Fields, Particles and Frontiers of Physics Flashcards
Define capacitance
Charge stored per unit potential difference
What are the two assumptions of Olber’s paradox
- static/homogeneous
- infinite/infinite number of stars
Describe how the fate of the Universe depends on its average density. (3 marks)
- if average density of universe is less than critical then it will be too small to stop it expanding
- if the average density of the universe is greater than the critical value it will cause the contraction
- close to critical value and therefore expands at an ever decreasing rate asymptotic
Describe the evidence for the hot big bang model of the Universe
- galaxies moving apart
- if galaxies have always been moving apart at some stage they must have been closer together.
- evidence in red shift microwave
- further away the galaxy the faster the speed of recession
- the existence of a (2.7K) microwave
- there is more helium in the universe than expected.
Electric Field Strength (equation)
Voltage (p.d.)/ separation of plates
Define magnetic flux
a measure of the strength of the magnetic field
Explain why the magnitude of the magnetic flux through the coil varies as the coil rotates (generator)
The component of B perpendicular to the area changes.
Maximum when the field is perpendicular to B
Depends on cos(feta)
State Faraday’s Law of Electromagnetic Induction
The induced e.m.f is directly proportional to the rate of change of flux linkage.
(On a graph of MF against time) Describe and explain the variation with time of the induced e.m.f across the end of the coil.
- e.m.f. max when MF is zero…(give figure)
- e.m.f. zero when MF is a max…(give figure)
- e.m.f and MF have the same frequency
Explain why the activity of a radioactive material is a major factor when considering the safety precautions in the disposal of nuclear waste.
- material with small decay constant/ long half life activity will last for a long period period hence need long term disposal.
- material with large decay constant/ short half life have initial high activity hence precautions needed for initial period of disposal.
In beta-minus decay you also get an….
antineutrino
In beta-plus decay you also get a…
neutrino
Beta-plus decay in terms of quark model
uud -> udd
Beta-minus decay in terms of quark model
udd -> uud
Name the force responsible for beta decay
weak
Nuclear reaction: Explain how this reaction is able to produce energy
- binding energy of the products is greater than uranium
- energy available is the difference between the binding energies uranium and the sum of products
- E=mc^2
Nuclear Reaction: state what form the energy is released in such a reaction
KE
Explain why the neutron has no binding energy
the neutron is a single nucleon/ cannot be split further
(Two protons are accelerated at each other) The two protons fuse together. Explain how they are able to remain together.
Attractive strong force
(Proton accelerated towards other) Explain why the proton must have a very large velocity for the fusion to occur and the protons to remain together.
- as proton travels towards the stationary proton it experiences a repulsive force that slows it down
- therefore it needs a high velocity to get close to the proton.
Explain how image intensifiers are used to improve the quality of the X-ray image.
- Absorption of x-rays by photographic film.
- uses of fluorescent/scillinator
- photons release electrons
- no. of electrons increased/multiplied
Explain how contrast media are used to improve the quality of the X-ray image.
- different soft body tissue produce a little difference in contrast/attenuation
- high atomic number/ iodine or barium- gives greater contrast
- liquids injected or swallowed into soft tissue areas.
Collimator
gamma (ray photons) travel along the axis of lead
tubes or allows parallel gamma (ray photons travel to the
scintillator)
Electric Field Strength
force per unit charge
Charge (equation) (C)
current x time
No. of electrons (equation)
charge / elementary charge (1.60x10^-19)
Energy transferred by electrons (E)
Voltage x Charge
Torque of a Couple
size of one of the forces (in N) x perpendicular distance between the forces (in m)
Force on a coil in an magnetic field (equation)
BIL
Magnetic field, force on an ion (equation)
F=BQv (v is speed)
Mass on an ion in a circular path
F= mv^2 / r
The oxygen-16 ion is replaced by an oxygen-18 ion. The oxygen-18 ion has the same speed and charge. Explain why this ion describes an arc of greater length.
BQv = mv^2/r
Hence, radius is proportional to mass
Magnetic Flux
measure of the strength of the magnetic field (no. of field lines per unit area)
If straight line on graph of magnetic flux density (B) against time (t). Explain why the induced e.m.f across the ends of the coil has a constant value.
constant rate of change of magnetic flux.
(alternator) The alternating voltage supply is replaced by a battery. Explain why the p.d. across the (electrical component) is zero some time after the battery is connected.
There is no change in flux density/ magnetic flux.
Capacitance
Charge per unit voltage stored by a capacitor. (charge/p.d.)
