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