value, the flux change is zero, so no current is induced in the secondary coil. The compass returns to its original position.

What happens when the switch is opened after being closed?

Comparing step-up and step-down transformers

A step-down transformer has less turns on the secondary coil than on the primary coil

By comparison, a step-up transformer has more turns on the secondary coil than on the primary coil


Step-down transformers are found in all electronic devices that can be run from the domestic 240 V AC domestic supply, since all electronic devices require low voltages to operate the semiconductor components.

Step-down transformers are used in the electricity supply grid to reduce the high voltages (up to 330 kV) used when transmitting energy to lower voltages (240 V) for domestic use


Step-up transformers are used at power stations to convert the lower voltage generator output (~600 V) to higher voltages (up to 330 kV) for transmission across the grid.

Step-up transformers are needed to convert 240 V to several thousand volts needed to operate fluorescent lights.

TVs and other devices containing a cathode ray tube (CRT) require high voltages (up to 50 kV) for their operation.

X-ray machines require high voltages, requiring the use of step-up transformers for their operation.


Compare the number of turns on the secondary coil to the number of turns on the primary coil of this transformer

This transformer has less turns on the secondary coil than the primary coil since it is a step-down transformer

The ratio of the number of turns on the primary to the secondary coil is 240:6.3


The induction coil

A low-voltage, pulsed DC (6 V) applied to the primary coil (typically having less than a hundred turns), produces a high voltage (~30 kV) across the secondary coil (having thousands of turns)

syllabus

Induction coil

Pulsed DC is used because the rate of change of flux is much greater than that produced by a 6 V alternating current.

+

–

I

The induction coil voltage supply*

* not explicit in HSC syllabus

• the applied DC voltage causes a current to flow

+

–

+

–I

iron cored coilreed switch

DC supply

• the current produces a magnetic field• the field attracts the magnetic reed switch• the circuit is broken, switching off the current• the reed switch springs back, completing the circuit

Induction coil

This induction coil is producing a voltage of about 20 kilovolts

spark

A device similar to this is used to produce the spark to ignite the petrol in a car engine.

Transformer primary-secondary voltage relationship

The voltage input and output of a transformer are related to the number of turns on the primary and secondary coils by

np = number of turns on the primary coil

ns = number of turns on the secondary coil

Vp = primary voltage (input)

Vs = secondary voltage (output)

VpVs

=npns

syllabus

Solving primary-secondary voltage problems

A transformer was found to be transform 220 V AC to 5.5 V AC. If the number of turns on the secondary coil was 20, how many turns must there have been on the primary coil?

np = 800 turns

VpVs

=npns

2205.5

=np20

220 V 5.5 V


There are 126 of turns on the secondary coil of this transformer.

How many turns are there on the primary coil?

There are 4800 turns on the primary coil.

VpVs

=npns

2406.3

=np

126

Transformers

Identify the component labelled A and outline its function.

Describe one structural feature of this component and explain the reason for this feature.

Component A is a soft iron core. Its two main functions are (1) to increase the flux through the primary coil and(2) to provide a more effective flux linkage between the primary and the secondary coils.

The iron core is usually laminated to reduce eddy currents in the core caused by the flux changes through it, and thus to minimise heat losses and to increase the efficiency of the transformer.

A

Conservation of energy and voltage transformations

The law of conservation of energy

Energy cannot be created or destroyed, but only transformed from one form to another.

Step-up transformers increase the voltage applied to the primary coil, resulting in a greater voltage across the secondary, however the energy available at the output of the secondary coil is always less than the energy applied to the primary coil.

Analysing the energy relationships in a transformer…

Energy is the product of power and time.

Power is the product of voltage and current. P =VI

Conservation of energy and voltage transformationsEnergy cannot be created or destroyed, but only transformed from one form to another

• The energy output of a transformer is always less than the input

• Energy losses occur because eddy currents induced in the transformer core by the alternating current, result in resistive heat losses (the transformer core heats up)

• The ratio of the energy output to the energy input, expressed as a percentage is called the efficiency of the transformer.

energy input energy output

energy losses

Input240 V

Output12 V

transformer

Conservation of energy and voltage transformationsEnergy cannot be created or destroyed, but only transformed from one form to another.

Consider a simplified case in which…

• The transformer output remains at 12 V, regardless of the load

• The energy losses (mainly heat) are constant, say 30%i.e. the efficiency of the transformer is constant at 70%


energy losses

Input240 V

Output12 V

transformer


If the lamp is a 36 watt lamp (12 V) then the output current is 3 AWhat is the input current?


energy losses

Input240 V

Output12 V

transformer

efficiency=outputinput

0.7=36Winput

The input power is 51.4 watts

The input current is therefore 0.21 A [ I = P / V ]

P =VI


If a 48 W lamp (12 V) is connected instead of the 36 W lamp, then the output current is 4 A. What is the input current?


energy losses

Input240 V

Output12 V

transformer

efficiency=outputinput

0.7=48Winput

The input power is 68.6 watts

The input current is therefore 0.29 A [P=VI]

P =VI


• The output current of a step-down transformer is more than the input current

• The output power of a step-down transformer is less than the input power

• If the load (resistance) on the output is decreased, the output current increases accordingly and the input current increases


energy losses

Input240 V

Output12 V

transformer


What would be the effect of increasing the number of turns on the secondary coil, with the same light globe attached?


energy losses

Input240 V

Output12 V

transformer

VpVs

=npns

Conservation of energy and voltage transformationsEnergy cannot be created or destroyed, but

only transformed from one form to another.

