Defintions Exam Acresdited Paper 2 Flashcards
Why does centripetal force not change speed of particle exhibiting circular
and only direction is chnaged !!
Because the Force is perpendicular to the velocity vector at all times
- hence no component of force that acts in the same direction of velocity vector
- hence no WORK IS DONE ON THE PARTICLE
Faraday law
Emf induced is equal to the rate of change of magentic flux LINKAGE
What’s the defintion of flux linkage
Either flux x number of turns
Or BAN (where everything already perpendicular)
Describe the string nuclear force (3)
So better way ti say it is it acts between quarks and nucleons
- short range of 3 fm
- from 0 to 0.5fm = repulsive , 0.5 to 3 attractive
Decay constant
PROBABILITY of a nuckei decaying per unit of time
What is the piezoelectric effect?
When a pad is applied across opposite faces of a crystal, it caused it to expand or contract
(To produce ultrasound etc)
Ocr COMPTON EFFECT (2) marks
Simple
Photo
Pair production
X ray photon interacts with the electron
1) energy from x ray photon is used to EJECT electron from atom
2) but not all of its energy, it then scatters with REDUCED energy (increased wl, lower f)
Simple = photon interacts with electron but does not have enough energy to eject, thus scattered elastically
Photoelectric = photon interacts and transfers all energy to eject the electron from atom
Pair production = photon interacts with nucleus and via string nuclear force energy is used to produce electron position pair
Advantage of CAT over x ray 2 marks !!
1) provides better contrast when objects are similar coefficients of attenuation
2) can produce a 3D IMAGE A
(Can’t say 2D slice because x ray does that too)
MUST SAY CONTRAST
How does a PET scan identify fluorine 18 nuclei in a body (4 marks)
Key point for annhilation ?
1) the Florine 18 emits positrons which close to emission annihilate with electrons within the patient (1)
2) the ANNHILATION OF ONE POSITRON AND AN ELECTRON produces two gamma photons that travel in opposite directions to each other due to conservation of momentum (2)
3) there is a ring of gamma cameras around the patient that detect these gamma photons . After processing them, the computer uses the DIFFERENCE IN ARRIVAL times (delay time) to determine the point of annihilation thus nuclei! (1)
Photomultiplier job 1 mark
Light photons produce an electric signal which the computer then processes
SIGNAL/ PD / current
Advantages of using technetium 99m (2 marks)
Check last two points
It emits gamma photons by decay
- this is least ionising So relatively safe
- highly penetrative so unlike other radiation can actually be detected outside body
- small half life (so doesn’t stay high cinc for long time)
- can be used to diagnose the FUNCTION OF THE ORGAN
- or detects tumours
How to do acoustic matching ocr (2)
Talk about the skin
This is when you apply a gel that has similar impedance of skin between the transducer snd the skin
This allows for minimal reflection at the skin because now the gel skin boundary have very similar impedances
Describe a B scan and tell how can you differntiate between boundaries (3)
1) transducer emits ultrasound
2) and is moved around so that the B scan takes place in DIFFERENT DIRECTIONS
3) where reflections of ultrasounds at boundaries take place, the computer marks a dot. The more intense these reflections are, the brighter the dots. This creates a 2d image
- THE INTENSITY IF REFLECTIONS depends on the acoustic impedances of the media . (The More different they are, the higher the intensity of reflections)
Thus you can differntiate between boundaries by looking at the brightness of the spots
Describe how CAT scan and components produce 3d image (5)
1) x ray tube and detector are oppsite esch other in the gantry and they rotate together around the patient
2) a THIN FAN X-ray beam is emitted
3), and data collected by the detectors are used to produced a 2d slice of the patient
4) as the patient is moved into the gantry at CONSTSNT speed, the x ray tube and det cords SPIRAL AROUND THR WHOLE PATIENT, creating multiple 2d slices
5)all the information collected by detectors are used BY THE COMPUTER To make a 3d image
Doppler shift defintion
The apparent shift of frequency / wavelength due to the relative motion of the object wrt to the observer
How to determine speed of blood 4 marks
What must say
1) ultrassound transducer is used to emit and detected reflected ultrasound waves
2) the transducer is placed at an ANGLE to the the direction of blood flow
3) the ultrasound waves are reflected by THE BLOOD CELLS, and due to the relative motion between the blood and transducer the reflected ultrasound waves experience a change in frequency
4)Can work out using equation , WHERE THE change IN FREQUENCY PROPORTIONAL TO THE SPEED OF BLOOD
Must say angle , reflected by the blood …, change in frequency proprtional to speed of blood
2 ethical reasons to why PET scans being expensive raises issues with patient treatment
1) as expensive, doctors will have to make very difficult decisions to who gets one and how doesnt,
2) as expensive, not all hospitals will have them, so patients will either have to travel very far to get one, or not do it at all if they cant.
