2. Electromagnetism
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Transcript 2. Electromagnetism
Electromagnetism
Kyle Thornton
DMI 50B
Magnetic Attraction and Polarity
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
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
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
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
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
Application in Radiology
– Generators and electric motors used in
radiologic equipment work on this principle
Magnitude Of The Induced
Current
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
http://micro.magnet.fsu.edu/electromag/j
ava/faraday2
http://micro.magnet.fsu.edu/electromag/j
ava/faraday/index.html
Lenz’s Law
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
http://micro.magnet.fsu.edu/electromag/j
ava/lenzlaw/index.html
Self Induction
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
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
Generators
Motors
Rectifiers
Transformers
Motors and Generators
http://www.youtube.com/watch?v=d_aTC0iK
O68&feature=related
Generators
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
http://micro.magnet.fsu.edu/electromag/j
ava/generator/dc.html
http://micro.magnet.fsu.edu/electromag/j
ava/generator/ac.html
Motors
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
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
Electric Motor Animation
http://www.phys.unsw.edu.au/hsc/hsc/el
ectric_motors.html
Synchronous And Induction
Motors
Synchronous Motor
– Designed to rotate at the same rpm as the
supplied current
– Used for timing devices
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
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
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
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
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
http://micro.magnet.fsu.edu/electromag/j
ava/transformer/index.html
Transformers
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
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
•
•
•
•
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
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
Transformer power rating is generally
rated in kilowatts
This rating cannot be exceeded
– Overheating would occur
This must be taken into consideration
when purchasing a machine and
considering its intended use
Review Questions
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
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