Unit 9 Mag Nest Ism

8/14/2019 Unit 9 Mag Nest Ism

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Unit IX: MagnetismUnit IX: Magnetism

MagnetsMagnets Magnetic fieldMagnetic field ElectromagnetsElectromagnets Dc motorDc motor Hall effectHall effect Magnetic inductionMagnetic induction Ac generatorAc generator

TransformerTransformer


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The magnetic forceThe magnetic force

A free suspended magnetline up with the Earth

north-south line

Navigational compass like

this since 11th

centuryA compass is a small

magnet that can points the

direction by aligning its

needle under the presenceof the magnetic force of the

earth.


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The MagnetThe Magnet

Compass woks because :

The Earth is like a giant

magnet

It has two magnetic poles

a north pole, a south pole

Itsmagneticnorth pole is

nearly aligned with thesouth geographical pole of

the Earth


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Rules of magnetic forceRules of magnetic force

There are only two magnetic poles:north pole and south pole

Like poles repel, unlike poles attract

In magnetic substance,like iron, each atom is

a small magnet A larger magnetic

force is producedwhen these tiny

magnet are aligned in

the same direction


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Magnetic fieldsMagnetic fields

Existence of magneticfields:

Magnets attract small iron

particles

Compass needle rotateswith the Earth poles

The magnetic field is

stronger at the pole The direction pointing by

the trace of magnetic

needle: magnetic field

lines


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Magnetic field created by a wireMagnetic field created by a wire

carrying currentcarrying current

A wire carrying current can create a magneticfield. The direction of such created field is determined

by right hand rule.


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ElectromagnetsElectromagnets

The magnetic force can be increased if the wire is coiled, orsolenoid. The direction of such created field is determined by right hand rule

Putting an iron bar into the coil strengthens the magnet even more. This is a simple electromagnet.

The strength of an electromagnet can be enhanced by Increasing the number of turn of the coil Increasing the current intensity


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Current: The source of all magnetismCurrent: The source of all magnetism

(a) In the planetary model of the atom, an electron orbits a nucleus,

forming a closed current loop and creating a magnetic field with a

north and south pole.


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Current: The source of all magnetismCurrent: The source of all magnetism

(b) Electrons have spin and can be crudely pictured as rotating charge,

forming a current that produces a magnetic field with a north and south

pole.


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Circuit breakerCircuit breaker


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The circuit breaker is a typical application of theThe circuit breaker is a typical application of theelectromagnet. The electromagnet can create aelectromagnet. The electromagnet can create a

strong magnetic field. The electromagnet of the circuitstrong magnetic field. The electromagnet of the circuit

breaker is usually not strong enough to attract the ironbreaker is usually not strong enough to attract the iron

bolt under the normal current range. However, if therebolt under the normal current range. However, if there

is a fault which causes a current surge, the iron bolt isis a fault which causes a current surge, the iron bolt is

pulled out of the plunger by the electromagnet.pulled out of the plunger by the electromagnet.

Hence, the circuit is broken.Hence, the circuit is broken.


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Magnetic force on current-carryingMagnetic force on current-carrying

wirewire

Flemings left-hand (motor) rule

F

I

Fis proportional toI, B and l

M ti f t iMagnetic force on current carrying


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Magnetic force on current-carryingMagnetic force on current-carrying

wirewire

The magnetic field B (directed into the plane) exerts a force on the

current- carrying wires.

(a)I=0, (b)Iupward, (c)Idownward.

The magnetic force on wire carrying current is the basis for dc motor.

(a) (b) (c)

h i i lTh t i i l


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The motor principleThe motor principle

a rectangular loop carrying a currentI, in the presence of a

uniform magnetic fieldB. The force on both side ofa will be

cancelled.

The magnitude of force on the sides b is not zero. They are thesame magnitude but opposite in direction.These two forces will produce a torque about O that rotates the

loop clockwise.

Th i i lTh t i i l


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The motor principleThe motor principle

When the coil at vertical, the current should change its

direction, the coil continue to turn.A split ring ensures that the current flow changes

direction at the right time.This is the principle of a dc motor.


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Hall effectHall effectPrinciple

The magnetic force creates the separation of chargein nonmetallic conductor, which builds up until it is

balanced by the electric force, an equilibrium that is

quickly reached.

Blv=

Bl d l it M tBl d l it M t


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Blood velocity MeasurementBlood velocity Measurement

BlvE=

The blood tube is a

conductor-carrying vesse

Eis the emf in volts,

B is the magnetic strength

in Tesla (T)

l=2a is the vesselsdiameter in m


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Doppler ultrasoundDoppler ultrasound

The frequency shift of the reflected ultrasound :

c

fvf

cos2=

Wherefis the sourcefrequency, v the speed

of the moving blood,

and c is the speed of

sound in the tissue.

By measuring the frequency shift f, the average speed of the blood can

be calculate.

C i f Bl d V l it M tC i f Bl d V l it M t


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Comparison of Blood Velocity MeasurementComparison of Blood Velocity Measurement

The electromagnetic and ultrasound techniques are two most used

methods for the the measurement of the blood flow rate..

The features of ultrasound technique are as follows:In clinical application, it is most frequently utilized to detect

the presence or location of blood flow rather than to measure its

magnitude accurately

The frequency shift is in the audio range and is made audiblewith loudspeaker.

