electricity, magnetism, & electromagnetism (image production unit 1, ch. 5)

electromagnetism

the branch of physics that deals with the relationship between electricity & magnetism

current

a flow of electrons

-every electric current is accompanied by/induces a magnetic field

-current is any charged particle in motion

what did oersted discover?

-magnetic fields always surround a conductor in which an electrical current is flowing

(any current or charge in motion is accompanied by magnetic field)

oersted electromagnetic phenomena experiment

-wire carrying a current, magnetic compass points to wire

-so current makes the wire have magnetic field that's stronger than the pull of the earth's magnetic field on the compass needle

right hand thumb rule

-thumb is direction of current

-fingers are direction of magnetic field/flux lines

what does the right hand thumb rule tell you?

direction of magnetic field

electromagnet

a coil of wire carrying an electric current into which a soft iron core has been inserted

(solenoid w/ iron core)

helix

coil of wire

solenoid

coil of wire with current flowing through it

-the basis of an electromagnet, can be considered one itself

what does the iron core do in an electromagnet?

it enhances the strength of the magnetic field

inducing EMF/voltage in a wire by electromagnetic induction

1. wire may move across stationary magnetic flux

2. magnetic flux may move across stationary wire

3. magnetic flux may vary in strength while a stationary wire lies in it

(varies w/ # of coils & strength of magnet)

faraday found how to...

induce EMF/voltage in a wire by electromagnetic induction

-induced current w/o the wire hooked to power source

-did this by keeping 1 part stationary & the other part moving (magnet & coil)

-OR by varying the magnetic strength w/ a stationary wire (?)

faraday's law: magnitude of induced electromagnetic force

-speed the wire cuts magnetic flux

-strength of magnetic field

-angle between conductor & direction of magnetic field

-number of turns of wire

faraday's law is based on...

mechanical means, not electricity

(not hooked to power source)

(faraday) the faster the wire cuts flux lines...

greater the strength of voltage

(faraday) the stronger the magnet...

the higher the magnitude of/greater voltage

(faraday) angling or no angling gives greater voltage?

no angling between conductor & direction of magnetic field

-90* between cuts the most flux lines than any angling

(faraday) the greater the number of turns of wire...

the more lines it can cut thru, so the higher the voltage

electromagnetic induction

an electric current is induced in an electric circuit if some part of that circuit is in a changing magnetic field

(1 must be stationary & 1 must move or intensity of magnetic field must vary)

self-induction

occurs within a circuit containing a helix or exist when 2 or more conductors lie next to each other

-1 coil (helix or solenoid)

-needs AC current

-has back EMF

how self induction works

-coil is hooked to power, AC current goes thru wire

-magnetic flux lines expands out on positive & is cut thru by coil, (comes to rest, reverse polarity, collapses)

-this induces a voltage in the coil

what transformer has 1 coil of wire?

autotransformer

back EMF

-in self induction when voltage coming off interferes w/ voltage coming back in (b/c AC current oscillates, positive portion goes out & negative goes back in)

-runs into each other like back feed b/c only 1 wire, starts to come back in before positive portion expanded out all the way

self induction needs DC or AC current?

AC current

mutual induction

the generation of an alternating current in a secondary coil by supplying an alternating current to the primary coil

-2 coils

-no back EMF

mutual induction needs AC or DC current?

AC current

-DC would only expand, never comes back in

how mutual induction works

-primary coil is supplied w/ voltage, secondary is not

-p&s coils do not touch

-as current runs thru primary, flux lines expand & are cut thru by secondary coil if it is in close proximity

-the moving magnetic field over stationary wires induces EMF

-positive flux expands out, reverse polarity & negative comes back in (b/c AC current)

transformer base needs ________ to operate

AC current

electromagnetic devices

-electric generator

-electric motor

-transformers

electric generator converts

mechanical energy into electrical energy

-done thru electromagnetic induction

electric motor converts

electrical energy into mechanical energy

-done thru electromagnetic induction

transformer

"a device that operates on the principle of electromagnetic mutual induction in order to change the electric potential to a higher or lower voltage, which also raises or lowers the current"

-(purpose is to change voltage)

voltage & current relationship

inverse/indirect

-if voltage increases, current decreases

electric generators (dynamo)

produces an electric current by rotating loops of wire (armature) thru a fixed magnetic field

-AC or DC generator

electric generators consists of 2 components:

1. magnet (stationary)

2. armature between the magnets (moving)

armature

a coil of wire that rotates inside a magnetic field

difference between motor & generators

motor & generator have same parts, just differs in what is supplied & what is generated

AC electric generator

-crank that turns the armature on one end

-armature is between magnets & cuts the flux lines as it rotates

-two wires on other end where electricity comes out

-slip rings on ends of wires that are in constant contact w/ brushes/contacts

what happens when the armature cuts the flux lines? (generator)

this induces EMF in the armature b/c the magnetic field is stationary while the armature is moving thru it

-EMF produces current

-this electricity that is produced comes out at the end w/ the brushes

brushes/contacts (AC generator)

-are in constant contact w/ slip rings

-brushes are stationary, slip rings rotate w/ armature

-brushes is where electricity comes out of

DC electric generator

-same parts as AC generator, except commutator ring

-commutator ring is not in constant contact w/ brushes b/c of slit so you get pulsating DC (only the positive portion)

how many cycles per second w/ DC generator?

