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Electromagnetism
Kyle Thornton
DMI 50B
Magnetic Attraction and Polarity
http://micro.magnet.fsu.edu/electromag/java/magneticlines/index.html
http://micro.magnet.fsu.edu/electromag/java/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/java/faraday2
http://micro.magnet.fsu.edu/electromag/java/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/java/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_aTC0iKO68&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/java/generator/dc.html
http://micro.magnet.fsu.edu/electromag/java/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=Q2mShGuG4RY&feature=related
Electric Motor Electric Motor Animation http://www.phys.unsw.edu.au/hsc/hsc/
electric_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.html
• http://www.youtube.com/watch?v=HWrNzUCjbkk&feature=related
Rectification Electrons are made to flow inside the x-
ray 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 tuberectifiers
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/java/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=related
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&feature=fvwp
http://www.youtube.com/watch?v=xGNmGCCCQRg&NR=1&feature=fvwp