Energy stored in a capacitor (3 equations)
- W= 1/2 x V^2 x C
- W= 1/2 x Q xV
- W = 1/2 x (Q^2/C)
Time constant
37%
State one property common to all isotopes of an element
same no. of protons/ same charge
Name the two fundamental forces experienced by both neutrons and protons.
- Gravitational
- Strong
Suggest why the actual speed of the electron is much less than your answer.
The mass of the electron increases.
Describe carbon-dating and explain one of its major limitations.
- Plants / living things take in carbon(-dioxide) or plants / living things stop taking in carbon after death
- The ratio of carbon-14 to carbon-12 (nuclei) for the relic sample is determined
- The current ratio of carbon-14 to carbon-12 nuclei is determined
- The age of the relic is found using ‘ x x0et ’
Limitation: The ratio of carbon-14 to carbon-12 is assumed to be constant / count(-rate) from relic may be comparable to background count(-rate)
Binding energy definition
the energy needed to separate all of the nucleons in a nucleus (measured in MeV) (also equivalent to mass defect)
Describe in simple terms how X-ray photons are produced in a hospital X-ray machine.
- Electrons are accelerated through high voltage
- (High speed) electron(s) hit metal
- kinetic energy of electron(s) ‘produces’ X-ray (photons)
Photon definition
Quantum/ packet of electromagnetic radiation.
Explain why an X-ray photon has greater energy than a photon of visible light.
E=hf and x-rays have higher frequency.
In the treatment of patients, explain what is meant by a non-invasive technique. State one of its advantages.
No entry into body / no cutting/incision of patient / no surgery B1 Lower risk of infection / less trauma
Explain what is meant by a medical tracer. Name a medical tracer commonly used to diagnose the function of organs.
Radioactive substance that is ingested / injected (into patient)
Technetium(-99m) / fluorine(-18)
What is a collimator (gamma camera).
gamma (ray photons) travel along the axis of lead
tubes or allows parallel gamma (ray photons travel to the
scintillator)
Having thin / long / narrow (lead) tubes makes the image sharper / less blurred
What is a scintillator (gamma camera)
gamma ray photon produces many/thousands of photons of (visible) light
What is photomultiplier tubes (gamma camera)
An electrical pulse is / electrons are produced from the light (photons)
What is the computer for (gamma camera)
Signals (from photomultiplier tubes) are used to B1 produce an image
Describe how the ultrasound is used to determine the speed of the blood in the artery.
Ultrasound is reflected by (moving) blood (cells)
The frequency / wavelength (of ultrasound) is changed
The change of frequency is related to speed of blood / change of wavelength is related to speed of blood / ‘frequency speed of blood’
Describe the formation of the Sun.
- (Sun / star formed from) dust cloud /nebula / (hydrogen) gas
- Gravitational collapse (AW)
- Temperature of (dust) cloud increases / KE (of cloud) in
creases / (cloud) heats up - Fusion occurs (when temperature is about 107 K)
- Protons / hydrogen nuclei combine to make helium (nuclei)
- Stable size star is produced when thermal / radiation pressure is equal to gravitational pressure
State two properties of a white dwarf.
- Very dense star
- Hot star / high surface temperature / low luminosity
- No fusion reactions take place / leaks away photons (fromearlier fusion reactions)
- Its collapse is prevented by Fermi pressure / mass less than 1.4 solar masses
Define electric field
A region in which a charged particle experiences a force /acceleration
State one difference and one similarity between the electric field of a point charge and the gravitational field of a point mass.
Difference:
- gravitational field / force is attractive
- electric field / force can be either attractive or repulsive
Similarity:
- Force / field (strength) inversely proportional to distance squared
- Radial fields
Force of a wire in a Magnetic field (equation)
F=BIL or F=BILsin(feta) (B is Magnetic flux density, L is length)
Suggest why it would be impossible for overhead cables carrying an alternating current to float in the Earth’s magnetic field.
The force on the cables will keep changing direction
Derive an equation for the radius r in terms of B, m, Q and v.
F=BQv or F= mv^2 /r
BQv = mv^2/r
r= mv/ BQ
State, with a reason, whether or not protons and neutrons are fundamental particles.
They are not fundamental particles because they consist of quarks
State two fundamental particles that can be classified as leptons.
electron / positron / neutrino / antineutrino
Explain why energy is released when a single nucleus of potassium-40 decays.
- There is a decrease in mass
- Energy (released) given by E = mc^2
Explain how the experiments on the scattering of alpha-particles by a metal foil provided evidence for the nuclear model of the atom.