The popularity of the magnetic technique is the result of the

following factors:Utilized normally during surgical procedures in which blood

vessels are exposed .Producing accuracies up to 5%Accommodation of blood vessels of diameters from 1mm to

20 mm

l i d i h


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Electromagnetic Induction: PhenomenaElectromagnetic Induction: Phenomena

Phenomena of electric inductionPhenomena of electric induction

We have learned :

The induction phenomena deals with the creation of anelectric current (or electro-motif force emf) in a loop by

varying the magnetic fields. (either direction or magnitude).

A current-carrying wire in a magnetic field willexperience a force,A current loop in a magnetic field will experience a torque

How about a torque in a magnetic field to create a current?

The answer is YES.

Electromagnetic Induction: 1Electromagnetic Induction: 1stst


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Electromagnetic Induction: 1Electromagnetic Induction: 1 experimentexperiment

A moving magnet can induce an current in a loop even

there is no battery in the loop.

Electromagnetic Induction: 2Electromagnetic Induction: 2ndnd


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Electromagnetic Induction: 2Electromagnetic Induction: 2ndnd experimentexperiment

The current meter registers a current in the left hand loopjust as the switch S is closed or opened. No motion of the

coils is involved.

Electromagnetic Induction: 3Electromagnetic Induction: 3ndnd


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Electromagnetic Induction: 3Electromagnetic Induction: 3ndnd experimentexperiment

A current is induced when the rod moves to the right in a

uniform constant magnetic field.

Electromagnetic Induction: FaradaysElectromagnetic Induction: Faradays


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Electromagnetic Induction: FaradaysElectromagnetic Induction: Faraday s

lawlaw

cosBA

Faradays law of inductionFaradays law of induction

An potential difference can be induced in a loop, if thereAn potential difference can be induced in a loop, if there

is a change ofis a change of

magnetic fieldmagnetic field

loop arealoop area

angle between magnetic field and loopangle between magnetic field and loop

t

BA

=

)cos(

is called mangetic flux.


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represents the product of a areaA with the

perpendicular component of a magnetic field B

passing through it.

is the angle between the direction of the magnetic

fieldB and the normal the areaA.

The unit of magnetic flux is T m2

B

A

Magnetic fluxMagnetic flux

cosBA

Electromagnetic Induction: Lenz lawElectromagnetic Induction: Lenz law


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Electromagnetic Induction: Lenz lawElectromagnetic Induction: Lenz law

Lenz lawLenz law

An induced current has a direction suchAn induced current has a direction such

that it induces a magnetic field whichthat it induces a magnetic field which

opposes the changes in the magnetic fluxopposes the changes in the magnetic flux

i.e.i.e. cosBA

The Generation of AlternativeThe Generation of Alternative


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CurrentCurrentFaradays law is the basis of

an ac current generation

In order to generate ac currents not necessary to move magnet, rotation of a coil of wire

between poles of magnet

The induced potential difference (or current) is increased, if- the coil rotate faster, - the area of the coil is increased,

- there are more turns on the coil,

- the strength of the magnet is increased

sin)(cos BAt

BA =

=

Simple ac generatorSimple ac generator


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Simple ac generatorSimple ac generator

It has a fixed magnet and a

rotating coil

Coil connected to a conducting

ring The conducting rings rotate

together with coil

The rings come into contact withtwo fixed carbon brushes

As the coil turns, the induced

voltage changes direction for

each half turn of the coil, thiscreates an alternating current

Energy TransmissionEnergy Transmission


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Energy TransmissionEnergy Transmission

How a household circuit wired?How a household circuit wired?

Electricity produced in power stations is first step-up to

high voltage (> 10 kV) by a transformer and delivered to local

area through high tension cable towers.

A transformer later step-down the voltage to domestic

level (rms 220 V in Hong Kong).


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Power transmission requirementsPower transmission requirements

For safety reason, low voltages are required at both

generating end and receiving end in an energy transmission.

The device with which we can rise and lower the voltage is

called the transformertransformer.

energy lossI2

R in the transmission lineoutput power isIV,in order to minimizeI, we have to rise Vduring

transmission.

The Transformer PrincipleThe Transformer Principle


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The Transformer PrincipleThe Transformer Principle

There is a current in the 2nd coil only as the switch is on

oroffin the 1st coil.

A changing magnetic field in a fixed coil will induce acurrent in a second fixed coil.

The iron core provide a magnetic link between the two

coil.

TransformersTransformers


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Transformers

They use magnetic link between two coils to step-up or step-

down alternating voltage:

primary coil must uses alternating current,which produces a changing magnetic field in the iron corean alternating current induced in the secondary coilTransformer works with ac current only

R l f lt t f ti


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Rule of voltage transformation

V1 and V2 are the primary and secondary voltages,

N1 and N2 are number of turns on primary and secondary coils

Rule of current transformation

I1 and I2 are the primary and secondary currents,

N1 and N2 are number of turns on primary and secondary coils

E l A t f i d i d t t d 230 V t 11 5 V


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Example: A transformer is designed to step-down 230 V to 11.5 V.

There are 1000 turns of wire on the primary coil. Calculate

1) the number of turns on the secondary coil

2) For a input current of 0.01 A, what is the output current?

1)

2

2

1

1

N

V

N

V

=2

5.11

1000

230

N=

2)2211 NINI = 50100001.0 2 = I

N2= 50 Turns

I2=0.2 A

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Unit 9 Mag Nest Ism