Pastpaper Questions Flashcards
Why would you be unlikely to use carbon dating of a sample is more than 60,000 years old
Half-life of carbon-14 is 5740 years
Too few C-14 atoms
Define the Avogadro constant
The number of particles in 1 mole of a substance
Why would two different elements have different mean square speeds at the same temperature?
- mean kinetic energy is equal at the same temperature
- mass difference
Define the specific latent heat of vaporisation of water
- the energy required for 1kg of water to change to steam
- when at it’s boiling point temperature
A nuclear reactor core is contained in a steel vessel surrounded by concrete.
State and explain the purpose of the concrete other than it’s structural function
- for a a shield to reduce intensity of radiation
- for protection from neutron radiation
How would you store radioactive waste that has these properties:
- half life = 20 days
- emits gamma and beta minus radiation
- needs significant screening (lead)
- highly active therefore produces heat
- lasts for a short time ~ 80 days
How would you store radioactive waste that has these properties:
- half life = 20 years
- emits alpha particles
- easy to screen, metal container
- active for a very long time
- problems over container fatigue
How is radioactive waste treating at a reactor site and how could it be stored safely?
- initially place in cooling pond
- water acts as a shield and cools down temperature
- (long term) after 1-3 years will be less active and can be stored underground in steel containers
Why would you be unlikely to use carbon dating of a sample is less than 200 years
Half-life of carbon-14 is 5740 years
Hard to measure such a small drop in activity
In a thermal nuclear reactor, one fission reaction typically releases 2 or 3 neutrons.
Describe and explain how a constant rate of fission is maintained in a reactor by considering what events or sequence of events may happen to the released neutrons (6 mark)
- neutron is slowed down by moderator
- about 50 collisions to reach thermal speeds
- absorbed by U(235) to cause fission
- one neutron released goes on to cause a further fission is the critical condition
- reaction may leave the reactor core without further interaction
- neutron could be absorbed by control rod
Uranium is an alpha emitter. Explain why spent fuel rods present a greater radiation hazard than unused uranium fuel rods
- easy to stay out the range of the a source
- most fission fragments are more radioactive
- initially most likely to be beta emitters (neutron rich)
- ionising radiation damages body tissue
What is meant by the decay constant?
Probability of decay per unit time
Give two reasons why it is difficult to obtain a reliable age of an ancient boat using carbon dating
- possible contamination
- boat may have been made from the wood a long time after the tree was cut down
Make I the subject of this equation
50dB = 10log(I/Io)
10^5 x Io
A sound has a dBA value is higher than the dB value, explain why?
- dBA scale is frequency dependant to match the response of the ear
- ear is more sensitive for a range of frequencies between 1kHz and 6kHz
What is the rest action potential for a nerve cell?
-70mV
Name the processes of the cycle of a nerve cells and state values of action potential
- depolarisation (-70mV to 0mV)
- reverse polarisation (0mV to +30mV)
- repolarisation (+30mV to -70mV)
Explain the process of a nerve cells
- at resting potential, high concentration of K+ ions inside and
Na+ outside - when stimulated, membrane becomes permeable to Na+ ions entering the core increasing membrane potential to 0mV
- reverse polarisation to +30mV
- at +30mV membrane becomes impermeable to Na+ ions and permeable to K+ ions leaving the core
- reducing membrane potential
to -70mv - much slower process to return axon to original K+/Na+ ratio
Outline the basic principles of a magnetic resonance (MR) scanner to scan a patient’s brain
- head placed in a strong magnetic field
- supplied radio pulse excite H nuclei
- when H nuclei de-excite they emit photons
- these signals are detected and passed to computer
State and explain two advantages of using an MR scanner to scan a patient’s brain compared with a CT scanner
- MR non-ionising radiation, ionising radiation in CT more danger to living cells
- MR gives real time image, CT scan needs to rotate to produce final image
Give a range of alpha decay in air
0.03-0.07m
Give the range of B decay in air
0.2-3m
State the main interaction when an alpha particle is scattered by a gold nucleus
Repulsion between the alpha particles and the nuclei
Would the scattering distribution still be the same if the gold foil was replaced with isotopes of gold?
Yes because the alpha particles react with a nucleus that has the same number of protons and therefore charge
Explain how energy is released when some nuclides undergo fission and when other undergo fusion
(using the binding energy per nucleon vs nucleon number)
- energy is released when binding energy per nucleon is increased
- in fission a large nucleus splits, in fusion small nuclei join
- the most stable nuclei at the peak
- fusion occurs to the left of the peak and fission to the right
By considering the motion of the molecules explain how a had exerts a pressure and why the volume of the container must change if the pressure is to remain constant as the temperature increases
- molecules are rapid random motion
- molecules change their momentum
- F = rate of change of momentum
- Newton’s 3rd Law
- P=F/A
- Crms proportional to T
- as temp increases so does change of momentum
- compensated for by longer time between collisions as the temperature increases
- as the volume increases the surface area increases which reduces the pressure
Explain what is meant by persistence of vision and state a practical situation where it is important
- image remains after stimulus is removed
- e.g optical illusions
Define the threshold of hearing, Io
- minimum intensity heard by a normal ear
- at 1kHz
How would you reduce the contact resistance in an ECG trace?
