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Radiation Physics > Week 4 - Work, Energy and Power > Flashcards

Week 4 - Work, Energy and Power Flashcards

1
Q

X-Ray Tube

A
  • EPE is transferred to the electrons
  • Electrons are accelerated and gain KE
  • The electric field (voltage) does not work on the electron
  • Electron KE converted to electromagnetic energy (x-rays) and thermal energy heat in the x-ray target

At filament –> converting EPE to electrons within the coil –> gain energy through thermionic emission

Majority of energy is converted to heat

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2
Q

Force

A

o Ability to move a stationary body or change the speed of a moving body

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3
Q

Newtons Second Law

A

F = ma

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4
Q

Units of Force

A

o Newton (N) = 1 kg x ms^-2

o The amount of force required to move a mass of 1 kg with an acceleration of 1 ms^-2

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5
Q

Work

A

o Required to be done to move an object
o Ability to move an object over a distance
o SI Unit: Joules (J)
o 1J of work would be done if a force of 1N is applied to an object over a distance of 1 metre
o Work = Force x Distance

In radiation physics, medical imaging and radiotherapy (electrical work)
 Electron Volt (eV)
 1 eV = 1.602176565 x 10^-19
 1 eV is the work done to move an electron through a potential difference (voltage) of 1 V

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6
Q

Energy

A

o Measure of the ability to do work

o	Forms include: 
	Mechanical 
	Electrical 
	Heat (thermal) 
	Light (radiant) 
	Nuclear 
	Gravitational

o Stored Energy = Potential Energy
o Energy due to Motion = Kinetic Energy

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7
Q

Conservation of Energy

A

 Energy cannot be created or destroyed; it can only be converted from one form to another

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8
Q

Work and Energy Examples

A
  • At the cathode (heated filament) electrons have potential energy due to the high electric potential difference
    o Electrical Potential Energy = charge x electric potential voltage

PE = e V
o E.g., if voltage is 125 000 V (125 kV)
 PE = 125 keV

  • At the anode target all of the potential energy will have been converted to electron kinetic energy
  • This will then be converted to x-ray energy in the target + heat
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9
Q

The Electron Volt (eV)

A
  • The energies of sub-atomic particles and electromagnetic waves usually very small (in J)
  • Amount of work that is done to move an electron with charge 1.60217662 x 10^-19 Coulombs through a potential difference of 1 Volt
  • Use the derived unit: electron volt

Diagnostic Imaging
- X-Ray Energies (approx. 50-150 keV)

Radiotherapy Linear Accelerator
- X-Ray Energies (approx. 6-20 MeV)

Cobalt-60 Gamma Rays
- Approx. 1.25 Me

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10
Q

Power

A
  • The rate at which energy is used
  • Units: Joules per second or the Watt
  • 1 J/s = 1W
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11
Q

Heat

A
  • The flow of energy from a higher temperature object to a lower temperature because of the difference in temperatures
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12
Q

Temperature

A
  • Measure of how much thermal energy an object has
  • Atoms/molecules are vibrating
  • Higher Thermal Energy = Larger vibrations
  • Increase in temperature and expansion
Temperature Scales 
o	Fahrenheit 
o	Centigrade or Celsius 
o	Kelvin 
o	0®K = - 273.15®C

0 degree K = -273.15 degree C

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13
Q

Transfer of Heat

A
  • Three mechanisms for transfer of heat
    o Conduction
    o Convection
    o Radiation
  • All three are important or a problem in x-ray production
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14
Q

Conduction

A
  • Heat is transferred directly though a material
  • Occurs when the atoms or molecules in a hotter pat of the material vibrate or move with greater energy than those in the cooler part
  • The more energetic molecules pass on some of their energy to their less energetic neighbours
  • Thermal Conductors = Conduct Heat Well
  • Thermal Insulators = Conduct Heat Poorly
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15
Q

Factors Affecting the Conduction of Heat

A
  • The time which conduction takes place
  • The temperature difference
  • The cross-sectional area
  • The length
  • The materials (insulators and conduction materials)
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16
Q

Convection

A
  • Heat is carried from one place to another by the bulk movement of a fluid (liquid or gas)

