Work and Energy - xraykamarul · Slide 11 of 52 TOPIC CHAPTER 5: Electromagnetism 5.1 Magnetism The...
Transcript of Work and Energy - xraykamarul · Slide 11 of 52 TOPIC CHAPTER 5: Electromagnetism 5.1 Magnetism The...
HDR102
SCHOOL OF MEDICAL IMAGINGFACULTY OF HEALTH SCIENCES
PREPARED BY:MR KAMARUL AMIN BIN ABDULLAH
CHAPTER 5
PHYSICS FOR RADIOGRAPHERS 1
ELECTROMAGNETISM
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TOPIC
CHAPTER 5: Electromagnetism
LEARNING OUTCOMES
At the end of the lesson, the student should be able to:-
Explain what is magnetization.
Describe the magnetic field including factors affecting it.
List the measurement Unit involved.
Explain the electromagnet and its application to DC Motor.
Explain the application of magnet in electromagnetic switches and magnetic
resonance imaging (MRI).
Explain the electromagnetic induction laws.
Explain the electromagnetic induction.
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TOPIC
CHAPTER 5: Electromagnetism
TOPIC OUTLINES
INTRODUCTION
5.1 Magnetism
5.1.1 Magnetic Laws
5.1.2 Magnetic Induction
5.2 Electromagnetism
5.2.1 Electromagnetic Induction
5.2.2 Electromagnetic Devices
5.3 References
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TOPIC
CHAPTER 5: Electromagnetism
5.1 Magnetism
Magnetism is fundamental property of some forms of matter.
Any charged particle in motion creates magnetic field.
Magnetic field of a charged particle such as electron in motion is
perpendicular to the motion of the particle.
The intensity of magnetic field is represented by imaginary lines.
Figure 1: A moving
charged particle induces
a magnetic field in a
plane perpendicular to its
motion.
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TOPIC
CHAPTER 5: Electromagnetism
5.1 Magnetism
If the electron’s motion is a closed loop, as with electron circling a nucleus,
magnetic field lines will be perpendicular to the plane of motion.
Figure 2: When a charged particle moves in a circular or elliptical path,
the perpendicular magnetic field moves with charged particle.
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TOPIC
CHAPTER 5: Electromagnetism
5.1 Magnetism
Electrons behave as if they rotate on an axis clockwise or counterclockwise.
This rotation creates a property called electron spin.
Therefore, atoms that have an odd number of electrons in any shell exhibit a
very small magnetic field.
Figure 3: The electron spin creates
a magnetic field, which is
neutralized in electron pairs.
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TOPIC
CHAPTER 5: Electromagnetism
5.1 Magnetism
Spinning electric charges also induce a magnetic field.
The proton in a hydrogen nucleus spins on its axis and creates a nuclear
magnetic dipole called a magnetic moment.
This forms the basis of MRI.
Figure 4: A basic MRI
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TOPIC
CHAPTER 5: Electromagnetism
5.1 Magnetism
The lines of a magnetic field do not start or end as the lines of an electric
field do.
Such field called bipolar or dipolar always has a north and south pole.
The small magnet created by the electron orbit is called a magnetic dipole.
Figure 5: The magnetic lines.
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TOPIC
CHAPTER 5: Electromagnetism
5.1 Magnetism
An accumulation of many atomic magnets with their dipoles aligned creates
a magnetic domain.
If all the magnetic domains in an object are aligned, it acts like a magnet.
Under circumstances, magnetic domains are randomly distributed.
Figure 6: In ferromagnetic
material, the magnetic dipoles
are randomly oriented.
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TOPIC
CHAPTER 5: Electromagnetism
5.1 Magnetism
When acted on by an external magnetic field, however such as the Earth in
the case of naturally occurring ores or an electromagnet in the case of
artificially induced magnetism, randomly oriented dipoles align with the
magnetic field.
Figure 7: This changes when
the dipoles are brought under
the influence of an external
magnetic field.
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TOPIC
CHAPTER 5: Electromagnetism
5.1 Magnetism
The magnetic dipoles in a bar magnet can be thought of as generating
imaginary lines of the magnetic field.
If a non magnetic material is brought near such magnet, these field lines are
not disturbed.
However, if ferromagnetic material such as soft iron is brought near the bar
magnet, the magnetic field lines deviate and are concentrated into the
ferromagnetic material.
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TOPIC
CHAPTER 5: Electromagnetism
5.1 Magnetism
Figure 8: A, Imaginary lines of force. B, These lines of force are undisturbed
by nonmagnetic material. C, They are deviated by ferromagnetic material.
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TOPIC
CHAPTER 5: Electromagnetism
5.1 Magnetism
There are three principal types of magnets:-
a) Natural Magnet
b) Permanent Magnet
c) Electromagnet
5.1.1 Types of Magnets
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TOPIC
CHAPTER 5: Electromagnetism
5.1 Magnetism
The Earth itself.