- use sandpaper to remove hair and some dead skin
- apply conducting gel
- securely attach more than one electrode
How would you remove any unwanted signals in an ECG trace?
- electrodes should be none reactive
- patient needs to remain relaxed and still
- shielded leads
Give 3 properties of an amplifier used in an ECG trace
- has large input impedance
- frequency response should be even across the frequency range
- it’s voltage gain must be of the order of 1000 over a frequency range up to 20Hz
In an ECG trace, name the electrical events which cause the 3 peaks
- depolarisation of atria
- depolarisation of ventricles (repolarisation of atria)
- repolarisation of ventricles
Explain the process which gives vertical spikes in an X-ray intensity vs photon energy graph
- electrons strike anode and excite the target atoms
- excited electrons fall to inner energy level
- fixed energy gaps produce fixed energy photons
Explain how the intensifying screens in the film cassette achieve their purpose and state their benefit to the patient
- convert X-ray photons to light photons
- light photons expose film in correct place die to closeness of the screens to the film
- reducing the intensity of radiation to the patient (less contact time)
Explain what is meant by a chain reaction, naming the materials and particles involved. (with uranium)
- U(235) captures a neutron and splits into two smaller nuclei, releasing more neutrons
- at least one of these neutrons go on to cause further splitting of U(235)
Explain the purpose of a moderator in a thermal nuclear reactor
- slows down neutrons
- so neutrons are efficiently absorbed by uranium fuel
Explain why the shielding becomes radioactive
When shielding around the core is used to protect from all types of radiation
- neutrons are absorbed
- converts atoms of shielding into unstable isotopes
Discuss one experiment that can be performed using a gas which would enable you to explain absolute zero and determine it’s value.
Including:
- what must remain constant
- what needs to be measured
- how to use results to find absolute zero
- justify why the value obtained is absolute zero
- constant mass of gas and volume of gas
- record volume for a range of temperature
- how temperature is maintained
- graph of volume against temperature is linear. Reference to pressure law.
- extrapolate results to lower temperatures which has 0 volume
- absolute zero is obtained using any gas (ideal)
Give 3 sources of background radiation
- rocks
- cosmic rays
- nuclear waste
How would you work out the ratio of no. of Y photons incident on detector vs no. Y photons produced by source ?
Ratio = Area/(SA of sphere between the source and detector)
How would you work out the activity of a source if you had a count rate?
Divide the count rate by (the ratio of the area of the detector/ SA of sphere surrounding source to detector)
How would you work out a new count rate if the detector was moved x meters away?
Use inverse square law to work out k, then use k with new distance to work out new count rate
Describe a graph of nuclear radius R vs nucleon number A
Initial steep gradient, then gradient slowly decreases
Name the basic principle of the Rutherford scattering experiment, an equation relating to it and limits in accuracy of the experiment in regards to measuring the radius of a nucleus
- alpha scattering involves electrostatic repulsion
- Ea = Qq/4pixepsilon0xr
- only measures the least distance of approach, not the radius
- alpha particles have a finite size which must be taken into account
- need a thin sample of gold foil to prevent multiple scattering
Electron diffraction experiments, basic principle, limits in accuracy for measuring the radius of a nucleus and an equation
- electron diffraction treats the electrons as a wave, having de Broglie wavelength
- lamda = h/mv
- electrons need to have high speed
- have a small wavelength as it determines resolution
- wavelength needs to be of the same order as the nuclear diameter for significant diffraction
Compare advantages and disadvantage between using Rutherford scattering vs electron diffraction for estimating the radius of a nucleus
- a particle measurements disturbed by nuclear recoil
- electrons are not subject to the strong nuclear force
- electrons are leptons
- a particles are only scattered by protons and not all nucleons that make up the nucleus
State the changes in a normal eye, when focusing on a distant object to a near object, in bright light
- ciliary muscles contract
- producing a lens of greater power
State the changes in a normal eye, when you view an object in very dim light to the same object in bright light
- pupils become smaller
- comes turn on and rods become inactive
State the effect of astigmatism on the image produced by the defective eye
Image is focused in a given plane and out of focus in perpendicular plane
What would be the dBA of a sound with the power of 60dB in a frequency of 1kHz
60dBA as 1kHz is the reference frequency at the threshold of hearing
How would you calculate the dB on a sound meter that is 5m from a drill that produces sound of power 2.0W?