In Pot
o Warmer water moves to the top –> cooler water moves down to replace it

17
Q

Radiation

A
  • The process in which energy is transferred by means of electromagnetic waves (infra-red)
  • Material that is a good absorber is also a good emitter
  • A material that absorbs completely = Perfect Blackbody
18
Q

Waves

A
  • Is a travelling disturbance

- Carries energy from place to place

19
Q

Longitudinal Waves

A
  • Sound Waves
  • Compressions propagates along the sling
  • Any point at the spring is just moving back and forth
20
Q

Transverse Waves

A
  • Electromagnetic Waves
  • Disturbance is propagating along the spring
  • At any point the spring is just moving up and down
21
Q

Properties of a Wave

A

Amplitude
o Maximum excursion of a particle from an undisturbed position

Wavelength
o Horizontal distance of one cycle of the wave

Period
o Time required for one complete cycle

Frequency
o Related to the period and has unit’s Hz or S^-1
o F = 1/T

22
Q

Waves

A

Generally, travel in straight lines

Can be: 
o	Reflected 
o	Deflected (scattered) 
o	Amplified 
o	Absorbed
23
Q

The Principle of Linear Superposition

A

o When two or more waves are present simultaneously at the same place, the resultant disturbance is the sum of the of the disturbances from the individual

o If negative and positive meet, they will cancel out (disruptive interferences)
 Zero disturbances

o If two waves come together –> overlap will double amplitude –> continue in original amplitude (constructive interference)

24
Q

Sound Waves

A
  • Sound is created by a vibrating object that compresses and decompresses the atoms/molecules in a material
  • Longitudinal pressure waves that propagates through air molecules
    o Air molecules are being vibrated  not moved
25
Q

Application of Sound in Medicine

A

Ultrasound
- Typical Frequencies (approx. 2-18 MHz)

Human hearing sensitive to frequencies
- 20 Hz - 20 KHz (cycles per second)

Pulses of US sent into patient, ‘echoes’ measured to reconstruct image

When the Sound is Reflected from RBC’s

  • Its frequency is changed in a Doppler Effect as the cells are moving
  • If stationary –> frequency would be reflected back as equal
26
Q

Magnetism

A
  • The needle of a compass is a permanent magnet that has a north magnetic pole (N) at one end and a sound magnetic pole (S) at the other
27
Q

Magnetic Field

A
  • The behaviour of magnetic poles is similar to that of like and unlike electric charges
28
Q

Magnetism

A
  • Surrounding a magnet there is a magnetic field
  • The direction the field at any point in space is the direction indicated by the north pole of a small compass needle placed at the point
29
Q

Magnetic Material

A
  • The intrinsic ‘spin’ and orbital motion of elections gives rise to the magnetic properties of materials
  • In ferromagnetic materials groups of neighbouring atoms, forming magnetic domains, the spins of electrons are naturally aligned with each other
30
Q

Magnetic Resonance Imaging

A
  • Humans are composed of billions of protons within H20
  • If placed within a big magnetic field (MRI), protons will align themselves along magnetic field or against the magnetic field
    o Net Magnetism
    o Slight magnetisation of patient
  • Burst of electromagnetic waves causes them to flip 90 degrees
  • Once the electromagnetic waves are tuned off, protons flip back to original position
  • Transmission of radio waves can then be measured
  • Protons relax at different rate within different tissues
  • Allows us to construct an image
31
Q

Electric Current

A

o Flow of electrons around a circuit

o Free electrons in the conducting metal wire

32
Q

Potential Difference

A

o Applying a potential difference across the circuit causes the electrons to flow
o Electrons will then flow from the negative ‘electrode’ to the positive ‘electrode’

33
Q

Units

A

o Ampere (1A = flow of 1 Coulomb (C) of charge per second

34
Q

Electrical Conductors

A

Have an abundance of loosely bound electrons in their outer shell
 E.g., copper and silver

35
Q

Electrical Insulators

A

Electrons strongly bound to atom

 E.g., plastics and rubber

36
Q

Electrical Power

A

Is determined by the current and voltage on an electrical circuit
o P = IV

  • Units: kilowatts (kW)
  • Direct Current = electrons will flow in one direction (fixed voltage)
  • Alternating Current = electrons flow in both directions (alternating voltage)

Radiation Physics (9 decks)

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