Download 2. Electromagnetism

Electromagnetism
Kyle Thornton
DMI 50B
Magnetic Attraction and Polarity
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http://micro.magnet.fsu.edu/electromag/j
ava/magneticlines/index.html
http://micro.magnet.fsu.edu/electromag/j
ava/magneticlines2/index.html
The Relationship Between
Electricity And Magnetism
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Oersted’s Discovery
– It was noted that a wire carrying a current
also has a magnetic field
• However:
– Only when the charge is in motion
– Stationary charges produce only electric fields
– Direction of the magnetic flow is
determined by the right hand rule
Magnetic Field Of A Current
Carrying Solenoid
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A wire wound into a coil would behave
as a magnet (North and South pole)
If the current is reversed, magnetic
polarity will reverse
Current direction is determined by
application of the right hand rule
The Electromagnet
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The magnetic field of a current-carrying
solenoid is concentrated through the
center of the coil
Inserting a bar of ferromagnetic material
into the coil intensifies the magnetic
The field lines would be the same as a
bar magnet
An electromagnet can be turned off or
varied by varying the current flow
through the wire
Faraday’s Discovery
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If an electric current can induce a
magnetic field, can a magnetic field
induce a current in a conductor?
Yes, if:
– The magnet is moving
– The wire is moving
– The magnetic fields are changed
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Application in Radiology
– Generators and electric motors used in
radiologic equipment work on this principle
Magnitude Of The Induced
Current
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Depends on four factors
– The strength of the magnetic field
– The velocity of the field as it moves past
the conductor
– The angle of the conductor to the magnetic
field
– The number of turns in the conductor
Faraday’s Law
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http://micro.magnet.fsu.edu/electromag/j
ava/faraday2
http://micro.magnet.fsu.edu/electromag/j
ava/faraday/index.html
Lenz’s Law
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Determines the direction of the induced
current flow
Occurs when a changing magnetic field is
used to induce current flow
– The induced current flow will oppose the
action that induced it
– This is known as self-induction
– This law establishes magnetic polarity
Application in Radiology
– This principle is applied to rotating anodes
Lenz’ Law
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http://micro.magnet.fsu.edu/electromag/j
ava/lenzlaw/index.html
Self Induction
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Autotransformers
work on this
principle
The generating of
an opposing voltage
or current in a single
coil as result of
changing magnetic
fields when an AC is
run through the coil
Mutual Induction
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An electromagnet (primary coil) is
placed near a coil of wire (secondary
coil)
Current is passed through the primary
coil
The resulting changing magnetic fields
induce a current in the secondary coil
Most transformers work on this principle
Basic Electromechanical Equipment
Used In Radiologic Machines
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Generators
Motors
Rectifiers
Transformers
Motors and Generators
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http://www.youtube.com/watch?v=d_aTC0iK
O68&feature=related
Generators
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Converts mechanical energy to electrical energy
This utilizes the principle of a changing magnetic field
to induce an electric current
The simplest type of generator is a coil of wire that
rotates through a magnetic field
– This is called a single phase generator
– The current produced is alternating current
High voltage generators are necessary
to produce x-rays in the x-ray tube
Inside a Generator
Retrieved from: http://www.energyquest.ca.gov/story/chapter06.html
DC/AC Generators
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http://micro.magnet.fsu.edu/electromag/j
ava/generator/dc.html
http://micro.magnet.fsu.edu/electromag/j
ava/generator/ac.html
Motors
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Converts electrical energy to mechanical
energy of motion
Simplest type is a battery powered DC motor
The components are the same as a generator
– Single wire inside a magnet
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The electric current is sent through the wire
The resulting magnetic fields induce motion
– http://www.youtube.com/watch?v=Q2mSh
GuG4RY&feature=related
Electric Motor
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Electric Motor Animation
http://www.phys.unsw.edu.au/hsc/hsc/el
ectric_motors.html
