A-Level Physics G485 Flashcards
Define Electric Field Strength
Electric field strength at a point in space is the force per unit positive charge
Define Magnetic Flux Density
a measure of the strength of a magnetic field at a given point, expressed by the force per unit length on a conductor carrying unit current at that point.
Define Tesla
One Tesla is the uniform magnetic flux density which, acting normally to a long straight wire carrying a current of 1 ampere, causes a force per unit length of 1Nm-1 on the conductor.
Define Magnetic flux
magnetic flux = magnetic flux density x area (perpendicular to field direction)
B x A (normal to B). Magnetic flux=magnetic field x Area
Define The Weber
One Weber is equal to one Tesla metre²
Define Magnetic Flux Linkage
The change of magnetic flux linkage is equal to the product of the change in magnetic flux and the number of turns N of a conductor involved in the change in flux
Define Capacitance
Capacitance = charge per (unit) potential differences
Ratio of charge to potential for a conductor
Define The Farad
coulomb per volt
A farad is the charge in coulombs which a capacitor will accept for the potential across it to change 1 volt
Define The Time Constant Of A Circuit
Time for the charge to have decreased to “1/e” of its initial charge
Define Proton Number
The number of protons found in the nucleus of an atom
Define Nucleon Number
The mass number, the sum of the number of neutrons and protons in an atomic nucleus
Define Isotopes
Isotopes are different forms of the same element which have the same number of protons but different numbers of neutrons in their nuclei
Define Activity (Radioactivity)
Spontaneous emission of a stream of particles or electromagnetic rays in nuclear decay
Define The Decay Constant
The probability of decay of a nucleus per
unit time
Reciprocal of decay time
Define Half Life
The half-life of a radioactive nuclide is the time taken for the number of un-decayed nuclei to be reduced to half its original number
Define Binding Energy
The energy equivalent of the mass defect of a nucleus. It is the energy required to separate to infinity all the nucleons of a nucleus
Define Binding Energy Per Nucleon
Binding energy per nucleon is defined as the total energy needed to completely separate all the nucleons in a nucleus divided by the number of nucleons in the nucleus
Define Intensity
Power per unit area(W/m²)
Define The Distance Measured In Astronomical Units (AU)
The astronomical unit is defined as the radius of the circular path round the sun followed by a body in 365.25 days
Define The Distance Measured In Parsecs (pc)
Distance from a base length of 1 AU that subtends an angle of 1 (arc) second
Define The Distance Measured In Light-Years (ly)
The distance travelled by electromagnetic radiation (light) in one year
Define Critical Density
The average density of the universe
- Above which the universe will collapse (closed universe)
- Below which the universe will expand forever (open universe)
- If the density is equal to the critical universe get a flat universe.
State that electric fields are created by electric charges
Electric charges exert forces on each other when they are a distant apart. An electric field is a region of space where a stationary charge experiences a force
State and use Fleming’s left-hand rule to determine the force on a current conductor places at right angles to a magnetic field.
If the first two fingers and thumb of the left hands are placed at right angles then the first finger is in the direction of the field, the second in the direction of the current and the thumb in the direction of motion
State and use Faradays Law of electromagnetic induction
Induced e.m.f is proportional to the rate of change of (magnetic) flux
State and use Lenz’s law
The direction of the induced e.m.f is such as to cause effects to oppose the change producing it
State and use the equation for the total capacitance of two or more capacitors in parallel
C= C1+C2+C3…….
State and use the equation for the total capacitance of two or more capacitors in series
1/Ct =1/C1+1/C2+1/C3 ……
State the quantities conserved in a nuclear decay
The charge, the total number of neutrons and protons, total energy, the total momentum of the system and the total lepton number.
State that there are two types of β decay
β- decay produces an emission of electrons. (Electron) antineutrinos are produced.
A β- particle IS an electron.
β+ decay produces an emission of positrons. (Electron) neutrinos are produced.
A β+ particle IS a positron.
State which Group electrons and neutrinos are members of.
Electrons and neutrinos are members of a group of particles called “Leptons”.
(There are six members of this group in all, the Electron, Muon and Tao, and their associated neutrinos)
State the approximate magnitude of the parsec in meters
3.1 x 1013km
or
3.1 x 1016m
State the approximate value in meters of the light year.
9 x 1015m
(sometimes it is taken to be
10 x 1015 the true value is
9.46 × 1015 [to 3sf])
State Olbers’ paradox
Based on: the universe being static, homogeneous and infinite - there should be an infinite number of stars visible (the night sky should be as bright as at day).
State and interpret Hubble’s law
All objects observed in deep space (intergalactic space) are found to have a Doppler shift observable relative velocity to Earth, and to each other; and that this Doppler-shift-measured velocity, of various galaxies receding from the Earth, is proportional to their distance from the Earth and all other interstellar bodies.
