Work and Energy - xraykamarul · Slide 11 of 52 TOPIC CHAPTER 5: Electromagnetism 5.1 Magnetism The magnetic dipoles in a bar magnet can be thought of as generating imaginary lines

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 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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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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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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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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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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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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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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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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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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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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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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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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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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5.1 Magnetism

Figure 9: A method for using an

electromagnet to render ceramic bricks

magnetic.

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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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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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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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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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5.1 Magnetism

Figure 10: Developments in permanent magnet design have

resulted in a great increase in magnetic field intensity.

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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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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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5.1 Magnetism

Figure 12: If a single magnet is broken into

smaller and smaller pieces, baby magnets result.

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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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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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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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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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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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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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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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Figure 14: Right Hand Rule

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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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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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Figure 16: The DC motor.

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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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Figure 17: The switch application.

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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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Figure 18: The MRI machine.

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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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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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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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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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Figure 19: Schematic description of Faraday’s

experiment shows how a moving magnetic field

induces an electric current.

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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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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

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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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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

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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.

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NEXT SESSION PREVIEW

CHAPTER 6: ALTERNATING CURRENT

In chapter 6, students will be taught alternating current and its

benefits in the medical imaging field.

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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.

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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

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Work and Energy - xraykamarul · Slide 11 of 52 TOPIC CHAPTER 5: Electromagnetism 5.1 Magnetism The magnetic dipoles in a bar magnet can be thought of as generating imaginary lines