Synchronous And Induction
Motors
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Synchronous Motor
– Designed to rotate at the same rpm as the
supplied current
– Used for timing devices
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Induction Motor
– This type of motor spins the anode used to
produce x-rays
• http://www.phys.unsw.edu.au/hsc/hsc/electric_motors1.ht
ml
• http://www.youtube.com/watch?v=HWrNzUCjbkk&feature
=related
Rectification
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Electrons are made to flow inside the xray tube via a high voltage AC generator
AC in the x-ray tube would be
problematic
AC must be changed to DC
This is accomplished via rectifiers
Two types
– Vacuum tube
– Solid state
Rectifiers
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Allows current to flow in one direction only
Vacuum tube
– Similar to x-ray tube
– Allows electrons to flow during positive half of AC cycle
Solid state
– Consists of a semiconductor and an added impure
substance
– Results in more efficient x-ray production
– More commonly used in modern machines
– Longer life spans than vacuum tube rectifiers
Silicon Rectifier
Vacuum tube
rectifiers
Transformers
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Principles of operation
– Power supply is normally 110 - 220
– Necessary to increase incoming voltage to
kilovoltage for efficient x-ray production
– It is also necessary to decrease incoming
voltage to 5 - 10 volts
– This is accomplished by transformers
– Transformers use interactions between
electricity and magnetism to accomplish
their tasks
Transformers
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Transformers have two circuits
Primary circuit contains the power source
Current is induced into the secondary circuit
by changing magnetic fields in the primary
circuit
A transformer takes incoming voltages or
currents and increases or decreases them
The output voltage depends on the turns ratio
of the transformer
Transformers
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http://micro.magnet.fsu.edu/electromag/j
ava/transformer/index.html
Transformers
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Turns ratio
– Ns/Np
– This expresses the relationship of turns in each
coil
– A transformer with less turns in the secondary coil
is a step-down transformer
• These decrease incoming voltage
– A transformer with a turns ratio > 1 (more turns in
the secondary coil) is a step-up transformer
• These increase incoming voltage
Transformers
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Types of transformers
– Air-core
• Two insulated coils placed in close proximity to each
other
– Open-core
• The coils are wrapped around an iron bar
– Closed-core
• The coils are wrapped around a square or circle of iron
– Autotransformer
• An iron core with only one winding of wire
– Shell-type
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Each core plate contains two holes
The coils are wrapped around the center post of the core
This traps more magnetic field lines
Results in more a more efficient transformer
Transformer Law #1
– Voltage related to turns ratio
• Vs/Vp = Ns/Np
– Work this problem
• An incoming voltage of 220 is placed across
the primary side of a transformer that has 100
primary turns and 40,000 secondary turns
• X/220 = 40,000/100
– Answer
• 88,000 V or 88 kV
Transformer Law #2
– Current related to voltage
– Vp/Vs = Is/Ip
– Work the problem
– The input V is 220. The output is 22kV.
The current in the primary is 20A. What is
the current in the secondary?
– 220/22000 = X/20
– Answer
• 0.2 A
Transformer Law #3
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Current related to turns ratio
– Ip/Is = Ns/Np
– Work the problem
– The turns ratio is 50. The current in the
secondary is 5 A. What is the primary
current?
– X/5 = 50/1
– Answer
• 250 A
Safety Issues With Transformers
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Transformer power rating is generally
rated in kilowatts
This rating cannot be exceeded
– Overheating would occur
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This must be taken into consideration
when purchasing a machine and
considering its intended use
Review Questions
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What is the relationship between magnetism
and electricity?
Under what circumstances can a magnetic
field induce current in a conductor?
What effect does the number of turns in the
secondary coil of a transformer have upon
current, and voltage?
What is the purpose of a rectifier?
What is the purpose of a motor?
What is the purpose of a generator?
Websites
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http://www.youtube.com/watch?v=uYfTzCa71SE&feature=relate
d
http://www.youtube.com/watch?v=UsT-qWAzTHg
http://www.youtube.com/watch?v=XTjiqF_FOPI&NR=1
http://www.youtube.com/watch?v=Q2mShGuG4RY&NR=1&feat
ure=fvwp
http://www.youtube.com/watch?v=xGNmGCCCQRg&NR=1&fea
ture=fvwp