Physics Exam #3

The purpose of an x-ray imaging system is __

to convert electrical energy into electromagnetic energy

Electrostatics

the study of static (or stationary) electric charges

The universes’ smallest units of electrical charge

proton and electron

• can never have a 0.5 - or + charge

How is a proton equal and opposite to an electron charge?

they have the same magnitude, but opposite signs

Most electrical charges deal only with ____

free electrons

An electrified object has ___

too many or too few electrons, causing a net electric charge

Electricity

moving electrons, with protons stuck in the nuclei

How does static electrification occur?

when an object has too many or too few electrons; flowing from high to flow concentration

What happens when a negatively charged objects meets something positively charged?

extra electrons move toward the deificency, causing a shock to occur

Electrons can be freed from atoms through____

friction or heating

Static sometimes gets converted to ___

light or sound , by flowing to the electron (negative) deficiency

Lightening is considered ….

static electrical discharge (induction

What is considered the universal reciever of electrons?

the earth; “electric ground”

What is electric charge measured in?/ What is its SI unit?

coulomb — “a certain number of charges”

1 Coulomb =

6.3 × 10 ^18 electron charges

The Four Laws of Electrostatics

1. unlike charges attract, like charges repel

2. electric charge of a conductor is concentrated along the sharpest curvature of a surface

3. electric charge distribution is uniform throughout or on the surface

4. electrostatic force is determined with coulomb’s law

Electrostatic force

• the force of attraction between unlike charges or repulsion between like charges

• moves toward a negative charge and away from a positive charge

Coulomb’s law

• follows the inverse square relationship

• how electrostatic force is determined

Neutral (uncharged)

• have no electric field

• if there is no electrical field, they cannot interact

What is another name for electric potential?

voltage (V) = J/C

the higher the voltage…

the greater potential to do work

How can electricity have potential energy?

• if electrons build up on one end of a system ready to be moved to the other end

• the greater the charges, the greater the potential energy

Electrodynamics

the study of electric charges in motions (dynamic)

What happens to electrons when voltage is applied ?

electrons move

Current (I)

• the rate of electron movement; the number of charges per second

• measured in amps (C/sec)

Resistance

• the resistance of materials to the flow of electrics

• measured in Ohms

4 Electric States of Matter

1. conductor

2. insulator

3. semiconductor

4. superconductor

Conductor

• electricity flows easily

• requires a voltage

• copper, water, aluminum

Insulator

• electricity does not flow due to a high resistance

• necessary with high voltage

• rubber, glass, clay, other earthlike materials

Semiconductor

• lower temperature can eliminate resistance all together, allowing electrons to flow freely

• no electric potential required

• at room temperature, materials resist the flow of electricity

• niobium, titanium

When the temperature of the material is reduced

resistance decreases

Which electric state of matter does ohm’s law not apply to?

superconductors

Electric Circuits

• when resistance is controlled and the conductor is made into a closed path

• a closed loop of electric current

What demontrates the relationship between current, resistance, and voltage ?

Ohm’s law

Electrons at high voltage have»

• high potential energy

• high capacity to do work

If electron flow is inhibited

the electric circuit resistance is high

Alternating Current (AC)

• circuits in which electrons flow back and forth in both directions

• electrons first flow positively with increasing voltage, than in a negative direction

• created by generatorswhen plugged into a wall

• Nicola Tesla

Direct Current (DC)

• circuits in which electrons flow in only one direction

• created by batteries and rectifiers

• charges keep going around

• Thomas Edison

• X-ray tubes

Electric Power

• the rate of consumption of electrical energy

• measured in watts (W)

• P = IV

Where was magnetite (loadstone/ leading stone) found?