Observations:
- Most of the alpha particles went straight / un-deflected through (the atom(s) / foil)
- (Some of the) alpha particles were scattered / repelled / deflected through large angles
Conclusions:
- showed that most of the atom is empty space
and
- showed the existence of small / dense / positive nucleus
You can figure out the speed of a particle, by using which equation?
KE= 1/2 x m x v^2
The strong force is….
…..attractive (cheeky I know)
Explain why fusion of helium requires higher temperatures than the fusion of hydrogen (protons).
- The helium nucleus has greater charge / The helium nucleus experience greater repulsive force
- Helium nuclei need to get close together (for the strong force to initiate fusion)
KE= 1/2 x m x v^2 or 3/2 x k x T
therefore, 1/2 x m x v^2= 3/2 x k x T
A property of a photon.
Can travel in a vacuum / has speed of 3 x 10^8 m s-1
in a vacuum / has no charge / has no (rest) mass / causes ionisation/ has momentum
Describe and explain a technique that can be used to reveal these blood vessels in an X-ray scan.
Contrast material / iodine is injected (into the vessels)
The contrast material:
- large attenuation / absorption coefficient
- has high Z (atoms)
(and hence reveal the outline of the blood vessels)
Discuss the advantages of using a gamma- emitting tracer in the patient rather than a beta-emitting tracer.
- Gamma radiation will pass through the patient (and hence can be detected) / beta particles will be absorbed by the patient (and hence cannot be detected)
- Gamma radiation is not (very) ionising / gamma radiation does little damage to cells / beta particles are (very) ionising/ beta particle damage cells
Describe how the components of a CAT scanner are used to produce a 3D image of a patient.
-X-ray tube rotates around (the patient) / X-ray beam passes through the patient at different angles
-A thin X-ray beam is used
-Image(s) of slice(s) / (cross) section(s) through the patient are taken
-X-ray tube moves / spirals along (the patient)
-The signals / information / pulses / data (from the detectors) are used by the computer (and its software) to produce
a 3D image
What is meant by Doppler effect.
Change in the frequency / wavelength because of source / ‘observer’ moving
Describe how high-frequency ultrasound can be used to determine the speed of blood through the arteries of a patient.
- Ultrasound transducer / device / probe emits and detects ultrasound
- The transducer / device / probe is placed at an angle (to the artery)
- Ultrasound is reflected by the blood / cells
During an ultrasound scan it is important that most the ultrasound from the transducer is transmitted into the patient. Describe and explain how this is achieved.
- Gel is used (between transducer and skin).
- The acoustic impedance / Z of gel is similar to that for skin hence less reflection (at the skin)
Explain what is meant by white dwarf when describing the evolution of a star.
A core / ‘star’ left behind after a red giant (has shed its
outer layers)
What is meant by intensity.
power per (unit) area
State Hubble’s Law
recessional speed / velocity of galaxy is proportional to its distance (from us)
Explain what is meant by the big bang and suggest two other observations that support the big bang model of the universe.
Big bang:
Creation / birth / expansion / evolution of the universe
or
The universe was very hot / very dense / singularity (at the start)
Evidence: Any two from:
- Microwave / background radiation / 3 K (or 2.7 K)
- Existence of (primordial) helium / lithium / lighter elements
-Tiny variation (or ripples) in (background) temperature
Explain why the ratio of the electric field strength E to the gravitational field strength g at any point around this charge is independent of its distance from the particle.
Both E and g vary with 1/distance2.
State two features common to all isotopes of hydrogen.
One proton / (same) charge / (same) element and
(same) chemical property
Explain why the total mass of the individual nucleons of the deuterium nucleus is different from the mass of the mass of the deuterium nucleus.
mass of nucleus < (total) mass of nucleons
Energy must be supplied to the nucleus to free the nucle- ons / energy released when nucleons combine (to form the nucleus).
(  )E = (  )mc2 and (  )E is the (binding) energy and ( )m is the mass defect or the difference in mass.
Explain what is meant by half-life of a free neutron.
(Average) time taken for half of the neutrons (in a sam- ple) to decay.
How can you find the repulsive electrical force.
F= qq/ 4(pie)(epicelont 0) r^2
Fusion occurs at much lower temperature. Suggest a reason why.
Some nuclei will be travelling faster / have greater (kinet- ic) energy (to overcome electrostatic repulsion and hence cause fusion).
How can you calculate the change in mass in a single fusion reaction.