-still 60 cycles/sec (60 hz/sec) or 1 cycle every 1/60th second (same as US system), but only uses a portion b/c slit in ring

-so, 60 total pulses per cycle (US system has 120 pulses per cycle)

only difference between AC & DC generator

-AC: slip rings in constant contact w/ brushes (gets the electricity in the positive & negative sides of cycle)

-DC: commutator ring is not in constant contact w/ brushes b/c of slit (get pulsating DC of only the positive portion of the cycle)

fleming's right hand rule

(for generator)

-looking for current flow (MIDDLE)

-thumb: direction of force/armature

-index: direction of field/flux lines

-middle: current

electric motors

difference from generator is that motors are hooked to electric power source instead of a crank to cause the armature to turn

-(electric energy to mechanical)

2 conditions must exist for electric motors

1. conductor must carry a current

2. current must be located within a magnetic field

AC & DC electric motor

same as AC/DC generator, but motor is supplied w/ electricity

fleming's left hand rule

(for motor)

-looking for direction of the conductor/movement of armature (THUMB)

-thumb: direction of force/armature

-index: direction of magnetic field/flux lines

-middle: direction of current

EMF allows...

flow of current, which induces a magnetic field

induction motor is based on...

electromagnetic induction

stators

stationary electromagnets arranged in pairs around the rotor on the outside of the anode

rotor

-located in the middle of the stators, made of an iron core surrounded by copper coils

-has ball bearings to help it spin

-is on inside of glass envelope

in the induction motor, which part is supplied w/ EMF/current?

stators

in the induction motor, what is being induced upon?

rotor

(done thru electromagnetic induction!)

are the stators & the rotor touching?

no

x-ray tube spins at

3,400-3,600 rpm

CT spins at

10,000 rpm

what is the purpose of the rotor?

rotates the anode

how does the induction motor cause the anode to spin?

-corresponding stators are sequentially supplied w/ electricity which produces electromagnetic field between the 2 stators across each other, creates N & S poles

-this causes the internal magnetic field to align w/ that outside magnetic field (same poles align - does not like that)

-it wants opposites to attract so the rotor flips to the next ones

-rotor keeps rotating as the corresponding stators are shut on & off

transformers

a device that operates on the principle of electromagnetic mutual induction to change the electric potential to higher or lower voltage (also raises/lowers current)

what is the only adjustable transformer?

autotransformer

how are transformers identified?

by what it does to the voltage

(they also affect current, but name goes by its effect on voltage)

all transformers have...

a primary side & a secondary side

(even if only 1 coil like on autotransformers)

step-up transformer

increases voltage, decreases current

-more turns on secondary coil than primary

step-down transformer

decreases voltage, increases current

-less turns on secondary coil than primary

turn ratio (T/R)

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

(Ns/Np) or (Ns:Np)

if turn ratio is greater than 1

it's step up

if turn ration is less than 1

it's step down

transformer law

the EMF induced in the secondary coil is to the EMF in the primary coil,

as the number of turns in the secondary coil to the number of turns in the primary coil

transformer law formula

Vs/Vp = Ns/Np

number of turns & voltage relationship (transformer law)

direct relationship

transformer law for current

Ns/Np = Ip/Is

Vs/Vp = Ip/Is

current has inverse relationship to voltage & # of turns

types of transformer configurations

-air core

-open core

-closed core

-shell-type

(make sure to know the pictures)

air core transformer

-no physical core; hollow

-two coils in proximity

open core transformer

-both the primary & secondary coils are filled w/ an iron core strengthening the magnetic field

-(iron enhances b/c it's ferromagnetic material)

closed-core transformer

-coils are wound around a square or circular core to provide a continuous path for magnetic field

-it is insulated to help contain & direct the magnetic force lines

shell-type transformers

-most commonly used transformer

-it "traps" the magnetic forces more efficiently in order to induce the strongest possible current in the secondary coil

-primary & secondary are intertwined in the middle of a square

what does the square core do?

-helps reduce waste of heat, doesn't let it spread out

-everything is connected in the 1 square

-more efficient, creates a better voltage

which autotransformer do we use / is most efficient?

shell type

autotransformer

consists of an iron core w/ only one winding of wire around it

-the single wire acts as both primary & secondary winding

autotransformer is controlled by

-kVp selector on control panel

-which lowers/raises voltage in the primary circuit

autotransformer operates on the principle of...

self induction

autotransformer taps

taps controls amount of voltage

-same thing as # of turns of wire on secondary

-(outside wires are attached at different points along the coil, the induced voltage varies depending on where the connections are made)

transformer efficiency

make transformer more efficient so it produces less heat & uses more power/energy; reducing heat

-affected by:

1. copper loss

2. eddy current loss

3. hysteresis loss

copper loss

due to resistance in copper conductor, results in some energy loss

copper loss solution

using a wire of adequate or appropriate diameter

-thicker wire=less resistance

-adjust diameter of secondary wire based on if need step up or step down

(step down: larger diameter = more current, less voltage)

(step up: smaller diameter = less current, more voltage)

eddy current loss

swirling currents produced in core due to the changing current (due to AC current); results in heat loss

eddy current loss solution

-use sheets of iron/steel for the core & laminated w/ silicon to decrease size of eddy currents

hysteresis loss

heat loss due to the constant re-arrangement of the magnetic domains in the core

hysteresis loss solution

use laminated sheets of iron/steel core (same as for eddy currents)

-has higher resistance to magnetic energy

-reduces fluctuation of alignment