The earth has a magnetic field because it spins on an axis.
Lodestone in the Earth exhibit strong magnetism presumably because they
have remained undisturbed for a long time within the Earth’s magnetic field.
5.1.1.1 Natural Magnet
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TOPIC
CHAPTER 5: Electromagnetism
5.1 Magnetism
5.1.1.2 Permanent Magnet
Are available in many sizes and shapes but principally as bar or horshoe-
shaped magnets, usually made of iron.
A compass is a prime example of an artificial permanent magnet.
Typically produced by aligning their domains in the field of an electromagnet.
It can be destroyed by heating or hitting that causes magnetic domains be
jarred and magnetism is lost.
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TOPIC
CHAPTER 5: Electromagnetism
5.1 Magnetism
Figure 9: A method for using an
electromagnet to render ceramic bricks
magnetic.
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TOPIC
CHAPTER 5: Electromagnetism
5.1 Magnetism
5.1.1.3 Electromagnets
Consist of wire wrapped around an iron core.
When an electric current is conducted through the wire, a magnetic field is
created.
The intensity of the magnetic field is proportional to the electric current.
The iron core greatly increases the intensity of the magnetic field.
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TOPIC
CHAPTER 5: Electromagnetism
5.1 Magnetism
Many materials are unaffected when brought into a magnetic field.
It can be classified into:-
a) Diamagnetic
b) Ferromagnetic
c) Paramagnetic
5.1.2 Materials Interaction
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TOPIC
CHAPTER 5: Electromagnetism
5.1 Magnetism
Weakly repelled by either magnetic pole.
They cannot be artificially magnetized .
They are not attracted to magnet.
Examples: Plastic and Water.
5.1.2.1 Diamagnetic Material
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TOPIC
CHAPTER 5: Electromagnetism
5.1 Magnetism
Include iron, cobalt, nickel.
Strongly attracted by a magnet .
Usually can be permanently magnetized by exposure to a magnetic field.
An Alloy of aluminium, nickel, and cobalt is called alnico and are very useful
magnets produced from ferromagnetic materials.
Rare earth ceramics have been developed recently and are considerably
stronger magnets. (figure 5-21)
5.1.2.2 Ferromagnetic materials
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TOPIC
CHAPTER 5: Electromagnetism
5.1 Magnetism
Figure 10: Developments in permanent magnet design have
resulted in a great increase in magnetic field intensity.
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TOPIC
CHAPTER 5: Electromagnetism
5.1 Magnetism
Lie somewhere between ferromagnetic and nonmagnetic.
Very slightly attracted to a magnet and are loosely influenced by an external
magnetic field.
Contrast agents employed in MRI are paramagnetic.
5.1.2.3 Paramagnetic Materials
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TOPIC
CHAPTER 5: Electromagnetism
5.1 Magnetism
All magnetic materials have two poles
Labeled: North and South
Just as in electrostatics:
Like poles repel each other and opposite poles attract.
N repels N
S repels S
N attracts S
Figure 11: The interactions between poles.
5.1.3 Magnetic Laws
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TOPIC
CHAPTER 5: Electromagnetism
5.1 Magnetism
Figure 12: If a single magnet is broken into
smaller and smaller pieces, baby magnets result.
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TOPIC
CHAPTER 5: Electromagnetism
5.1 Magnetism
Some materials can be made magnetic by induction.
The imaginary magnetic field lines just described are called magnetic lines of
induction and the density of these lines is proportional to the intensity of the
magnetic field.
Ferromagnetic objects can be made into magnets by induction.
5.1.4 Magnetic Induction
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TOPIC
CHAPTER 5: Electromagnetism
5.1 Magnetism
Examples:-
When ferromagnetic material, such
as a piece of soft iron, is brought
into the vicinity of an intense
magnetic field, the lines of
induction are altered by attraction
to the soft iron and the iron is made
temporarily magnetic.
If copper, diamagnetic material,
there would be no such effect.Figure 13: Ferromagnetic material
such as iron attracts magnetic lines of
induction, whereas nonmagnetic
material such as copper does not.
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TOPIC
CHAPTER 5: Electromagnetism
5.1 Magnetism
This principle is employed with many MRI systems that use an iron magnetic
shield to reduce the level of the fringe magnetic field.
Ferromagnetic materials act as a magnetic sink by drawing the lines of the
magnetic field into it.
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TOPIC
CHAPTER 5: Electromagnetism
5.1 Magnetism
The SI unit of magnet field strength is the tesla.
An older unit is the gauss.
