Paper 2 Exploring Flashcards
describe how to accurately determine the threshold voltage of an LED in a brightly-lit laboratory
- Circuit diagram with LED, voltmeter and ammeter connected to a supply with correct polarity and some means of adjusting the p.d.
- Increase the p.d. until the LED is just lit / current is shown in the circuit
-To observe the light from LED use a tube / turn lights off
why does a p.d decrease across thermistor
- (Current causes) increase in temperature of thermistor
- Resistance of thermistor decreases (and hence V
decreases)
or - Current in the circuit increases, p.d. across resistor
increases (and hence V decreases)
Describe and explain how you could demonstrate the diffraction of sound waves in the laboratory.
- Loudspeaker, microphone/ear and slit
- Sound spreads from the slit AW
- Size of slit comparable to the wavelength (of sound)
What does coherent mean?
Constant phase difference
State the principle of superposition of waves.
resultant displacement is equal to the sum of the individual
displacements
Explain the maxima and minima in the variation of the intensity.
Maxima is when constructive (interference) occurs / phase
difference is zero / path difference = nλ
Minima is when destructive (interference) occurs / phase
difference is 180(°) or π( rad) / path difference = (n + ½)λ at
minima
Explain why maximum KE of electrons decreases as wavelength increases, during photoelectric effect
Photon mentioned / one-to-one interaction (between electron and photon)
(Maximum KE of electrons decreases as wavelength increases because) KE(max) =ℎ𝑐/𝜆 − 𝜙
(When λ < λ0) energy (of photon) > work function /
f > threshold frequency and electrons emitted
Define refractive index of a material
(refraction index) = speed of light in vacuum ÷ speed of
light in materia
explain how x-rays are produced in an x-ray tube
- a current is passed through the cathode, so it becomes hot, and electrons gain KE to escape
- anode produces an electric field to accelerate electrons towards the target
- electrons collide with orbital electrons then decelerate, and when they de-excite 1% of photons are desired x-rays
attenutaion
Decrease in intensity of radiation through material in original direction due to interaction with matter
explain 4 attenuation mechanisms
Scattering - x-rays are absorbed by electron and reemitted in different direction with same energy
Compton effect - high energy x-rays are absorbed and remitted as low-energy (or longer wavelength) x-rays and electron is removed
Photoelectric effect - x rays are absorbed by electron and gain enough KE to escape
Pair production - high energy x rays produce positron and electron (E=2mc^2)
attenuation coeffecient
the probablitlity that an xray photon will be attenuated in the next unit distance
decay constant
the probability that a nuclei wil deacy per unit time
explain the use of a contrast media
When doing CAT scan, organs and tissue have very similar attenuation coefficient, so contrast media is used as it has a high attenuation coeffcient.
e.g. barium for digestive organs and iodine for blood
explain how CAT scan produces 3d images
- x-ray tube rotates around patient’s body
- Thin fan-shaped beam of x-rays is emitted
- images of slices are taken
- x-ray tube moves along patient
- computer uses multiple slices to produce 3D image
Explain how PET scan produces images
- There is a ring of gamma detectors around the body
- Fluorine18 emits positrons which annihilate with an electron to form two gamma photons moving in opposite directions
- gamma detectors pick up photons, and the delay time is used to determine the location of annihilation
- computer uses this information to form an image
describe the features and functions of gamma camera
- Collimator makes sure all incident photons are parallel to the axes, it has thin long tubes produce a sharp image
- scintillator absorbs gamma photons and converts it to thousands of visible light photons
- photomultiplier detects these photons and produces an electric signal
- computer uses this information to form an image
Describe piezoelectric effect and how transducer can use this to emit and detect ultrasound
- emf applied to material causing it to contract or contracting material induces emf
- ac emf at the resonance frequency of piezo crystal causes it to oscillate at large amplitude
- sound waves passing through induces an ac emf which can be measured
describe Ultrasound A scan
Pulse of ultrasound is emitted into patient.
reflections occur at front and back of (eye/bone…)
the detected signal is displayed on an oscilloscope .
Thickness is determined by time difference between pluses and speed of ultrasound
describe ultrasound B scan
Scans taken in different directions. Reflected signals from differnt boundaries are measured for their intensity, which is turned into an image by a computer. Intensity is dependent on the difference in acoustic impedence
Explain what happens to resistance when connected to a higher p.d.
- the resistivity/resistance (of a metal) increases with temperature
- R is greater when hot(ter)
- at 6V/low I little heating effect or at 230 V/high I large heating effect
When a clean metal surface in a vacuum is irradiated with ultraviolet radiation of a certain
frequency, electrons are emitted from the metal.
(a) (i) Explain why the kinetic energy of the emitted electrons has a maximum value (2)
hf is energy available/received or same energy from photons (1)
energy required to remove the electron varies (hence kinetic
energy of electrons will vary) (1)
When light of a certain frequency is shone on a particular metal surface, electrons are
emitted with a range of kinetic energies.
