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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