One tesla (T) = 10000 gauss (G)
5.1.4 Unit
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TOPIC
CHAPTER 5: Electromagnetism
5.2 Electromagnetism
Electromagnetism concerned with the forces that occur between electrically
charged particles.
Electromagnetism manifests as both electric fields and magnetic fields.
Both fields are simply different aspects of electromagnetism, and hence are
intrinsically related.
A changing electric field generates a magnetic field; conversely a changing
magnetic field generates an electric field.
This effect is called electromagnetic induction, and is the basis of operation
for electrical generators, induction motors, and transformers.
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TOPIC
CHAPTER 5: Electromagnetism
5.2 Electromagnetism
A current flowing in a wire always gives rise to a magnetic field round it.
The magnetic effect of current is called electromagnetism which means that
electricity produces magnetism.
The magnitude of magnetic field produced by a current-carrying wire at a
given point is:-
1. Directly proportional to the current passing in the wire, and
2. Inversely proportional to the distance of that point from the wire.
5.2.1 Relationship Electricity and Magnetism
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TOPIC
CHAPTER 5: Electromagnetism
5.2 Electromagnetism
To assess the direction of the emf (voltage) generated in a moving conductor
if the direction of the magnetic field and the direction of motion of the
conductor in the field are known.
Point the thumb in the direction of motion of the conductor relative to the
field, and point the forefinger in the direction of the flux (magnetic lines).
The second finger will then point along the conductor in the direction of the
positive end of the conductor.
5.2.2 Right Hand Rule
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TOPIC
CHAPTER 5: Electromagnetism
5.2 Electromagnetism
Figure 14: Right Hand Rule
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TOPIC
CHAPTER 5: Electromagnetism
5.2 Electromagnetism
The DC motor is a machine that transforms electric energy into mechanical energy in
form of rotation.
Its movement is produced by the physical behavior of electromagnetism.
DC motors have inductors inside, which produce the magnetic field used to generate
movement.
Figure 15: The DC
motor.
5.2.3 Electromagnet and its application to DC Motor
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TOPIC
CHAPTER 5: Electromagnetism
5.2 Electromagnetism
An electromagnet, which is a piece of iron wrapped with a wire coil that
has voltage applied in its terminals. If two fixed magnets are added in both
sides of this electromagnet, the repulsive and attractive forces will produce a
torque.
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TOPIC
CHAPTER 5: Electromagnetism
5.2 Electromagnetism
Figure 16: The DC motor.
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TOPIC
CHAPTER 5: Electromagnetism
5.2 Electromagnetism
An electrical switch that opens and closes electrical circuit.
A relay has at least two circuits. One circuit can be used to control another
circuit.
When the 1st circuit is closed, the iron armature will attract to
electromagnet.
Then, it will close the 2nd switch and allows current flows in the second
circuit.
5.2.4 Application of Magnet
5.2.4.1 Electromagnetic Switches
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TOPIC
CHAPTER 5: Electromagnetism
5.2 Electromagnetism
Figure 17: The switch application.
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TOPIC
CHAPTER 5: Electromagnetism
5.2 Electromagnetism
5.2.4.2 MRI Machine
MRI machine uses a powerful magnetic field to align the magnetization of
some atomic nuclei in the body, and radio frequency fields to systematically
alter the alignment of this magnetization.
This causes the nuclei to produce a rotating magnetic field detectable by the
scanner —and this information is recorded to construct an image of the
scanned area of the body.
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TOPIC
CHAPTER 5: Electromagnetism
5.2 Electromagnetism
Figure 18: The MRI machine.
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TOPIC
CHAPTER 5: Electromagnetism
5.2 Electromagnetism
Magnetic flux can be thought of as the total number of magnetic field lines
passing through a particular area.
Φ = BA
B = magnetic flux density, A = area
Magnetic flux is a scalar quantity and Unit = weber (Wb)
The magnetic flux is 1 Weber if the magnetic flux density over an area of
1m2 is 1 Tesla.
5.2.5 Electromagnetic Induction Law
5.2.5.1 Magnetic Flux Φ
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TOPIC
CHAPTER 5: Electromagnetism
5.2 Electromagnetism
Any change in the magnetic environment of a coil of wire will cause a voltage
(emf) to be "induced" in the coil.
No matter how the change is produced, the voltage will be generated.
The change could be produced by changing the magnetic field strength,
moving a magnet toward or away from the coil, moving the coil into or out of
the magnetic field, rotating the coil relative to the magnet, etc.
5.2.6 Faraday’s Law
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TOPIC
CHAPTER 5: Electromagnetism
5.2 Electromagnetism
When an emf is generated by a change in magnetic flux according to Faraday's
Law, the polarity of the induced emf is such that it produces a current whose
magnetic field opposes the change which produces it.
The induced magnetic field inside any loop of wire always acts to keep the
magnetic flux in the loop constant.