- work out I, which would be power/surface area of sphere between drill and meter
- then use intensity level = 10log(I/Io)
Explain the method of obtaining an image using ultrasound
- US reflected at interface between two different acoustic impedances
- each transducer emits pulse in turn and receives the echoes from the interfaces directly in line with it
- each echo displayed as a bright spot on a screen
- brightness depends on intensity of echo
- y position determined by the time taken from transmission to the echo
- x position determined by position of transducer
- images are produced at about 25 per second and thus appear as real time moving image
Give practical considerations of ultrasound
- probe has a line of transducers
- high frequency ac pulse applied to each transducer in turn
- each transducer has piezoelectric crystal to generate ultrasound
- use of gel between probe and skin to eliminate air
- transducer as receiver
Explain why the pulses of ultrasound used in medical imaging must be short
- allows the transducer to damped before the echo returns to allow the transducer to act as a receiver
- distance travelled short
- emitted pulse must cease before echo arrives so that there is no interference
Describe briefly how a CT scanner produces an image
- narrow beam of X-rays
- X-ray generator rotated around patient
- detectors arranged around outside of the path
- detector opposite generator registers transmitted intensity
- detectors connected to computer which produces cross sectional image
Define intensity of sound
- intensity is the power per unit area
- at normal incidence
How does a piezoelectric crystal work?
- alternating p.d applied across crystal
- causes crystal to expand and contract
- creating pressure waves in the crystal
- f of alternating p.d equal to that of the crystal
- above 20kH- short application of A.C to produce short pulse
- backing material to damp and stop pulse
Explain why a coupling gel is needed and state the property of the gel that ensures a good quality image
- gel is between the probe and the skin to exclude air
- gel should have acoustic impedance equal to that of the skin
Define atomic mass unit
1/12 the mass of an atom of C(12)
Explain why nuclei in a star have to be at a high temperature for fusion to take place
- nuclei need to be close together for fusion to take place
- but the electrostatic force is repulsive
- the nuclei have to high kinetic energy to overcome repulsion
Describe the changes made inside a nuclear reactor to reduce it’s power output and explain the process involved
- insert control rods into the nuclear core
- which will absorb neutrons
State the main source of the highly radioactive waste from a nuclear reactor
Spent fuel rods
Describe and explain the nature of the radiation that may be emitted from an excited nucleus of the moderator
- gamma is emitted
- as the nucleus de-excited down discrete energy levels
How would tell from a ray diagram if someone is suffering from myopia?
The image falls short of the retina
How would you tell from a ray diagram if a person had hypermyopia
The image would form outside/beyond of the retina
The unaided near point of the defective eye is 0.15m, with the lens of focal length of -0.75, what is the distance of the aided near point from the eye?
1/f = 1/v + 1/u 1/-0.75 = 1/-0.15 + 1/v 1/v = 1/-0.75 + 1/.15 v= 0.19
In a fluoroscopic image intensifier, what is the first fluorescent screen used for?
Converts X-ray photons to light photons
In a fluoroscopic image intensifier, what is the photocathode used for?
Uses energy of each light photon to release electrons from the surface of the cathode
In a fluoroscopic image intensifier, what are anodes used for?
- to accelerate the electrons
- focuses electron beams
In a fluoroscopic image intensifier, the last fluorescent screen used for?
Converts energy of electrons to many light photons
Why would a patient be asked to swallow barium sulfate before an X-ray?
- Barium has a high density which provides a better contrast between organs and soft tissue
- usually organs have poor contrast with soft tissue
How to work out the mass of 1 atom of an element?
Mass of 1 mole/ Avogadro’s constant
What does this image distance represent?
Negative figure, in hypermyopia
Unaided near point
Why is a logarithmic scale used for dB scale?
- response of ear is logarithmic
- allows a large range of intensities to be put in a suitable scale
Describe how hearing loss varies with frequency over the audible range.
- due to excessive noise
- loss increases up to 4kHz
- then decreases after this frequency
Points to talk about explain the process of the heart pumping blood
- SAN and AVN
- valves/delay(0.1s)
- contraction and relaxation
- type of blood in each side (Right deoxygenated blood from body to lungs, left is opposite)
- depolarisation and reverse polarisation, from movement of Na+ into axon causes contraction
- -70mv to +30mV
- repolarisation of K+ out of axon causes relaxation
State the use of lead sheets in X-rays
Protects areas of body not being scanned
State the use of the aluminium filter in X-rays
Filters out most low energy photons(high energy photons can pass through)
State two reasons for using ultrasound over X-rays for scanning an unborn fetus
- Ultrasound is not ionising, won’t harm the fetus
- ultrasound gives good images of soft tissue
Give the ranges of the cones
Blue: 375 to 500
Green: 425 to 675
Red 475 to 725
Describe how rods and cones are distributed over the surface of the retina
- only cones at fovea
- as you get further away from fovea, fewer cones and more rods
Why is an image in better resolution in bright light a opposed to dim light?
- rods used in dim light, cones in bright
- size of cones are smaller than rods
State one condition that must be satisfied for two objects to be resolved as individual images on the retina
The two images need to fall on receptors with a least - I stimulated receptor in between them
Units of intensity of sound?
Wm^-2
