Electricity & electromagnetism - Q2

magnetism

the ability of a material to attract, iron, cobalt, or nickel in pure form

predominately one direction

the nature of magnetism is that the orbital electrons of their atoms spin ___________

magnetic dipoles

atoms that behave like small magnets bc they have an unpaired e- (H)

magnetic domains

when these dipoles or ‘‘atomic magnets’’ form groups of similarly aligned atoms, they create ___________ ________

1. natural (Earth)

2. artificially induced (permanent- horseshoe magnet, compass needle)

3. electromagnets (wire wrapped around a soft iron core)

3 kinds of magnets

1. every magnet has a north and south pole

2. like poles repel and opposites attract

3. Gauss’s law

3 laws of magnetism

flux lines

invisible magnetic field lines of force passing through a magnet (they dont move)

1. travel from S to N pole inside the magnet and N to S pole outside the magnet

2. lines of flux traveling in the same direction repel each other and lines going in the opposite direction attract each other

3. are distorted by magnetic materials & are unaffected by nonmagnetic materials

3 characteristics of flux lines

Tesla (T)

SI unit of the strength of the magnetic field

Gauss

traditional unit of magnetic field in cgs system

10⁴ guass

1 T = how many gauss

30,000

MRI’s are approximately _____________ times stronger than the earth’s magnetic field

nonmagnetic materials (glass, wood, plastic)

are not attracted to magnetic fields at all

are weakly repelled by magnetic fields (water, mercury, gold)

diamagnetic materials

are weakly attracted to magnetic fields (platinum, gadolinium, aluminum)

paramagnetic materials

are strongly attracted to magnetic materials (iron, cobalt, nickel)

ferromagnetic materials

electromagnetism

• any flow of electrons, whether in space or in a conductor, is surrounded by a magnetic field

• a moving magnetic field can also create an electric current

Hans Oersted

discovered that electricity flowing through a conductor produces a magnetic field

solenoid

coiling a conductor creates a

electromagnet

adding an iron core to a solenoid creates a

michael faraday

discovered electromagnetic induction

electromagnetic induction

moving a conductor through a magnetic field induces an electric current in that conductor

mutual induction

• induction of electricity in a 2° coil by a moving magnetic field (AC current induces a changing magnetic field in the primary coil)

• when a 1° coil is placed near a 2° coil, alternating current/electricity is induced due to the moving magnetic field in the 1°

  • increases incoming voltage to x-ray machine to kilovoltage

self induction

• Lenz’s law

1. a magnetic field is created in a coil carrying electrical current & expands outward from center of the coil

2. as it does so, it ‘‘cuts’’ through the turns of the coil & creates a current within the same conductor that opposes the original current

3. used in x-ray circuit in the autotransformer design to choose a variable kVp from the incoming voltage

coulomb’s law

force between charges is directly proportional to the product of the quantity and inversely proportional to the square of the distance between them (law of electrostatics)

Ohm’s law

V=IR (circuit rules)

Gauss’s law

force between poles is directly proportional to their strength and inversely proportional to the square of the distance between them (magnetism)

Oerstad

flowing current produces a magnetic field (electromagnetism)

Faraday

moving a conductor through a mag field will induce a current in another conductor without touching it (electromagnetic induction - Mutual transformers)

Lenz’s law

an induced current flows in the opposite direction as the action that induced it (self-induction - autotransformers)

electric generators

devices that convert mechanical energy into electrical energy

electric motors

devices that convert electrical energy into mechanical energy

transformers

devices that increase/decrease voltage

turns ratio

number of turns on primary coil divided by the number of turns on the secondary coil

air core

• simplest

• 2 insulated coils laying side by side

• large power loss

open core

• iron core inserted into coils

  • iron core incr. mag so flux inc

• appreciable power loss due to loss of mag flux at ends of core

closed core

• insulated coils called windings are wound around a square or circular iron core

  • provides a continuous path for mag flux, increasing efficiency

• older xray equip has this

shell type

• most advanced & efficient

• note primary & secondary windings on 1 laminated core

• commercial power & modern xray systems

autotransformer

• operates on the principle of self-induction

• has only 1 coil of wire around an iron magnetic core serving as both 1° and 2° coil

• the taps allow to adjustment of the # of windings to be used (vary kVp)