If the B field is increasing, the induced field acts in opposition to it. If it is
decreasing, the induced field acts in the direction of the applied field to try
to keep it constant.
5.2.7 Lenz’s Law
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TOPIC
CHAPTER 5: Electromagnetism
5.2 Electromagnetism
Electricity can generate magnetic fields.
Micheal Faraday found the theory and concluded that electric current cannot
be induced in circuit only by the presence of a magnetic field.
He discovered that when magnet is moved, the coil wire does have a current
and emf is induced. i.e: to induce the current with magnetic field, the
magnetic field cannot be constant but must be changing.
5.2.8 Electromagnetic Induction
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TOPIC
CHAPTER 5: Electromagnetism
5.2 Electromagnetism
Figure 19: Schematic description of Faraday’s
experiment shows how a moving magnetic field
induces an electric current.
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TOPIC
CHAPTER 5: Electromagnetism
5.2 Electromagnetism
An emf will be induced in a conductor which is situated in changing magnetic
field.
It happens not only to secondary coil but also to primary coil.
It experiences the changing magnetic flux that varying current Ip produced
which then an emf is induced across primary coil.
This phenomenon known as self-induction, occurs irrespective of whether
there is a secondary coil present.
5.2.8.1 Self Induction
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TOPIC
CHAPTER 5: Electromagnetism
5.2 Electromagnetism
According to the Lenz’s Law, if the primary-coil current (Ip) is increasing, the
induced emf (Ep) will tend to oppose the increase and is known as back emf
and vice versa.
The value of self induced emf can be calculated from:-
The SI unit of self inductance is the henry.
L = self inductance
Slide 47 of 52
TOPIC
CHAPTER 5: Electromagnetism
5.2 Electromagnetism
The production of induced emf does not depend on the use of a permanent
magnet.
a magnetic field from any source will have the same effect.
Consider there are two coils, Primary coil (P) and Secondary coil (S). If a
changing magnetic field is generated by passing a varying current (Ip) through
coil P, there will be changing flux linkage with coil S.
This will result in an induced emf Es across coil S.
This phenomenon is known as mutual induction.
5.2.8.2 Mutual Induction
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TOPIC
CHAPTER 5: Electromagnetism
5.2 Electromagnetism
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TOPIC
CHAPTER 5: Electromagnetism
Answer the question.
Test Your Knowledge
Activity 1
A magnetic field line is used to find the direction of
south-north
A
a bar magnet
B
magnetic field
C
Slide 53 of 52
TOPIC
CHAPTER 5: Electromagnetism
SUMMARY
Magnetism is fundamental property of some forms of matter.
Any charged particle in motion creates magnetic field.
Magnetic field of a charged particle such as electron in motion is
perpendicular to the motion of the particle.
There are three types of materials: diamagnetic, paramagnetic,
ferromagnetic.
There are three principal types of magnets: natural magnet, permanent
magnet, electromagnet.
Magnetic field can induce electrical current and vice versa.
Slide 54 of 52
TOPIC
CHAPTER 5: Electromagnetism
NEXT SESSION PREVIEW
CHAPTER 6: ALTERNATING CURRENT
In chapter 6, students will be taught alternating current and its
benefits in the medical imaging field.
Slide 55 of 52
TOPIC
CHAPTER 5: Electromagnetism
5.3 References
No. REFERENCES
1 Ball, J., Moore, A. D., & Turner, S. (2008). Essential physics for
radiographers. Blackwell.
2 Bushong, S. C. (2008). Radiologic science for technologists. Canada:
Elsevier.
Slide 56 of 52
TOPIC
CHAPTER 5: Electromagnetism
APPENDIX
FIGURE SOURCE
Figure 1 http://www.actors.co.ke/en/news/Energy1.jpg
Figure 2 http://intechweb.files.wordpress.com/2012/03/shutterstock_77399518.jpg
Figure 3 http://www.solarenergybook.org/wp-content/uploads/2009/12/solar-energy-
example.gif
Figure 4 http://www.petervaldivia.com/technology/energy/image/potencial-and-
kinetic.bmp
Figure 5 http://iws.collin.edu/biopage/faculty/mcculloch/1406/outlines/chapter%206/S
B7-2b.JPG
Figure 6 http://www.petervaldivia.com/technology/energy/image/potencial-and-
kinetic.bmp
Figure 7 http://www.physics4kids.com/files/art/motion_energy1_240x180.jpg
Figure 8 http://www.sciencebuilder.com/michigan/science/images/p/potentialenergy.j
pg
Figure 9 http://4.bp.blogspot.com/_V7DuEO3c2E8/S-
b2PZfOXZI/AAAAAAAAADk/KKXoueyon2I/s1600/One-balanced-rock.jpg