The interpretation of this law is that the universe is expanding.
State the cosmological principle
On large enough scales the universe is isotropic and homogenous. That is that there is no “special” point in the universe and no discernable centre.
Explain the effect of a uniform electric field on the motion of a charged particle
If E is uniform, then the acceleration of the charged particle is constant. If the particle has a positive charge, then its acceleration is in the direction of the electric field. If the particle has negative charge, then its acceleration is in the direction opposite the electric field
Explain the use of deflection of charged particles in the magnetic and electric fields of a mass spectrometer
Depending on the strength of the magnetic and electric fields the mass of charged particles detected can be changed. E.g: the smaller the electric field the larger massed particles can be detected
What does the area under a potential difference vs charge graph represent.
It is equal to the energy stored bvy a capacitor.
Explain the propertiers of an exponential decay graph
At first gradient is large in magnitude and this gradient then decreases in magnitude over time. THE MOST IMPORTANT property of exponential decay graphs is that they have a constant ratio property, in a certain time “t” they will always decay to half the previous value.
Explain why Protons and neutrons are not fundamental particles.
Protons and Neutrons (as well as all mesons and hadrons) are made of smaller constituent particles called “Quarks”, and so are not fundamental particles.
Explain how soft tissues like the intestines can be imaged using barium meal
In order to make soft tissue more visible, contrast media, such as barium, are used. The patient swallows a liquid rich in barium as it will readily absorb X-rays. The barium meal coats the wall of the tract enabling its outline to be seen in X-rays
Explain what is meant by the Doppler effect
Doppler effect is a change in frequency and wavelength of a wave. It is caused by the change in distance between the thing creating the wave and whatever is measuring, seeing or hearing the wave.
Explain qualitatively how the Doppler effect can be used to determine the speed of blood
Doppler effect uses ultrasound waves. Sound waves are reflected by the moving blood cells. Change in frequency/wavelength enables the speed of blood flow or rate of flow of blood to be found
Explain how ultrasound transducers emit and receive high frequency sound
Transducer is the name given to any device that converts energy from one form to another. In this case electrical energy is converted into ultrasound energy by means of a piezo-electric crystal such as a quartz
Explain that the standard model of the universe implies a finite age for the universe (hot big bang)
galaxies are moving apart / universe is expanding
if galaxies have always been moving apart then at some stage they must have been closer together / or started from a point
evidence in red shift either optical / microwave
further away the galaxy the faster the speed of recession
the existence of a (2.7 K) microwave
background radiation
there is more helium in the universe than expected
Explain that the universe can be ‘open’, ‘flat’ or ‘closed’, depending on its density
Open: Universe expands for all time
Flat: expands to a limit (but never reaches it)
Closed: Universe contracts / collapses
back
Explain that the ultimate fate of the universe depends on its density
if average density of the Universe is less than critical then it will be too small to stop it expanding / it goes on forever
if the average density of the Universe is greater than the critical value it will cause the contraction (and produce a big crunch)
close to critical value and therefore a universe expands that will go towards a limit / expands at an ever decreasing rate asymptotic
Explain that it is currently believed that the density of the universe is close to, and possibly exactly equal to, the critical density needed for a ‘flat’ cosmology
p0 = 3(H0)2/8πG
Estimates give values of 1 or 2 orders of magnitude less than critical. But rotation of galaxies show they have more mass than we can see, and the inflationary expansion theory suggests that the density is exactly equal to critical
Describe the difference between A-scan and B-scan
A-scan in one direction only / range or distance or depth finding
B-scan uses a number of sensors or a sensor in different positions / angles (to build up a 2D/3D image)
Describe the importance of impedance matching
There is a large difference in density between air and skin/fat/flesh. Because of this large difference roughly 99% of the Ultrasound would be reflected, resulting in a poor quality image. By use of a gel of similar “acoustic impedance” to the human body Ultrasound will transmit into the body and return to the transducer so an image can be processed
Describe the principle contents of the universe, including stars, galaxies and radiation
There are at least 1010galaxies in the universe. From a side view there is a disc shaped conglomeration of stars with a bulging central nucleus
Describe the solar system in terms of the Sun, planets, planetary satellites and comets
Solar system contains 9 well known planets and their satellites; it also contains a number of small or dwarf planets. These planets all orbit the sun. Comets have an elliptical orbit, which means that they return regularly often passing close enough to the earth to be visible
Describe the formation of a star, such as our Sun, from interstellar dust and gas.