(a) Explain
* in terms of photons why electrons are released from the metal surface, and
* why the kinetic energy of the emitted electrons varies upto a maximum value
energy is needed to remove an electron from the surface
work function φ (of the metal) is the minimum energy needed by an electron to escape from the surface
light consists of photons , each of energy E = hf
one photon is absorbed by one electron
an electron can escape (from the surface) if hf > φ
kinetic energy of an emitted electron cannot be greater than hf – φ
an electron below the surface needs to do work/uses energy to reach the surface
kinetic energy of such an electron will be less than hf – φ
The photoelectric effect suggests that electromagnetic waves can exhibit particle-like
behaviour. Explain what is meant by threshold frequency and why the existence of a
threshold frequency supports the particle nature of electromagnetic waves. (6)
- threshold frequency minimum frequency for emission of electrons
- if frequency below the threshold frequency, no emission
even if intensity increased - because the energy of the photon is less than the work function
- wave theory can not explain this as states that the energy of wave
increases with intensity - light must travel as photons
- as photons have energy that depends on frequency
- if frequency the is above the threshold, the photons enough energy to emit electrons
- also there is no time delay when the photon emitter is turned on and when the electrons are emitted suggesting a 1-1 relationship between photon and electron
when light is shone on a surface the photoelectric effect takes place. Explain the effect on the emitted electrons when the frequency of the light is increased (2)
the (maximum) kinetic energy/speed/velocity/momentum
of released electrons increases (1)
this is because increasing the frequency of the photons increases
their energy (1)
Explain the effect on the emitted electrons when the intensity of the light is increased (2)
the number of electrons emitted (per second) increases (1)
because there are now more photons striking the metal surface (1)
Explain why the emitted electrons have a range of kinetic energies up to a maximum
value. (4)
energy of photons constant (1)
one to one interaction between photon and electron (1)
Max KE = photon energy – work function (1)
more energy required to remove deeper electrons (1)
How can ultrasound measure the speed of blood
Transducer used to produce ultrasound
Ultrasound reflected by moving blood cells
Transducer is placed at different directions/angles
Frequency is altered
Speed can be measured using delta f/f = 2vcos theta/c
One major advantage is that it’s non invasive
define magnetic flux
magnetic flux density multiplied by the area normal to the field
explain why the force acting on the electron doesn’t change the speed
the force acts perpendicular to the direction of travel therefore no work is done
define emf
energy transferred from one form of energy to electrical energy per unit charge
define p.d.
energy transferred from electrical energy to other forms per unit charge
explain how an emf can be induced in secondary coil
- connect the primary coil to alternating voltage supply
- changing magnetic flux is produced in primary coil
- iron core links primary coil and secondary
- rate of change of flux linkage induced an emf/ changing magnetic flux induces an emf in secondary coil
faraday law of induction
The induced emf is proportional to the rate of change of flux linkage
lenz law
induced e.m.f.is set up in a direction to produce effects that oppose the change causing it
advantages of technetium99 in the brain
it emits gamma
least ionising
can be picked up outside of brain
can identify tumour
short half life
what is acoustic impendence matching and how it can be achieved
- coupling gel has same impedance as skin
- this reduced the reflection at boundaries
similarities and differences between stationary and progressive waves
Similarities:
can both be longitudinal and transverse
both use v=fλ
Differences:
one stores energy the other transfers
one has nodes and antinodes the other doesn’t
observation of photon emission
1:1 interaction with photon and electrons
photon is a packet of energy
Energy of photon is independent of intensity
intensity affect the rate of emission
E=hf or E=hc/λ
emission occurs when light incident is above the work function/ threshold frequency
differences and similarities between fusion and fission
fission is the breaking down of nuclei into smaller nuclei
fusion is the combining of nuclei to form heavier nuclei
Energy is produced in both reactions
More energy produced (per reaction) in fission
The (total) binding energy of ‘products’ is greater
In fusion, nuclei repel (each other)
Fusion requires high temperatures/ high KE
Fission reactions are triggered by neutrons
Chain reaction possible in fission
How can a uniform magnetic field be added to an electric field to select specific speed electrons
Apply a magnetic field at right angles to electric field
electric force = magnetic force
No resultant vertical force, so only beta-particles with a specific speed will travel horizontally
equations: Eq=Bqv; v=E/B
What is meant by the photoelectric effect
Emission of electrons form the surface of a metal when em waves are incident in the metal
explainwhy photoelectric cant be explain with wave-model but can with photon model
-wave model cannot explain threshold frequency
-photon model shows a 1:1 interaction
-energy of photon must be larger than work function to cause emission
-Ek is independent of intensity
describe how time constant can be determined experimentally
