Transcript P3 Exam Preperation

P3 Exam Preparation
Gases
• Absolute zero = -273 ºC, 0 ºC = 273 K
• Increasing the temperature of a gas
increases the speed of its particles
• The average kinetic energy of its particles
is proportional to the temperature of a gas
in Kelvin
Gases
• Gas pressure is caused by particles
colliding with the container wall
• The faster the particles (the higher the
temperature) the greater the pressure
• In all situations:
P1V1 / T1 = P2V2 / T2
Atoms and nuclei
• Nuclei contain protons and neutrons
• Neutrons are difficult to detect because
the have no charge
• As a result of β- or β+ decay nuclei often
undergo rearrangement with a loss of
energy as gamma radiation
• In nuclear equations:
– Mass is conserved (top number)
– Charge is conserved (bottom number)
Properties of Radation
Radiation
Mass
Charge
Penetration
Ionising
Ability
Alpha
4
2
low
high
Beta (β-)
1 / 2000
-1
medium
medium
Positron (β+)
1 / 2000
1
medium
medium
Neutron
1
0
high
not
directly
ionising
Gamma
0
0
high
low
N - Z plot for stable isotopes
α
βN=Z
βn-1
p+1
β+
α
n-2
p-2
β+
n+1
p-1
Fundamental and other Particles
• Fundamental particles are not made up of
other, smaller particles eg:
– Electron
– Positron (anti-electron)
– Quark
– Netrinos
– Muons
• A positron has the same mass as an
electron and all other properties are
opposite ie charge = +1
• Scientists are creating fundamental
particles, such as anti-matter, in particles
accelerators which smash particles into
each other providing enough energy for the
fundamental particles can exist on their own
• These project are normally international
collaborative projects due to the cost
• The proton and neutron are not
fundamental particles because they are
made up of quarks
Quarks
'Flavour'
'UP'
'DOWN'
Charge
+ 2/3 e
- 1/3 e
Mass
1/3 u
1/3 u
Particle
Proton
Neutron
Charge
+1
0
Quarks
UUD
DUD
Beta Decay and Quarks
• β- decay involves a down quark changing into
an up quark
(one neutron becomes a proton and an electron)
• β+ decay involves one up quark changing into
a down quark
(a proton becomes a neutron and a positron)
Electrons and Electron Beams
• Thermionic emission is when charged
particles are emitted ‘boiled off’ a filament
due to thermal energy
• Uses of electron beams include:
– TV picture tubes
– computer monitors
– oscilloscopes
– the production of X-rays
Cathode Ray Tubes
Cathode
(filament)
Accelerating
Anode
Vacuum
Electron
strikes screen
Electron
Accelerated
Thermionic
Emission
Steered by magnetic
or electric field
Kinetic Energy
converted to
Light Energy
Steering
plates
Heating
Current
Accelerating
Voltage
Increasing heating current
Increases numbers electrons boiled
Increasing accelerating voltage
Increases the KE of the electrons
Both increase the brightness of the screen
Cathode Ray Tubes
• kinetic energy = electronic charge ×
accelerating voltage
KE = e × V
• a beam of electrons is equivalent to an
electric current
I=(nxe)/t
You’ll be given: e = 1.6 x 10-19
Beam Deflection
• An electron beam, or a stream of charged
particles (for example ink drops), can be
deflected by the electric field between
parallel charged metal plates
• The amount of deflection increases when:
– Mass of particle is decreased
– The time in the field is increased
• Larger plates
• Slow particle
Methods of ‘seeing’ inside the body
Methods of ‘seeing’ inside the body
• Refraction of a wave, is the change in direction
(or bending) caused by the change in speed of the
wave
• This usually due to a change in density of medium
TIR – Fibre Optics
Radiation
• Radiation is the spreading out of energy
– Light (EM Spectrum)
– Radioactive radiation (Alpha & Beta particles)
– Sound
Radiation
• Medical applications of radiation:
– Reflection
• X-rays – bones may reflect the x-rays
• Ultrasound scan (echocardiogram)
– Total internal reflection
• Endoscopes (Keyhole surgery, colonoscopy)
– Absorption
• Pulse oximetry
• X-rays – bones may absorb the x-rays
• Radiotherapy
Remember the light is absorbed by the medium
not the other way round
Pulse Oximeter
Energy and the body
• Work done is equal to energy transferred
• work done = force × distance
(moved in the direction of the force)
W=F×s
• power = work done / time taken
P=W/t
• basal metabolic rate (BMR) is the minimum
amount of energy required to stay alive
Electricity in the body
• frequency = 1 / time period
f=1/T
• Action potentials can be measured with an
Electrocardiogram (ECG) to monitor heart
action
ECG Probes
• The probes are able to
measure the potential
differences between the
heart and the rest of the
body
• This potential difference
is known as the action
potential and makes the
heart muscles contract
Normal ECG
Contraction
of the atria
Contraction
of the
ventricles
Relaxation of
the ventricles
Heart Problems
Bradycardia = low heart rate
Tachycardia = high heart rate
Arrhythmia = uneven heart rate
Positron Emission
Tomography (PET)
•
•
•
•
Radioactive tracer is injected into blood
Tracer emits positron
Positron annihilates an electron
Emits a pair of gamma rays in opposite
directions
• Gamma rays are detected by an array of
gamma cameras
• 3D map of body is created showing where
the tracer accumulated
• Ionising radiation may cause:
– Tissue damage
– Mutations
• The larger the dose of radiation the bigger
the risk
• Risk minimised by minimising the:
– Intensity
– Duration
of exposure
• Tumours irradiated by radiation are
affected more than normal cells
• Palliative care is the treatment of the
symptoms when the cause can not be
cured
• Social and ethical issues of (new/newer)
techniques in medical physics:
– Cost of treatment
– Geographical availability
– Potential risks
Physics theory in medical care
• intensity = power of incident radiation/area
I = P/A
• Double the distance => Quarter the Intensity
I α 1 / r2
r = distance from source
• Intensity depends on the nature of the
medium the radiation is travelling through:
Higher density => Higher absorption
=> Lower Intensity of radiation
Physics theory in medical care
• balancing nuclear equations that use thermal
neutrons
• In nuclear equations:
– Mass is conserved (top number)
– Charge is conserved (bottom number)
235
92
U +
1
0
n →
92
36
Kr +
141
56
Ba + 3
1
0
n
Collisions
• Energy conservation
Total Energy Before = Total Energy After
½ m1 v12 + ½ m2 v22 = ½ m1 v1’2 + ½ m2 v2’2
+ Sound & heat Energy etc
• Momentum conservation
Total Momentum Before = Total Momentum After
m1 v1 + m2 v2 = m1 v1’ + m2 v2’
Physics theory in medical care
• The bombardment of certain stable
elements with proton radiation can result in
making them into radioactive isotopes that
usually emit positrons
• The production of gamma rays by
annihilation of electron and positron as the
rest energies (E = mc2) are converted from
matter into (lots of) pure energy – gamma
rays
Physics theory in medical care
• Annihilation of electron and positron to form
gamma rays is an example of momentum
and mass energy conservation:
– Energy of gamma ray
= rest energies of particles + KE of particles
– Pairs of gamma rays are given out in opposite
direction to maintain momentum