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Flashcards in Week 4 - Work, Energy and Power Deck (36)
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1

X-Ray Tube

- 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

2

Force

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

3

Newtons Second Law

F = ma

4

Units of Force

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

5

Work

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

6

Energy

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

7

Conservation of Energy

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

8

Work and Energy Examples

- 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

9

The Electron Volt (eV)

- 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

10

Power

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

11

Heat

- The flow of energy from a higher temperature object to a lower temperature because of the difference in temperatures

12

Temperature

- 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

13

Transfer of Heat

- Three mechanisms for transfer of heat
o Conduction
o Convection
o Radiation

- All three are important or a problem in x-ray production

14

Conduction

- 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

15

Factors Affecting the Conduction of Heat

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

16

Convection

- 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

Radiation

- 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

Waves

- Is a travelling disturbance
- Carries energy from place to place

19

Longitudinal Waves

- Sound Waves
- Compressions propagates along the sling
- Any point at the spring is just moving back and forth

20

Transverse Waves

- Electromagnetic Waves
- Disturbance is propagating along the spring
- At any point the spring is just moving up and down

21

Properties of a Wave

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

Waves

Generally, travel in straight lines

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

23

The Principle of Linear Superposition

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

Sound Waves

- 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

Application of Sound in Medicine

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

Magnetism

- 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

Magnetic Field

- The behaviour of magnetic poles is similar to that of like and unlike electric charges

28

Magnetism

- 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

Magnetic Material

- 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

Magnetic Resonance Imaging

- 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