- Gas / dust (cloud) drawn together by gravitational forces
- Loss in (gravitational) PE / KE increases / PE changes KE / temperature increase
- Fusion of protons / hydrogen nuclei (produces helium nuclei and energy)
- A stable star is formed when radiation pressure is equal to gravitational pressure
Describe the Sun’s probable evolution into a red giant and white dwarf
Sun is on the main sequence but becoming redder, it will take 5x109 years before becoming a red giant
In the red giant stage no further energy can be obtained so the star will shrink under its internal gravitational attraction. As it gets smaller emitted radiation becomes whiter, it now becomes a white dwarf
Describe how a star much more massive than our Sun will evolve into a super red giant and then either a neutron star or black hole.
Supernova followed by neutron star / black hole
Great mass, so the core collapses under its gravity. The KE and temperature are so high that fusion takes places; there is enough energy to turn electrons and protons into neutrons. Core contracts until it becomes entirely neutron, it is then a neutron star (high density)
However gravitation force on neutrons could become so strong it prevents the light being emitted, no radiation emitted means it will appear black, this is a black hole.
Describe and interpret Hubble’s red shift observations
Observations that the wavelengths of identifiable spectral lines in the spectra of light from distant galaxies did not correspond with wavelength measured on earth. Column of light seemed to be shifted towards the red end of the spectrum. This was interpreted as a continuous expansion of the universe
Describe and explain the significance of the 3K microwave background radiation
The 3K CMB radiation is a remnant of the “Big Bang”. The early universe was very energetic and “hot”. It was enhabited by high energy gamma rays, as the universe expanded these gamma rays were redshifted so that their wavelength was increased and their energy and frequency reduced. This energy now exists as microwaves which pervade the whole universe.
Describe qualitatively the evolution of universe 10-43s after the big bang to the present
10-43stemperature was high 1032K and any particles will be moving with average KE of 1019GeV
At 10-43s Change occurred; gravitational force appeared as a separate force. Still no distinction between quarks and leptons 10-35s
The universe had expanded, the temperature fallen to 1027K, strong nuclear force distinguished as were quarks and leptons. Leptons included electrons, neutrinos and all corresponding antiparticles. Quarks were initially free but soon began to form nucleons and hadrons
10-6s Temperature fell to 1013K, average KE was insufficient to create particle-antiparticle pairs from nucleons. Annihilation continued leading to a universe with far fewer nucleons and more particles than antiparticles
Describe how electric field lines represent an electric field
The direction of the electric field is defined as the direction in which a positive charge would move if it were free to do so. So the lines of force can be drawn with arrows that go from positive to negative
Describe the similarities and differences between the gravitational fields of point masses and the electric fields of point charges
Both fields decrease in magnitude as the distance from the point charge/mass decreases in accordance with the r2 law.
The magnitude of an electric field depends on the amount of charge, the magnitude of a gravitational field depends on the amount of mass.
Electric fields are in general stronger than gravitational fields because the gravitational constant is much smaller than its equivalent.
Describe the magnetic field patterns of a long straight current-carrying conductor and a long solenoid
Magnetic field patterns due to a long straight wire are concentric circles centred on the middle of the wire. The separation of the line increases with distance from the wire.
A solenoid may be thought to be made up of many flat coils placed side by side. The field lines are parallel and equally spaced over the centre section of the solenoid indicating the field is uniform
Describe the function of a simple ac generator
An electric generator converts mechanical energy in the form of the rotation energy of a coil of wire into electrical energy. This occurs as the coil is rotated in a magnetic field, the change in magnet field induces an e.m.f. which produces a current in the coil.
Describe the function of a simple transformer
A simple transformer is two coil of insulated wire wound on to a laminated soft iron core. And alternating e.m.f is applied across the primary coil and an e.m.f is induced in the secondary
Describe the function of step-up and step-down transformers
Step up- when Vs is greater than Vp there are more turns in the secondary coil than the primary. Low to high voltage
Step down- when Vs is less than Vp there are more turns in the primary coil than the secondary. High to low voltage
Describe the uses of capacitors for the storage of energy in applications such as flash photography, lasers used in nuclear fusion and as back-up power supplies for computers
Capacitor takes a few seconds to charge then it is either discharged rapidly when connected to the flash bulb to give a short but intense flash, or it can be released slowly, when being used as a back up battery
Describe qualitatively the alpha-particle scattering experiment and the evidence this provides for the existence, charge and small size of the nucleus
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
OR electron scattering
High energy diagram with source sample, moveable detector / film
Vacuum
Electron accelerator or other detail
Most have zero deflection
Characteristic angular distribution with minimum
Minimum not zero
De Broglie wavelength
Describe the function of a simple transformer
A simple transformer is two coil of insulated wire wound on to a laminated soft iron core. And alternating e.m.f is applied across the primary coil and an e.m.f is induced in the secondary