• Increasing the number of turns on the secondary coil would increase the transformer output voltage

• With the same lamp, the current would be more and hence more power would be produced and the lamp would be brighter

• A greater output current would result in a greater input (primary) current

• Energy would still be lost in the core as heat [How much more?]


energy losses

Input240 V

Output12 V

transformer

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Electricity substations and transformers

Electricity generated at a power station is usually produced at a voltage ranging from a few hundred volts to 10s of kV (Eraring power station at Lake Macquarie has four 660 MW generators with an output of 23 kV.

It is transformed to 330 kV or 500 kV for transmission over the grid.

High transmission voltages are used to minimise heat losses in the transmission lines.


Electricity substations are used to transform voltages in the grid

The voltage must be transformed to lower voltages, usually a few thousand for local distribution, and then to 240 volts for domestic use.


Electricity substations are used to transform voltages in the grid

The distribution of electrical energy involves substations responsible for stepping up voltage for transmission and stepping it down for use

Domestic appliances and transformersMany domestic appliances today have semiconductor (electronic) components requiring low voltage DC for their operation.

Electronically operated domestic appliances require both a

• transformer to change 240 volts to about 5-20 volts

• rectifier to change the low voltage AC to DC

Appliances with no transformer

kettle, hot water heater, toaster, older room heaters, hair dryers, incandescent lights, old model refrigerators, some clothes dryers

Appliances with a transformer

TV, stereo, computer, CD player, clock radio, fluorescent lights, home security systems, microwave oven, answering machines, air conditioner, fax machines, washing machines, microwave oven

Domestic appliances and transformersA 2002 transformer development…

The need for transformers in the electricity grid

Transformers are required at the power station to step up the relatively low voltage from the generators (100 V) to high voltages (330 kV) for distribution over the grid.

Transformers are required at local substations to step down the very high voltages from transmission lines to lower voltages (11 kV) for suburban distribution. Finally, local transformers step the voltages down further for domestic use (240 V)

High voltage transmission

syllabus

Suburban step down transformer


Voltage produced by the power station generators ~ 23 kV


MANY transmission lines to the step-up transformers


Step-up transformers at the power station ~ 330 kV


Step-down transformers for industry ~ 415 volts


Local distribution~ 110 kV - 33 kV


Domestic transformer~ 220 - 240 volts


All energy comes in via the home meter box


Below-ground distribution is becoming popular

Above-ground is still common

The need for transformers in the electricity gridsyllabus

Step-up transformer at a power station. A suburban step-down transformer

Eddy currents and energy losses in transformers

To increase the magnetic flux produced by the primary coil of a transformer, a soft iron core is used

The changing flux induces eddy currents in the iron core, which results in resistive heat losses, and therefore inefficiency of the transformer.

To reduce the eddy currents, the core of a transformer is usually laminated, that is, made up of many layers of soft iron, electrically insulated from each other

syllabus

Transformers and their impact on society

Analyse the impact on society of the development of transformers

• The first practical transformer, using AC, was developed in 1883

• Prior to this, direct current was seen as being the logical way to distribute energy using electricity

• AC triumphed, and by the early 1900s, its future impact on society was inevitable

• Transformers permitted the long-distance transfer of electrical energy with low resistive energy losses

• Without the high voltages possible through the use of transformers, the electrical wires required to transmit large amounts of electrical energy would have to have been too large to be practical

Transformers and their impact on societysyllabus

Animation! electricToaster.avi

Transformers and their impact on society

Transformers were a key to establishing electrical energy as the driving force behind technological and industrial development in the 20th century.

• Electrical energy rapidly became the means of lighting homes and cities, with its distribution facilitated by the use of transformers

• Electrically operated machines thus replaced less efficient machines, resulting in the rapid growth of industry and commerce

• Communication networks grew rapidly as a result of electrical energy and its intimate association with radio, then television and ultimately the computer revolution of the late 20th century

• Every home has dozens of appliances that make use of transformers, permitting a host of electronic devices to be operated from the mains

syllabus

Describing the main features of the AC electric motor

The main features of AC motors

• current direction is reversed by the alternating voltage used, rather than by a split ring commutator as in a DC motor

• the motor speed is determined by the AC frequency, rather than by the magnitude of the applied voltage, as with a simple DC motor

There are two common types of AC motor

• synchronous motors

• induction motors


The frequencies may be different, but they bear a simple relationship to each other.