Why is a collimator used ? (2)
To reduce the spread of angles the emitted photons are detected at
- this increases certainty of the where the position of photon came from (directly below it)
Which INCREASES SHARPENESS OF PICTURE! (Reduces blur)
What is centre of mass
Where is centre of mass for a uniform distributed object
1) the point at which all mass can be thought to be concentrated
Point where if force is applied through the centre of mass no rotation is produced, only straight line motion
2) point at WHCIH ALL LINES OF SYMMETRY across
EQUILBKRUM 2 mark definition
No resultant force about any plane
No resultsnt moment about any point (sun of clockwise moments = sum of anti-clockwise )
Principle of moments
For a body in rotational equilibrium the sum of clockwise moments = sum of anti-clockwise moments
Couple
Further maths ?
Is when there is no resultant force, but resultant moment , due to two forces of same magnitude acting in opposite direction separated by a perpendicular distance
Couple = F x Pepe distance
Torque = moment if couple
2) moment of couple in general just means total moment, ignore the couple stuff
Fraction of object under fluid ratio?
Density of object / density of fluid
Kirchhoff 1st law WHATS conserved and why
Sum of current entering a junction = sum of current exiting
To do with conservation of charge which states chsrge cannot be destroyed , so all the charge entering a point in a unit of time = charge leaving , this is current
Explain mean drift velocity using electrons (3 marks)
- free electrons in a wire make collisons with positive metal ions in the wire giving them a RANDOM VELOCOTY
- free electrons will thus drift towards the positve end of the wire along the length
- the distance travelled per unit time along the lenght of the wire = mean drift velocity
Ohms law (2)
At cinstsnt temp
- the potential difference across a metallic conductor is prorotinsl to its current across it too
Kirchhoff second law what is this conserving
- in any circuit the sum of emf must = the sum of pd in a CLOSED LOOP
This refers to conservation of energy
What is polarisation , why can’t longitudinal waves be polarised?
Polarisation is confining transverse waves into a single plane. This is the plane that energy transfer happens in
As longtidunal waves are already confined to a single plane , which is the direction of energy transfer, they can’t be polarised
Radio wave wavelength ?
Above 10cm
Microwave wavelength
1mm to 10 cm
Visible light wavelength
700nm to 400nm
Ultraviolet ?
X ray gamma
10 nm to 400nm
X ray gamma just very very small
Why is a single slit needed for a double slit experiment (1)
Why laser don’t need
To ensure ligjt going into and thus coming out of the double slitd are coherent
Laser light is coherent anyways so don’t need
Principle of superposition DONT LACK
When two or more waves meet at a point in space, the resultant waves displacment is equal to the VECTOR sum of individual displacements or the waves meeting!
Must say displacement, not necessarily but can be amplitude
What does intensity of phtojdn relate to?
What must we say in any question to get mark (key term)
The RATE OF EMISSIONS ( not the number of photons emitted)
Energy of a photon does not link to the intensity but rather the frequency , due to ONE TO ONE INTERACTIONS
Why are electron level energies NEGATIVE ?
(3 reasons, but 1 mark)
Correvt word for highest energy level?
1) they represent the energy needed to be GAINED for the electron to leave the atom
- because potential values are negative for attractive forces
- because the HIGHEST energy level = ionisation level is defined as 0 potential , so all lower levels must be neagtive values
When removing dielectric between CAPPACITORs why does energy stored increase in terms of forced (2)
The capacitor molecules accumulate charge by induction where they touch the cappator, positve and negative, and attractive
Thus work must be done AGAINST this attraction, hence energy of CAPPACITOR increases
Explain why capacitor gains equal AND OPPOSITE CHARGE
1) electrons flow non conventional current build up at A
2) induction repels at B, so removed from B and eventually deposited at A
3) as thr same number of electrons repelled are added, the charge is equal but opposite
Why do higher intensities not cause photoelectric
Energy of photon needs to exceed meet work function
Intensity is indepent of electron energy, due to ONE TO ONE INTERACTIONS, and not storing energy
As intensity is linked to rwte of photon emission, nothing to do with energy , hence won’t cause photoelectric
What is the role of control rod/ moderator
Control rod = absorb neutrons to prevent exponential fissomm
Moderator = Slow down Neutrons , reduce their ke
Random vs spontaneous
Random =
- can’t predict when next will decay
- each habe same chance
Spontaneous
- not influenced by presence of more
- not influecned by any other fsctor
Hence random times are random
All comparisons between stationary and progressive
4
1) stationary no net energy transfer , progressive is
2) phase difference changes across one cycle of progressive, same between two nodes and antiphase on each other end of node
3) amplitude is the SAME across a progressive wave, but is max at antinides and min at nodes
4) wavelength is between two successive poitns in phase in 2 cycles , but in stationary = to 2x the length between adjacent nodes
(Remember all points have the same amplitude mean all points can reach the same amplitude )
Work function
Min energy required to REMOVE an electron
How to di velocity selector (3)
Apply mag field perpendicualr
No vertical disaplcmetn when e force = b force
Only v at E/B are selected
