Week 4 - Work, Energy and Power

Question 1

X-Ray Tube

Answer 1

• 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

Question 2

Force

Answer 2

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

Question 3

Newtons Second Law

Answer 3

F = ma

Question 4

Units of Force

Answer 4

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

Question 5

Work

Answer 5

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

Question 6

Energy

Answer 6

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

Question 7

Conservation of Energy

Answer 7

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

Question 8

Work and Energy Examples

Answer 8

• 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

Question 9

The Electron Volt (eV)

Answer 9

• 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

Question 10

Power

Answer 10

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

Question 11

Heat

Answer 11

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

Question 12

Temperature

Answer 12

• 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

Question 13

Transfer of Heat

Answer 13

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

Question 14

Conduction

Answer 14

• 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

Question 15

Factors Affecting the Conduction of Heat

Answer 15

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

Question 16

Convection

Answer 16

• 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

Question 17

Radiation

Answer 17

• 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

Question 18

Waves

Answer 18

• Is a travelling disturbance

- Carries energy from place to place

Question 19

Longitudinal Waves

Answer 19

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

Question 20

Transverse Waves

Answer 20

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

Question 21

Properties of a Wave

Answer 21

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

Question 22

Waves

Answer 22

Generally, travel in straight lines

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

Question 23

The Principle of Linear Superposition

Answer 23

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)

Question 24

Sound Waves

Answer 24

• 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

Question 25

Application of Sound in Medicine

Answer 25

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

Question 26

Magnetism

Answer 26

- 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

Question 27

Magnetic Field

Answer 27

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

Question 28

Magnetism

Answer 28

- 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

Question 29

Magnetic Material

Answer 29

- 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

Question 30

Magnetic Resonance Imaging

Answer 30

- 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

Question 31

Electric Current

Answer 31

o Flow of electrons around a circuit | o Free electrons in the conducting metal wire

Question 32

Potential Difference

Answer 32

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’

Question 33

Units

Answer 33

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

Question 34

Electrical Conductors

Answer 34

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

Question 35

Electrical Insulators

Answer 35

Electrons strongly bound to atom |  E.g., plastics and rubber

Question 36

Electrical Power

Answer 36

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)