Basic Physics Flashcards
X-rays are a form of what kind of radiation?
Electromagnetic radiation
The flow of energy from electromagnetic radiation is created by what? What is it represented by?
Simultaneously varying electrical and magnetic fields Represented as a sine wave
What are the properties of electromagnetic radiation? (4)
- no mass
- no charge
- Always travels at the ‘speed of light’ (3x108 ms-1 = 671 million mph)
- Can travel in a vaccum
Describe the EM spectrum (3 main points).
- consists of all the diff types of EM radiation
- Each type has diff properties depening on its energy/wavelength/frequency
- 7 main groups
What are the 7 EM groups?
- gamma ray
- X-ray
- Ultraviolet
- Visible light
- Infrared
- Microwave
- Radio
Describe gamma rays in terms of wavelength, frequency and energy.
Shorter wavelength, higher frequency and higher energy
What is the wavelength?
How ‘big’ one cycle is
What is the frequency of a wave? What is it measured in?
How many times the wave’s shape repeats per unit time
Hertz (Hz) [one hertz = one cycle per sec]
Speed = frequecny x wavelength
BUT EM radiation speed is constant (3x108ms-1)
What does this mean for the relationship between frequency and wavelength in EM radiation?
If frequency increases then wavelength must decrease (and vis versa)
photons and mocement of enery etc
What are the 2 types of X-rays you get?
- Hard x-rays
- Soft x-rays
Describe hard x-rays.
Higher energies and are able to penetrate human tissues
(mediacal imaging mainly uses these types of x-rays)
Descibe soft X-rays.
Lower energies and easily absorbed
NOTE: dont want soft x-rays if want radiographs as you need some x-rays to reach the receptors
What are the basic properties of X-rays? (4)
- form of EM radiation (no mass, no charge, very fast, can travel in a vaccum)
- undetectable to human senses (contrasts with visible light)
- Man-made
- Cause ionisation (displacement of electrons from atoms/molecules)
How are x-rays and gamma rays similar?
They are identical except that gamma rays ocur naturally and X-rays are man-made
Gamma rays also generally have higher energies
Describe the basic production of X-rays.
- Electrons fired at atoms at very high speed
- On collision, the kinetic energy of these electrons is converted to electromagnetic radiation (ideally X-rays) and heat is produced as a side-effect
- The X-ray phoons are aimed at a subject
What is the basic structure of an atom?
Describe neutrons, protons and electrons. (charge, mass, location)
What is the overall charge of the nucleus?
Positive
What dictates the atomic number and mass number of an element?
AN = number of protons
Mass number = protons + neutrons
The number of electrons determines what in an atom?
Its chemical properties
What is the innermost electron shell?
K (then it foes L, M, N, O etc.)
How do you figure out the max no. of electrons a shell can hold?
max. no. = 2n2 (n=shell number)
e. g. L shell = 2x22 = 8 electrons
Max no.’s for first shell’s go (2, 8, 18, 32)
Orbiting electrons are held within their shells by what?
Electrostatic force
(-ve charge of electrons attracted to overall +ve charge of nucleus)
To remove an electron from its shell, a specific amount of energy is required to overcome the electrostatic force. What is this energy called?
Binding energy (additional energy required to exceed electrostatic force)
The energy required to move anelectron to a more outer shell requires what energy?
The difference between the 2 binding energies of the shells
If an electron drops to a more inner shell, what energy is required?
none - the difference in the 2 binding energies is released
Can be released in the form of heat or x-rays (if sufficient energy)
What are the main components of a dental X-ray unit?
Tubehead
Collimator
Positioning arm
Control panel
Circuitry
What is the definiion of a current?
Flow of electric charge, usually by the movement of electrons
What are currents measured in? Describe this measurement.
Amps
Measure of how much charge flows past a point per second
Currents can also be defined by their direction. What are these 2 categories? Describe them.
Direct current (DC) = constant unidirectional flow e.g. batteries
Alternating current (AC) = flow repeatedly reverses direction
When it comes to powering machienary, does it matter what direction a current is flowing in?
No - as long as it is flowing
X-ray production requires what kind of current? (direction)
Unidirectional current
X-ray production requires a unidirectional current BUT x-ray units are powered by mains electricity (AC - alternating currents). How is this problem resolved? What is the process called?
X-ray units have generators which modify the alternatig current so that it mimics a constant directional current
Process called rectification
What is the voltage?
Difference in electrical potential beween 2 points in a n electrical fiels
Related to how forcefully a charge will be pushed through an electrical field
NOTE: potential differenece is the same as voltage and the unit for voltage is volt
Describe the electrical supply to an X-ray unit in the UK mentioning the current and volts.
From mains supply:
- alternating current (less than or equal to 13 amps) [need DC but fixed by rectification)
- 220-240 volts
What voltages does an X-ray machiene need?
2 diff voltages:
- one as high as 10s of thousands of volts
- one as low as around 10 volts
(mains supply only gives a voltage of 220-240 volts)
How is the problem of volatge (mains supply not sufficient) for the X-ray machines resolved?
Transformers alter the voltage (and current) from one circuit to another
What transformers are found in the X-ray unit?
2 diff ones:
- Step-up transformer for the X-ray tube
- Step-down transformer for the filament
Describe the step-up transformer.
increases potential difference across x-ray tube
Usually 60,000-70,000 volts (60-70kV)
Current reduced to milliamps (mA)
Describe the step-down transformer.
decrease potential difference across filament
approx 10 volts
approc 10 amps
What are x-ray beams made up of?
Millions of x-ray photons directed in the same general direction
Describe how the photons in X-ray beams move/travel?
Effectively travel in straight lines but diverge from the X-ray source (dont travel in parallel)
What is the x-ray beam inensity defined as?
READ PIC AND UNDERSTAND
The quantitiy of photon energy passing through a cross-sectional area of the beam per unit time
Increase in number and/or energy of phtons = increase in intensity
The dose from the X-ray decreases with what?
Distance from the x-ray
What is the inverse square law that describes intensity of x-ray beam at a point?
answer = 0.25 grays
What is the difference between other types of ionising radiation and x-rays?
others produced by radioactive decay of unstable atoms unlike x-rays which are directly man-made