(change in) E= (change in) mc^2
Suggest why the probability of reaction at a given temp is smaller for (element a) and (element b)
(element b) (nucleus) has greater charge / more protons.
The (electrostatic) repulsive force (between the a and b nuclei) is greater (hence smaller chance of fusion).
Define the time constant of a capacitor-resistor discharge unit.
The time taken for the p.d / current / charge to decrease to 1/e of its (initial) value.
State Faraday’s law of electromagnetic induction.
The induced e.m.f. is (directly) proportional / equal to the rate of change of (magnetic) flux linkage.
Use the ideas about energy conservation to state and explain the polarity of the face nearer the magnet. (north end of a magnet moving towards coil)
North / N (pole). There is a repulsive force (between magnet and coil and the work done against this repulsive force is transferred to electrical energy in the coil).
What is meant by induced nuclear fission.
A (thermal / slow-moving) neutron splits the nucleus into two (smaller) nuclei and (fast-moving) neutron(s).
Explain the purpose of using a moderator and control rods in the core of a nuclear reactor.
- Fission reactions produce fast neutrons.
- The moderator / water slows down (the fast-moving)
neutrons. - Slow-moving neutrons have a greater chance of caus-
ing fission (of U-235). (ora) - The control rods absorb (some of the) neutrons.
- (On average) one neutron survives between succes-
sive (fission) reactions.
-The neutrons make collisions with the (moderator) nuclei and transfer (some of) their (kinetic) energy.
What is meant by an energy level of an atom.
Discrete energy (of electrons in an atom) / quantised en- ergy (of electrons in an atom) / permitted energy (states of electrons in an atom).
Explain how the wavelength from the X-ray tube changes when the accelerating potential difference of the X-ray tube is doubled.
The wavelength is halved. as E= hc/ wavelength or E is prop to 1/ wavelength.
Describe and explain the principles of a B-scan.
Ultrasound reflected at boundary (between materials). B-scan takes place in different directions.
The intensity of the reflected ultrasound depends on the acoustic impedances of the materials (and this is greater when the difference between the acoustic imped- ances is greater).
Explain the basic principles of PET scanning, including how the image is formed.
- The brain / body is surrounded by a ring of (gamma)
detectors /gamma camera(s). - The positrons (from the F-18 nuclei) annihilate elec-
trons. - The annihilation of a positron and an electron produc-
es two (identical gamma) photons travelling in oppo-
site directions. - The delay time between these two photons / gamma
rays is used to determine the location of the annihila-
tion / F-18 / tracer. - Computer connected to detectors / gamma camera
and an image is formed by the computer (using the electrical signals from the detectors).
State the cosmological principle.
The universe is homogeneous. and isotropic (on a large scale).
State some of the properties of the microwave background radiation observed from the Earth. Discuss how the background microwave radiation is linked to the big bang model of the universe.
The intensity of the microwaves is the same in all direc- tions.
These microwaves correspond to a temperature of 2.7 K or The temperature of the universe is 2.7 K.
The expansion of the universe following the big bang led to cooling and hence we observe microwaves rather than short wavelength e.m. waves / gamma waves.
Define electric field strength at a point in space.
Force per unit charge.
Gravitational force on a proton (equation)
F= -GMm/ r^2
How can momentum of an electron relate to its’ wavelength.
p=mv
wavelength= h/ mv
Name the rule that may be used to determine the direction of the force acting on the ions. (magnetic field)
Fleming’s Left-hand rule.
Explain why the speed of an ion travelling in a circle does not change.
The force is at right angles to the velocity.
State one quantity that is conserved in this fission reaction.
momentum / mass-energy / charge / baryon number / nucleon number
Suggest why some fusion can occur at temperatures lower then this.
Some nuclei will have KE greater than the mean KE.
Describe the principles of PET.
- Annihilation of electron and positron
- Positron-electron annihilation produces two gamma
- The gamma photons travels in opposite directions
- The patient is surrounded by (a ring of) gamma
detectors - A 3-D image is created (using the detector-signals
with the aid of computer software)
-The arrival times / delay times of the photons (at diametrically opposite detectors) are used to pinpoint B1 areas of increased activity (AW)
Describe briefly how X rays are produced in an X-ray tube.
(Fast-moving) electrons hit a metal / an anode
The kinetic energy of the electrons is transferred into X-rays / photons / EM waves
Describe the Compton Effect in terms of an X-ray Photon.
An X-ray photon interacts an electron (within the atom)
The electron is ejected and the energy / frequency of the (scattered) photon is reduced
Describe the operation of a computerised axial tomography (CAT) scanner.