Heavy loads cause synchronous motors to slow down too much and the frequencies will no longer be synchronised - hence the motor does not work efficiently. It may fail altogether.

Synchronous AC motors

An alternating voltage is applied to the rotor coils via a pair of slip rings.

The stator field may be produced by either permanent magnets, or it may be produced by a DC electromagnet.

This type of motor is called “synchronous” because the speed is synchronised to the frequency of the applied voltage.


AC induction motor

• This is simplest and most rugged type of electric motor• Induction motors have current carrying coils wound on the stator, and a

rotor assembly which has no electrical connections to the power supply.

• The AC induction motor is named because the electric current flowing in the rotor is induced by the alternating current flowing in the stator.

• The power supply is connected only to the stator. The combined electromagnetic effects of the applied alternating current and the induced rotor current produce the torque

Main features of the AC electric induction motor


Rotor: The rotor is the rotating component of an induction AC motor

Stator: The fixed part of an AC motor, consisting of copper windings within steel laminations.


Rotor

The rotor is the rotating component of an induction AC motor.

The rotor consists of conducting cast-aluminium rods.

These rods are short-circuited by end plates completing the so called “squirrel cage”, which rotates when the moving magnetic field induces current in the conductors.

Stator: The fixed part of an AC motor, consisting of copper windings within steel laminations.


Rotor

The squirrel cage may have a laminated, cylindrical soft iron core, electrically insulated from the squirrel cage.

The iron core intensifies the magnetic field of the stator, inducing a larger current in the rotor, resulting in a larger torque.

The purpose of the laminations is to reduce heat losses due to eddy currents induced in the core, thus making the motor more efficient


A squirrel cage rotor

Describing the main features of the AC electric motorThese two photographs show two views of the rotor from an AC electric motor.

Shaft

squirrel cage rotor

These conductors run parallel to the shaft, the full length of

the squirrel cage

syllabus

Investigating the principle of the AC induction motor

The neodymium magnets rotating above the metal plate produce a changing flux through the plate.The flux change induces eddy currents in the plate.The eddy currents produce a magnetic field which results in an interaction with that of the rotating magnets, producing a torque on the metal plate, which rotates.

Principle of the induction motor

Discuss why most motors are AC induction motors

AC induction motors have a simple design, requiring no brushes, commutator or slip rings for their operation, since there is no electrical contact between the rotor and the power supply.

AC induction motors, because of their simplicity are cheaper to manufacture as well as being very reliable.

They are well suited to applications requiring a constant torque and rotational speed - common criteria in applications such as fans, fridges, washing machines, clothes dryers, air conditioners.

syllabus

AC electric motors and power tools

Despite what the syllabus states…

AC motors are used for high power applications.

Three phase AC induction motors are widely used for high power applications, including heavy industry.

However, such motors are unsuitable if multiphase is unavailable, or difficult to deliver, as in the case of electric trains.

Many electric train systems run on DC, because it is easier to build power supply lines requiring just one active conductor for DC.


Power tools and some appliances use synchronous AC motors with brushes.

Brushes introduce energy losses (plus arcing and ozone production).

Power tools using AC induction motors produce low torque at low speeds - this can be a problem.

Eddy currents induced in the rotor core result in energy losses and the possibility of overheating.

These motors are sometimes called ‘universal motors’ because they can operate on DC as well as AC.

DC operated electric drill

syllabus


Power tools can operate on DC as well as AC however the simplest type use a DC motor.

syllabus

AC induction motors and their advantages

Simple design

Low cost

Reliable operation

syllabus

Conversion of electrical energy

Gather, process and analyse information to identify some of the energy transfers and transformations involving the conversion of electrical energy into other forms in the home and in industry

Discussion in class!

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Coils from an AC fan motor (right)

AC vs DC motors*

AC most common type for power applications, simple, cheap, constant speed operation

DC easily controlled, variable speed operation, with or without brushes, also function as generator

Internal view of the “Universal” motor used in the SKIL electric hand drill - the so-called universal motor can be operated using either AC or DC voltage

commutator

Brushes

Stationary windings (stator)

Armature (rotating unit)

* Not specifically required by syllabus, however AC motor advantages must be understood

syllabus

The end

Westinghouse and Tesla

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George Westinghouse was a famous American inventor and industrialist who purchased and developed Nikola Tesla's patented motor for generating alternating current. The work of Westinghouse and Tesla gradually persuaded Americans that the future lay with AC rather than DC (Adoption of AC generation enabled the transmission of large blocks of electrical, power using higher voltages via transformers, which would have been impossible otherwise). Today the unit of measurement for magnetic fields commemorates Tesla's name.

What chemists do

The electrochemical cell shown here separates the two electrodes into separate solutions so that the chemistry of what is being done can be more easily understood.It is not necessary to have two different electrolytes in two containers in order to produce a potential difference.

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A word from the creator

This Powerpoint presentation was prepared by Greg Pitt of Hurlstone Agricultural High School.

Please feel free to use this material as you see fit, but if you use substantial parts of this presentation,

leave this slide in the presentation.

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