3.1 - Physics Basics Flashcards
What are radiographs created by?
X-ray photons that are projected through and object and then interact with the receptor on other side to form an image
What is black in radiographs?
Air
What is a lighter colour in radiographs?
dense enamel
What colour is amalgam in radiographs?
WHite
What colour is skin of cheek?
Darker image - like gingival
What to radiographs allow us to do?
Aid diagnosis, planning and monitoring
What are x-rays?
Type of EM radiation
What is electromagnetic radiation?
Flow of energy from one point to another and is created by simultaneously varying electric and magnetic fields
Describe properties of x-rays?
No mass
No charge
Travels at speed of light
Can travel in a vacuum
What is the electromagnetic spectrum?
Spectrum of all the types of EM radiation and they all have different properties depending on energy wavelength and freq
What does a shorter wavelength equal?
Higher frequency and higher energy
What does a longer wavelength equal?
lower frequency
lower energy
What is a wavelength
When top of wave moves up and down once
What is frequency?
How many times the wave shape repeats per unit time and is measured in hertz
What is freq measured in?
Hertz
What does one hertz equal?
One cycle per second
What is wavelength?
The distance over which the wave shape repeats itself and is in m
What is speed?
Freq x wavelength
What is speed of all EM radiation?
3x10 power 8
What happens as wavelength decreases?
Freq increases
What are photons?
Packets of energy that are measured in electron volts
What are photons measured in?
Electron volts (eV)
What are the types of x-rays?
Hard xray s
soft x-rays
What are hard x-rays?
higher energy x-rays that can penetrate human tissues and are used in medical imaging
What are soft x-rays?
Nit useful to medical imaging and are lower energy and easily absorbed
What are the properties of x-rays?
EM radiation - no mass no charge, travels at speed of light, can travel in a vacuum
man made
not detectable by human sense
causes ionisation
What is ionisation?
Displacement of electrons from atoms/molecules
How are x-rays produced?
Electrons are fired at atoms at very high speeds and on collision the kinetic energy of these electrons is is converted to electromagnetic radiation and heat - the photons are then aimed at the patient
What are atoms made of?
Protons - pos mass 1
Neutrons - 0 - mass 1
Electrons - neg - mass 0
What is the charge of nucleus?
Positive as composed of p and n
What is the atomic number?
Number of protons
What is mass number?
Number of protons and neutrons
What charge is atom in ground state?
Neutral
What do the number of electrons = when neutral?
No of protons
What happens during ionisation?
Adding or removal of electrons to an atom (adding - neg and removing - pos)
Where do electrons go?
They spin around the atom in shells - k l m n o shells
What shell is filled first?
Inner shell
Describe electron shells
k - 2
l - 8
m -18
n - 32
How do we work out how many electrons a shell can have?
2n squared
How are outer electrons held in shells?
They are held by electrostatic force
What is electrostatic force?
Neg charge of electrons held in by positive charge of protons
What is needed to remove electron from its shell?
Specific amount of energy to overcome attraction to positive nucleus
What is binding energy?
This is the additional energy required to exceed electrostatic force
When is electrostatic force greatest?
When electron is closer to nucleus
What does the energy required to move an electron to a more outer shell away from nucleus equal?
The difference in binding energy of the two shells
What happens if an electron drops to an inner shell?
Energy is released in form of x-ray photons if enough energy
How much energy is required to move electron from K shell to L shell in tungsten?
binding energy of k shell - 69.6
binding energy of l shell - 10.2
69.6-10.2 = 59.4
What is the dental x-ray unit composed of?
Tube head
Collimator
Positioning arm
Control Panel
Circuitry
What is current?
The flow of an electric charge usually by movement of electrons
What units is current in?
Amps (A)
What directions of current are there?
Direct current
Alternating current
What is DC?
Direct current is when current flows in one direction
What is AC?
when current flow repeatedly reverses direction over and over again
What does x-ray production require?
Unidirectional current - x-ray units provided by mains AC electricty so we have to use generators to modify AC so it mimics constant DC§
What is rectification?
This is when AC IS MODIFIED BY GENERATOR TO MIMIC CONSTANT DC
What is voltage?
Difference in electrical potential between 2 points in electrical field and is in volts
What volatge do dental x-ray units need?
2 different voltages
one as high as 10s of thousands of volts
one as low as 10 volts
What are transformers?
used to alter the voltage and current from one circuit to another
What transformers do X-ray tubes have?
there are two different ones
step up transformer going from mains to x-ray tube
step down transformer from mains to filament
What does a step up transformer do?
Increases potential difference across X-ray tube as is 60,000 to 70000 volts (60-7keV)
What does a step down transformer do?
It decreases the potential difference across filament and is around 10v and 10 amps
What is a filament?
The filament is the source of electrons (cathode) in x-ray tubes.
What is x-ray beam made up of?
Millions of x-ray photons directed in same general direction
What directions do photons travel in?
Straight line but diverge from source (not parallel)
What happens to dose of x-rays as you increase distance from x-ray source?
if close to the source then full intensity beam but further away some photons pass straight by and around
What is inverse square law?
if you double the distance you quarter the dose
