Defintions Exam Acresdited Paper 2

Question 1

Why does centripetal force not change speed of particle exhibiting circular

Answer 1

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

Question 2

Faraday law

Answer 2

Emf induced is equal to the rate of change of magentic flux LINKAGE

Question 3

What’s the defintion of flux linkage

Answer 3

Either flux x number of turns

Or BAN (where everything already perpendicular)

Question 4

Describe the string nuclear force (3)

Answer 4

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

Question 5

Decay constant

Answer 5

PROBABILITY of a nuckei decaying per unit of time

Question 6

What is the piezoelectric effect?

Answer 6

When a pad is applied across opposite faces of a crystal, it caused it to expand or contract

(To produce ultrasound etc)

Question 7

Ocr COMPTON EFFECT (2) marks

Simple

Photo

Pair production

Answer 7

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

Question 8

Advantage of CAT over x ray 2 marks !!

Answer 8

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

Question 9

How does a PET scan identify fluorine 18 nuclei in a body (4 marks)

Key point for annhilation ?

Answer 9

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)

Question 10

Photomultiplier job 1 mark

Answer 10

Light photons produce an electric signal which the computer then processes

SIGNAL/ PD / current

Question 11

Advantages of using technetium 99m (2 marks)
Check last two points

Answer 11

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

Question 12

How to do acoustic matching ocr (2)

Answer 12

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

Question 13

Describe a B scan and tell how can you differntiate between boundaries (3)

Answer 13

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

Question 14

Describe how CAT scan and components produce 3d image (5)

Answer 14

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

Question 15

Doppler shift defintion

Answer 15

The apparent shift of frequency / wavelength due to the relative motion of the object wrt to the observer

Question 16

How to determine speed of blood 4 marks

What must say

Answer 16

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

Question 17

2 ethical reasons to why PET scans being expensive raises issues with patient treatment

Answer 17

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.

Question 18

Why is a collimator used ? (2)

Answer 18

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)

Question 19

What is centre of mass

Where is centre of mass for a uniform distributed object

Answer 19

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

Question 20

EQUILBKRUM 2 mark definition

Answer 20

No resultant force about any plane

No resultsnt moment about any point (sun of clockwise moments = sum of anti-clockwise )

Question 21

Principle of moments

Answer 21

For a body in rotational equilibrium the sum of clockwise moments = sum of anti-clockwise moments

Question 22

Couple

Further maths ?

Answer 22

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

Question 23

Fraction of object under fluid ratio?

Answer 23

Density of object / density of fluid

Question 24

Kirchhoff 1st law WHATS conserved and why

Answer 24

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

Question 25

Explain mean drift velocity using electrons (3 marks)

Answer 25

• 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

Question 26

Ohms law (2)

Answer 26

At cinstsnt temp

• the potential difference across a metallic conductor is prorotinsl to its current across it too

Question 27

Kirchhoff second law what is this conserving

Answer 27

• in any circuit the sum of emf must = the sum of pd in a CLOSED LOOP

This refers to conservation of energy

Question 28

What is polarisation , why can’t longitudinal waves be polarised?

Answer 28

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

Question 29

Radio wave wavelength ?

Answer 29

Above 10cm

Question 30

Microwave wavelength

Answer 30

1mm to 10 cm

Question 31

Visible light wavelength

Answer 31

700nm to 400nm

Question 32

Ultraviolet ?

X ray gamma

Answer 32

10 nm to 400nm

X ray gamma just very very small

Question 33

Why is a single slit needed for a double slit experiment (1)

Why laser don’t need

Answer 33

To ensure ligjt going into and thus coming out of the double slitd are coherent

Laser light is coherent anyways so don’t need

Question 34

Principle of superposition DONT LACK

Answer 34

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

Question 35

What does intensity of phtojdn relate to?

What must we say in any question to get mark (key term)

Answer 35

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

Question 36

Why are electron level energies NEGATIVE ?

(3 reasons, but 1 mark)
Correvt word for highest energy level?

Answer 36

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

Question 37

When removing dielectric between CAPPACITORs why does energy stored increase in terms of forced (2)

Answer 37

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

Question 38

Explain why capacitor gains equal AND OPPOSITE CHARGE

Answer 38

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

Question 39

Why do higher intensities not cause photoelectric

Answer 39

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

Question 40

What is the role of control rod/ moderator

Answer 40

Control rod = absorb neutrons to prevent exponential fissomm

Moderator = Slow down Neutrons , reduce their ke

Question 41

Random vs spontaneous

Answer 41

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

Question 42

All comparisons between stationary and progressive
4

Answer 42

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 )

Question 43

Work function

Answer 43

Min energy required to REMOVE an electron

Question 44

How to di velocity selector (3)

Answer 44

Apply mag field perpendicualr

No vertical disaplcmetn when e force = b force

Only v at E/B are selected