• found by farmers near village of Magnesia (western Turkey)

• the rock pointed to water

What does any spinning or moving charged particle create?

a magnetic field perpendicular to itself

A magnetic field needs …

charged particles and a spin

What specifically creates the magnetic field?

the spin (diploe), which is neturalized in electron pairs

Diploe/ magnetic moment

• small magnet created by the electron orbit

• forms the basis of MRI

Amplified Field/ Magnetic Domain

happens when aligned magnetic fields get near each other

What happens to opposing magnetic fields?

they cancel each other out

A moving electron creates..

a disk shaped magnetic field

Three types of magnets

1. natural

2. permanent

3. electromagnets

Natural Magnets

• objects that have become magnetized by the earth, the largest natural magnet (made of iron)

• magnetite = load stone

• earth has a magnetic field because it spins on an axis

Permanent Magnets

• objects that have become magnetized artifically by aligning their domains with an electromagnet

• similar to natural, just man-made

• usually made of iron

• compass

Electromagnets

• wire wrapped around an iron core, uses the property of electromagnetic induction to create a strong magnetic field

• the wire itself becomes a magnet

• the iron core increases the intensity

Matter has one of three interactions with magnetic fields

1. diamagnetic

2. paramagnetic

3. ferromagnetic

diamagnetic

• paired electrons weakly repel a magnetic field

  • cannot be artifically magnetized

  • they are not attracted to a magnet

  • water, plastic

paramagnetic

• unpaired electrons are weakly attracted to a temporary magnetic field; allows a tiny magnetic domain

• contrast agents

Ferromagnetic

• unpaired electrons in certain materials can be locked into a configuration that is strongly attracted to a magnetic field

  • not a lot of this material: iron, cobalt, nickel, alnico

  • most metals are not this (term)

alnico

colbalt that is one of the more useful magnets pronduced from ferromagnetic material

What do magnetic fields work similary to?

• gravity, electrostatic, and nuclear forces

• allow the inverse square law

Maxwell’s field theory

combines electric and magnetic forces into electromagnetism

Electricty creates …

a magnetic field, inducing magnetism

By curving a wire…

magnetic fields overlap and intensify

What induces current?

a changing magnetic field

Faraday’s Law: the magnitude of the induced current depends on 4 factors

1. the strength of the magnetic field

2. the velocity of the magnetic field as it moves

3. the angle of the conductor of the magnetic field

4. the number of turns in the conductor (coil)

An electric current cannot…

be induced in a circuit merely by the presence of a magnetic field

Electric Motors

• electrical energy is supplied to the current loop to produce mechanical motion (the rotation of the loop in the magnetic field)

• the discovery of electromagnetic induction has led to many inventions

• induction motors, transformers, and generators inside the x-ray unit work based on the prinicples of induction

Transformers

• use mutual induction to change the intesnity of voltage and current

• transforms electric potential and electric current into higher or lower intensity

How do transformers work?

• by wrapping a coil around a ferromagnetic core, the magnetic field stays concentrated

• wrapping another coil around the other side induces a current in the secondary coil

Changing the number of turns in the secondary coil …

changes the voltage induced

an equal number of turns in each coil induces

a current with the same voltage

step up transformer

increase in the number of turns in the secondary coil

• increase voltage

• decreases current

step down transformer

decrease in the number of turns in the secondary coil

• decrease voltage

• increases current

closed core transformer

built around a square core of ferromagnetic material

autotransformer

• only good for small changes in voltage

• only one coil that acts as primary and secondary windings

• you can change how this functions, but not for big functions

Shell type transformer

• more efficient than closed core

• confines even more of a magnet field lines of the primary winding because the secondary winding is wrapped around it with two closed cores

• most commonly used

kVp

amount of energy of electrons

mA

intesnity, amount of photons

General use x-ray room kVp

25-150

General use x-ray room mA

100-1200

Imaging couch (table)

• for pt to lie on

• floating motorized top for easy movement

• lightweight radiolucent carbon fiber to reduce patient dose

• contains potter bucky tray to hold IR and grid

Grid usuage

• when patient is bigger, reduces the dose to the IR

• don’t need it, don’t use it — multiplies technique twice

• used for image quality