Week 4 - Work, Energy and Power Flashcards
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
Force
o Ability to move a stationary body or change the speed of a moving body
Newtons Second Law
F = ma
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
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
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
Conservation of Energy
Energy cannot be created or destroyed; it can only be converted from one form to another
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
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
Power
- The rate at which energy is used
- Units: Joules per second or the Watt
- 1 J/s = 1W
Heat
- The flow of energy from a higher temperature object to a lower temperature because of the difference in temperatures
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
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
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
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)
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
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
Waves
- Is a travelling disturbance
- Carries energy from place to place
Longitudinal Waves
- Sound Waves
- Compressions propagates along the sling
- Any point at the spring is just moving back and forth
Transverse Waves
- Electromagnetic Waves
- Disturbance is propagating along the spring
- At any point the spring is just moving up and down
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
Waves
Generally, travel in straight lines
Can be: o Reflected o Deflected (scattered) o Amplified o Absorbed
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)
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
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
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
Magnetic Field
- The behaviour of magnetic poles is similar to that of like and unlike electric charges
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
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
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
Electric Current
o Flow of electrons around a circuit
o Free electrons in the conducting metal wire
Potential Difference
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’
Units
o Ampere (1A = flow of 1 Coulomb (C) of charge per second
Electrical Conductors
Have an abundance of loosely bound electrons in their outer shell
E.g., copper and silver
Electrical Insulators
Electrons strongly bound to atom
E.g., plastics and rubber
Electrical Power
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)
