Physics Basics Flashcards
What type of radiation are x-rays?
electromagnetic
What are the properties of em radiation?
- No mass
- No charge
- Always travels at “speed of light”
- Can travel in a vacuum
What are the 7 types of em radation?
- gamma ray
- x-ray
- ultraviolet
- visible
- infrared
- microwave
- radio
How is the wavelength calculated?
Divide distance over cycle
in m
How is the frequency calculated?
Divided the cycles by the seconds
measured in hertz Hz
How is speed calculated and how does this affect other measurements?
Speed = Frequency x Wavelength
* BUT speed of all electromagnetic radiation is constant
* 3x108 ms-1
* Therefore if frequency increases then wavelength must decrease (& vice versa)
What is the energy usually measured in?
electron volts, eV
What is 1 ev?
energy (in joules) gained by 1 electron moving across a potential difference of 1 volt
What are the types of x rays?
- Hard X-rays (higher energies)
- Able to penetrate human tissues
- Soft X-rays (lower energies)
- Easily absorbed
What type of x-rays does medical imaging use?
hard >5kev
What are the properties of x rays?
- Note: gamma rays are identical except that they occur naturally (& generally have higher energies)
- Cause ionisation
- ie. displacement of electrons from atoms/molecules
How are X-rays produced?
electrons fired at atoms at very high speed
on collision the kinetic enegry of these electrons is converted to EM and heat
the x-ray photons are aimed at a subject
What is the structure of the atom (bohr)?
Atoms are the “building blocks” of matter
* Consist of:
* Central nucleus
* Protons (+ve charge)
* Neutrons (neutral)
* Orbiting “shells”
* Electrons (-ve charge)
What is the relative charge, mass and location of neutrons, protons, and electrons?
neutron = 0, 1, nucleus
proton = +1, 1, nucleus
electron = -1, 0, shells
What is the atomic number and what is the mass number?
- Atomic Number (Z) = number of protons
- Unique to each element
- Mass Number (A) = number of protons + neutrons
What does the number of electrons equal in a ground state atom?
number of electrons = number of protons
What is ionisation?
Ionisation = removing/adding electron(s) to an atom
* Atom - e- → positive ion
* Atom + e- → negative ion
What is the innermost shell called?
K
What are all the shells from innermost?
k, l, m, n, o
What is the max number each shell can hold?
- K=2, L=8, M=18, N=32
What force holds orbiting electrons?
electrostatic force
-ve charge of electrons attracted to overall +ve charge of nucleus
What is the binding energy?
To remove an electron from its shell, a specific amount of energy is required to overcome this attraction
* Binding energy = additional energy required to exceed electrostatic force
What shells have the strongest binding energies?
The closer the electron is to the nucleus, the greater the electrostatic force (& therefore binding energy)
* K shell electrons have the highest binding energies
* Then L, then M,
What increases the electrostatic force?
the more positively-charged the nucleus, the greater the electrostatic force
What is the energy required to move an electron to a more outer shell equal to?
The specific amount of energy required to move an electron to a more outer shell (i.e. away from the nucleus) equals the difference in the binding energies of the 2 shells
conversely if an electron drops to a more inner shell then this specific amount of energy is released
What are the components of an x-ray unit?
Tubehead
* Collimator
* Positioning arm
* Control panel
* Circuitry
What is current and what is it measured in?
- Flow of electric charge, usually by the movement of electrons
*SI unit: amp (or ampere), A - Measure of how much charge flows past a point per second
What are the two directions of current?
- Direct current (DC) = constant unidirectional flow
- e.g. batteries
- Alternating current (AC) = flow repeatedly reverses direction (mains)
What is AC?
How is the freqency found?
- Flow periodically reverses direction
- Number of complete cycles (reverse + reverse-back)
per unit time is the frequency - SI unit: hertz, Hz (cycles per second)
What current do x-rays require and what are they powered by?
X-ray production requires a unidirectional current
* But X-ray units are powered by mains electricity (AC)
How is the AC current in x-ray machines rectified?
X-ray units have generators which modify the AC so that it mimics a constant DC
* Process known as rectification
What is voltage and what is it related to?
Difference in electrical potential between 2 points in an electrical field
* Related to how forcefully a charge will be pushed through an electrical field
* SI unit: volt, V
What is the voltage and current of mains supply?
Alternating current (≤13 amps)
* 220-240 volts
What two voltages does a dental x ray unit require?
One as high as 10s of thousands of volts
One as low as around 10 volts
What do transformers do?
alter the voltage (& current) from one circuit to another
What are the two seperate transformers required for x-ray unit?
- One which takes the Mains supply and makes it appropriate for the X-ray tube (cathode-anode)
- And one which takes the Mains supply and makes it appropriate for the Filament
What is the step up transformer?
↑ potential difference across X-ray tube
* Usually 60,000-70,000 volts (60-70 kV)
* Current reduced to milliamps (mA)
What is the step down transformer?
- Step-down transformer
- ↓ potential difference across filament
- ~10 volts
- ~10 amps
How do photons travel from the x-ray beam?
Photons effectively travel in straight lines but diverge from the X-ray source (i.e. do not travel in parallel)
What is the x-ray beam intensity?
Quantity of photon energy passing through a cross- sectional area of the beam per unit time
What is the x ray beam intensity proportional to?
current in filament (mA) & potential difference across X-ray tube (kV)
↑ number &/or energy of photons = ↑ intensity
What is the inverse square law?
Intensity of X-ray beam is inversely proportional to the square of the distance between the X-ray source & the point of measurement
- Intensity ∝ 1/distance2
- Therefore, doubling the distance will quarter the dose
What are alpha, beta and gamma particles produced by and what are x-rays produced by?
All produced by radioactive decay of unstable atoms
- Unlike X-rays which are directly man-made
