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 PenetrationIonising Ability

Alpha 4 2 low high

Beta (β-) 1 / 2000 -1 medium medium

Positron (β+) 1 / 2000 1 medium medium

Neutron 1 0 highnot

directlyionising

Gamma 0 0 high low

N - Z plot for stable isotopes

N = Z

β-

β+

α

β-n-1p+1

β+n+1p-1

αn-2p-2

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' Charge Mass

'UP' + 2/3 e 1/3 u'DOWN' - 1/3 e 1/3 u

Particle Charge QuarksProton +1 UUD

Neutron 0 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 TubesCathode(filament)

AcceleratingAnode

Steeringplates

AcceleratingVoltage

HeatingCurrent

ThermionicEmission

ElectronAccelerated

Steered by magneticor electric field

Electronstrikes screen

Kinetic Energyconverted toLight Energy

Vacuum

Increasing heating currentIncreases numbers electrons boiled

Increasing accelerating voltageIncreases 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 = ( n x e ) / tYou’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 mediumnot 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 takenP = 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

U +1

n →92

Kr +141

Ba + 31

n92 0 36 56 0

Collisions• Energy conservation

Total Energy Before = Total Energy After

½ m1 v12 + ½ m2 v2

2 = 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