Physics Basics

Question 1

What type of radiation are x-rays?

Answer 1

electromagnetic

Question 2

What are the properties of em radiation?

Answer 2

• No mass
• No charge
• Always travels at “speed of light”
• Can travel in a vacuum

Question 2

What are the 7 types of em radation?

Answer 2

• gamma ray
• x-ray
• ultraviolet
• visible
• infrared
• microwave
• radio

Question 3

How is the wavelength calculated?

Answer 3

Divide distance over cycle

in m

Question 4

How is the frequency calculated?

Answer 4

Divided the cycles by the seconds

measured in hertz Hz

Question 5

How is speed calculated and how does this affect other measurements?

Answer 5

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)

Question 6

What is the energy usually measured in?

Answer 6

electron volts, eV

Question 7

What is 1 ev?

Answer 7

energy (in joules) gained by 1 electron moving across a potential difference of 1 volt

Question 8

What are the types of x rays?

Answer 8

• Hard X-rays (higher energies)
• Able to penetrate human tissues
• Soft X-rays (lower energies)
• Easily absorbed

Question 9

What type of x-rays does medical imaging use?

Answer 9

hard >5kev

Question 10

What are the properties of x rays?

Answer 10

• Note: gamma rays are identical except that they occur naturally (& generally have higher energies)
• Cause ionisation
• ie. displacement of electrons from atoms/molecules

Question 11

How are X-rays produced?

Answer 11

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

Question 12

What is the structure of the atom (bohr)?

Answer 12

Atoms are the “building blocks” of matter
* Consist of:
* Central nucleus
* Protons (+ve charge)
* Neutrons (neutral)
* Orbiting “shells”
* Electrons (-ve charge)

Question 13

What is the relative charge, mass and location of neutrons, protons, and electrons?

Answer 13

neutron = 0, 1, nucleus
proton = +1, 1, nucleus
electron = -1, 0, shells

Question 14

What is the atomic number and what is the mass number?

Answer 14

• Atomic Number (Z) = number of protons
• Unique to each element
• Mass Number (A) = number of protons + neutrons

Question 15

What does the number of electrons equal in a ground state atom?

Answer 15

number of electrons = number of protons

Question 16

What is ionisation?

Answer 16

Ionisation = removing/adding electron(s) to an atom
* Atom - e- → positive ion
* Atom + e- → negative ion

Question 17

What is the innermost shell called?

Answer 17

K

Question 18

What are all the shells from innermost?

Answer 18

k, l, m, n, o

Question 19

What is the max number each shell can hold?

Answer 19

• K=2, L=8, M=18, N=32

Question 20

What force holds orbiting electrons?

Answer 20

electrostatic force
-ve charge of electrons attracted to overall +ve charge of nucleus

Question 21

What is the binding energy?

Answer 21

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

Question 22

What shells have the strongest binding energies?

Answer 22

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,

Question 23

What increases the electrostatic force?

Answer 23

the more positively-charged the nucleus, the greater the electrostatic force

Question 24

What is the energy required to move an electron to a more outer shell equal to?

Answer 24

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

Question 25

What are the components of an x-ray unit?

Answer 25

Tubehead * Collimator * Positioning arm * Control panel * Circuitry

Question 26

What is current and what is it measured in?

Answer 26

* 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

Question 27

What are the two directions of current?

Answer 27

* Direct current (DC) = constant unidirectional flow * e.g. batteries * Alternating current (AC) = flow repeatedly reverses direction (mains)

Question 28

What is AC? | How is the freqency found?

Answer 28

* Flow periodically reverses direction * Number of complete cycles (reverse + reverse-back) per unit time is the frequency * SI unit: hertz, Hz (cycles per second)

Question 29

What current do x-rays require and what are they powered by?

Answer 29

X-ray production requires a unidirectional current * But X-ray units are powered by mains electricity (AC)

Question 30

How is the AC current in x-ray machines rectified?

Answer 30

X-ray units have generators which modify the AC so that it mimics a constant DC * Process known as rectification

Question 31

What is voltage and what is it related to?

Answer 31

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

Question 32

What is the voltage and current of mains supply?

Answer 32

Alternating current (≤13 amps) * 220-240 volts

Question 33

What two voltages does a dental x ray unit require?

Answer 33

One as high as 10s of thousands of volts One as low as around 10 volts

Question 34

What do transformers do?

Answer 34

alter the voltage (& current) from one circuit to another

Question 35

What are the two seperate transformers required for x-ray unit?

Answer 35

- 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

Question 36

What is the step up transformer?

Answer 36

↑ potential difference across X-ray tube * Usually 60,000-70,000 volts (60-70 kV) * Current reduced to milliamps (mA)

Question 37

What is the step down transformer?

Answer 37

* Step-down transformer * ↓ potential difference across filament * ~10 volts * ~10 amps

Question 38

How do photons travel from the x-ray beam?

Answer 38

Photons effectively travel in straight lines but diverge from the X-ray source (i.e. do not travel in parallel)

Question 39

What is the x-ray beam intensity?

Answer 39

Quantity of photon energy passing through a cross- sectional area of the beam per unit time

Question 40

What is the x ray beam intensity proportional to?

Answer 40

current in filament (mA) & potential difference across X-ray tube (kV) ↑ number &/or energy of photons = ↑ intensity

Question 41

What is the inverse square law?

Answer 41

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

Question 42

What are alpha, beta and gamma particles produced by and what are x-rays produced by?

Answer 42

All produced by radioactive decay of unstable atoms * Unlike X-rays which are directly man-made