-connect voltmeter across resistor and ammeter in series with capacitor
- use stopwatch to time how long it takes resistor to fall to 37% of original value of pd
- time constant is the taken take for pd or current to fall to 37% of initial value
similarities and differnces between gravitational and electric field on point mass
S: both produce a radial field; field strength inversely proportional to separtion^2
D: G fields are always attractive E fields can be both
discuss problems for PET scanners having particle accelerators, hence only available in specific locations
- decision on who can use needs a pet scan
- location will limited ppl who have access to one
- long waiting times
what is meant by piezo electric effect
material expands and contracts so a pd is applied across it
how to find max ke of electrons
-electrons are repelled in xray tube/travel against electrical field
-electrons have a range of speeds
-as V increases fewer elctrons reach C (read graph)
Define Electrical potential
work done per unit charge in bringing a positive point charge from infinity to a point in field
how is lamp lit across transformer
- there is a change in magnetic flux linkage
- magnetic flux in core and into secondary coil
- emf proportional to rate of change of magnetic flux linkage
why does lamp light up briefly when primary coil connected to battery
- there is a change in current/magnetic flux at the start
- current and flux linkage are then constant so no emf
nature of SNF
acts between quarks
short range force
attractive up to 3fm
why density of nuclei are roughly same
M is proportional to A
r proprtional to A^1/3
volume propritonal to r^3
therfore v proportional to A
P=m/v=A/A
why are high temps necessary for fusion in stars
nuclei repel each other
higher temps particles have more KE to overcome repulsive force so can get close
how to polarise light
- use a polarising filter
- rotation will change intensity
types of errors
systematic: zero error(line not through origin)
random error: data points spread around linie of best fit
parallax: reading off measuremnt in corectly
take multiple to reduce random error, calibrate apparatus for systematic error
max emf when B=0 ?
gradient when B is 0 is the maximum rate of change of flux
structure of atom and evidence to back it
- nucleus v small compared with size of atom
– most of alpha particles went through - positive charge at nucleus
– some particles reflected - most of mass is at the centre
– some particles scattered at angles greater than 90 degrees hence nucleus at centre
why are high speed proton needed for reaction
proton is repelled
proton needs to get close to nucleus
kirchoffs laws
1st law: sum of current in a junction is the same as current of sum out of junction - charge is conserved
2nd law: sum of emfs is equal to sum of pds in a loop - energy is conserved
describe basic structure of x ray tube
cathode and anode in a tube
high voltage connected across cathode and anode
electrons hit the target and their KE is transformed into x-ray photons
explain how an ultrasound transducer can emit ultrasound
alternating pd applied to crystal produces vibrations due the piezoelectric effect so ultrasound is produced
how is reflection of ultrasound dependent on physical properties
density of tissue
same Z, no reflction
quote formula
forces on two parallel current carrying wires
magnetic field around wire(right hand grip rule - field up/down)
flemings left hand rule
list direction of field current and force
how to measure max freq of hearing
loudspeaker and signal generator connected
frequency increased until the limit of hearing
frequency calculated using f=1/T
work function definition
min energy required to remove an electron of the surface of a metal
adv of cat scan
cat scan will give 3d image
better contrast in image
fundamental frequency orders
closed both ends: L=n/2λ (n:1, 2, 3, 4…)
open both ends: L=n/2λ (n:1, 2, 3, 4…)
fixed one end: L=n/4λ (n: odd 1, 3, 5…)
describe nature and range of the two forces acting between two hadrons
SNF: short range; range about 3fm; attractive; repulsive below 0.5fm
gravitational force: attractive; inverse square law; long range
application of gamma camera
detection of cancer; tumour
changes in light through different mediums
Ray is refracted AWAY form normal therfore the refractiveindex is less
speed of light in medium is greater
frequency remain constant
v=fλ therefore wavelength of light increases as it travels into medium
what is meant by decay constant
the probability of decay of a nucleus per unit time
magnetic and electric field applied to a positive point charge
M: force is at right angles to motion so follows a circular path
E: Particle will accelerate in the direction of the electric field and follow a parabolic curve
It moves to the right and either comes out or goes into the
plane of the paper
define induced nuclear fission
Splitting of nuclei as a neutron is absorbed into lighter nuclei
explain role of fuel rod control rod and moderotr in nuclear fission reactor
- Fuel rod: Contain the uranium (nuclei) / fissile material
- Control rods: Absorb (some of the) neutrons
- Controls rods prevent uncontrollable chain reactions, so reduce the rate of reactions that take place
- Moderator: Slows down the (fast-moving) neutrons
- Slow-moving neutrons have a greater chance of causing
fission / of being absorbed (by U-235)
Explain changes when separation if capacitor is doubled
charge on each plate remains the same
c inverse d hence c halves
energy doubles (E = ½ Q^2/C)
max and min emf
when parallel to field lines(flat) no field lines are cut therefore minimum emf