X-ray beam passes through the patient at different angles / X-ray tube rotates around the patient
A thin fan-shaped beam is used
Images of ‘slices’ through the patient (in one plane are produced with the help of computer software)
X-ray tube / detectors are moved along (the patient for the next slice through the patient)
State one of the advantages of a CAT scan image over a conventional X-ray image.
3D image / better contrast between different (soft) tissues
Describe briefly the sequence of events which occur in the formation of a star, such as our Sun, from interstellar dust and gas clouds.
- interstellar dust and gas cloud is drawn together by gravitational force / gravity
- Loss in (gravitational) PE / KE increases / temperature
increases - Fusion (of protons / hydrogen nuclei) takes place
- Energy is released in fusion reactions
- A stable star is formed when gravitational pressure is
equal to internal / gas / radiation pressure
Describe some of the characteristics of a white dwarf.
-(extremely) dense / (very) hot / low luminosity
-no fusion reactions occur
-it is a remnant of a low-mass star
-correct reference to Fermi pressure / electron
degeneracy / Chandrasekhar’s limit
Explain why, in its evolution, the star is brightest when at its coolest.
- Red giant
- (It is cooler but has) large surface area (and therefore radiates large amounts of energy)
State Olbers’ paradox and the two assumptions made about the Universe.
- The night sky should be bright / have uniform brightness (but it is not)
- The line of sight ends on (the surface of a star) or ‘number of stars prop. r^2 and intensity prop. 1/r^2’
- Infinite / uniformly distributed matter or stars throughout / static / infinite age
State Hubble’s law and explain how it resolves Olbers’ paradox.
(recessional) speed of galaxy prop. its distance (from the Earth)
The universe is finite / it is expanding / it has a beginning / visible light is red-shifted (because of expansion of space)
Suggest why it would be impossible for overhead cables carrying an alternating current to float in the Earth’s magnetic field.
-Force will keep changing.
Explain why the laser doesn’t heat the object as quick as you expect.
- Photons reflected/ not absorbed
- Thermal energy transferred to rest of metal.
Why does the temperature remain constant when changing states
- Specific latent heat of fusion/vaporisation.
- energy is being used to break intermolecular bonds
- PE goes to KE.
Internal energy=
sum of random KE + PE
Simple transformer: explain how an e.m.f is induced in the secondary coil.
- a changing magnetic flux is produced in the core.
- the iron core links this magnetic flux
- the changing magnetic flux through the secondary induces e.m.f
State how you could change the transformer to increase the maximum e.m.f induced in the secondary coil.
- more coils/ turns on secondary
- less coils/ turns on primary
- laminate the core
Number of turns calculation transformer
n(s)/n(p) = v(s)/v(p)
Q (charge) is a ………… in series
constant
Electric force (equation)
EQ
Strong Force range
0.5fm to 3fm (f is 10^-15)
Charge (equation using electrons)
N x e (N-no. of electrons) (e-charge of one of the electrons)
E (electric potential energy) (equation)
QV
Phase difference
how far ‘out of step’ (out of sync) the oscillations at two points on the wave
Lenz’s Law
the e.m.f is induced in a direction so as to oppose the change producing the e.m.f
State two factors that determine the value of acoustic impedance.
- density of medium
- speed of ultrasound in medium.
Why do you need to remove low energy X-rays, during an x-ray
Low energy X-rays are absorbed by the skin- greater ionising.
Explain how ultrasound is produced using a piezoelectric crystal such as quartz
- Alternating voltage
- at resonant frequency
Describe one scattering experiment to investigate the size of the nucleus of the atom.
α - particle scattering
suitable diagram with source, foil, moveable detector 2 or more trajectories shown
vacuum
most particles have little if any deflection
large deflection of very few
reference to Coulomb’s law /elastic scattering
alphas repelled by nucleus (positive charges)
monoenergetic
Describe Helium to Hydrogen fusion
- (4) hydrogen or light nuclei/protons are fused together to form a helium/heavier/larger nucleus;
- two positrons must also be released; to conserve charge;
- mass reduction provides energy release/ ∆m = ∆E/c2 (1)
- the process requires very high temperatures (to bring the protons together);
- normally achieved inside a star;
- Energy release in fusion is much greater than in radioactive decay;
- because mass reduction/change in fusion is much greater than in radioactive decay
- as the helium nucleus is so strongly bound
- also energy release from annihilation of positrons
The photoelectric effect
The emission of electrons from the surface of a metal when electromagnetic waves (of frequency greater than the threshold frequency) are incident on the metal.
