M8 From the Universe to the Atom Flashcards
Luminosity
rate at which energy is radiated, same as power. P = E/t.
Apparent brightness (b)
the power of a star’s radiation received by an observer per unit area. Inverse square law. b = L/4d2 where d is distance to the star, L is luminosity (power).
Measuring apparent brightness
- CCD camera on a telescope. Each pixel produces a potential difference that depends on the rate at which photons are collected.
- Photoelectric photometer.
- Chemical emulsion photography.
Solar Luminosity
total power output of the Sun, L⊙ = 3.85 x 10^26 W.
Classification of stars based on
colour, temperature, distance, luminosity, spectral class.
Temperature of stars
high temperature - emit higher frequency radiation and have a colour on the blue side of the spectrum. Colder - redder.
Spectral class of stars
Decreasing temperature: hot to cold (blue white yellow red) OBAFGKM (Oh Be A Fine Guy Kiss Me).
Wien’s displacement law
lambda(max)= b/T for blackbody. T in Kelvin.
Blackbody Radiator
an object that, when in thermal equilibrium with its surroundings, absorbs all electromagnetic radiation incident upon it, and emits all EM radiation in a spectrum entirely based on its temperature. Radiation curves: wavelength x-axis, intensity (spectral radiance) y-axis
Hertzprung-Russell diagram
plots luminosity (in L⊙) and temperature of stars against each other, both with log scales. Decreasing temp scale (O to M class).
Stefan-Boltzmann law
L = AT4. L is luminosity (W), is the Stefan-Boltzmann constant = 5.67 x 10-8 Wm-2K, A is surface area, T is temperature (K).
Region A of stars: Main Sequence
long-term stable conditions. In the hydrogen-to-helium fusion phase. E.g. our Sun. Majority of stars are here.
Region B of stars: Red Giants
low temperature, more luminous than main sequence, large surface area. Finished fusing hydrogen.
Region C of stars: Supergiants
similar to B, red giants, but bigger.
Region D of stars: White dwarfs
very hot, less luminous than main sequence. Small. Final state, remnants of medium-sized main sequence stars.
Redshift
Doppler shift in light. Decrease in frequency. Increase is blueshift (e.g. Andromeda, think galaxy is not moving through space, the universe is expanding).
Occam’s Razor
Among different models, the one with the fewest assumptions should be favoured.
Cosmological Red Shift
Observed by Edwin Hubble, galaxies are red-shifted, moving away from us. The greater the relative velocity, the greater the redshift. Conclusion: the space-time of the universe expands. Hubble’s 1929 data showed linear relationship between recessional velocity and distance.
Hubble’s Law
Evidence for Big Bang theory. v = H0d. v is recessional velocity. D is distance.
H0 = 67.74 km/sMpc where Mpc is megaparsec.
1/H0 is the age of the universe.
Models for the expansion of the universe
- Static. Size is not changing (disproven).
- Flat. Expansion is slowing and may eventually stop in the far, far future.
- Closed. The universe will soon stop expanding and start contracting, leading to the ‘Big Crunch’.
- Open. Expanding at an accelerating rate and will continue to.
Pressure of gases in a star’s atmosphere
The thickness of absorption lines in a star’s spectrum gives an indication of the pressure of gases in the star’s atmosphere. Larger stars’ surface is further away from the core (inverse square law with gravitational force), therefore gas pressure on the surface is low.
Globular cluster
A spherical collection of stars that orbits a galactic core. Very tightly bound by gravity. Spherical shape, relatively high stellar densities toward their centres.
Open cluster
a group of up to a few thousand stars that were formed from the same giant molecular cloud and have roughly the same age.
Key stages in a star’s life
- Nebula, cloud of dust and gas.
- Protostar, formation of star possible, gravity is pulling in more material from surroundings.
- Main sequence star: starts nuclear fusion of hydrogen into helium. When hydrogen fuel has been depleted, the core collapses.
3 options: 1 or less solar mass becomes a red giant, then a white dwarf. 1 to 3 solar mass becomes a red giant, then a neutron star/pulsar/supernova. >3 solar mass becomes a red supergiant, then a black hole.
Cepheid variables
1912, Henrietta Leavitt analysed a group of stars called Cepheid variables, whose apparent brightness changes periodically. She showed that the period was directly related to the star’s luminosity.
John Dalton 1803 atom model
solid sphere model, ‘billiard ball’ model.
William Crookes cathode ray properties
experiments in Crookes tubes, found 4 properties of cathode rays.
- It originates at the cathode and travels in a straight line.
- It possesses momentum (paddle wheel experiment).
- Is deflected by magnetic fields.
- Makes phosphorescent materials fluoresce.
Cathode Ray
A high voltage applied between electrodes accelerates electrons. As the electrons travel to the anode, they collide with gas and cause the particles to glow. Rays not observed to deflect by electric field, rays pass through foil, rays deflect by magnetic field, rays cause paddle wheels to turn, objects struck by the rays heat up.
JJ Thomson
showed that cathode rays were deflected by an electric field. Made it so that particle is undeflected by FE and FB in opposite directions. Plum pudding model, electrons embedded in positive sphere. Found q/m ratio for cathode rays, hence proved they are particles, not waves.
Stoke’s Law
F = 6pirvn. F is viscous drag force (N). n is viscosity of the fluid (Nsm-2). r is radius of the object (m). v is velocity of the object (ms-1).
Ernest Rutherford 1911
Geiger-Marsden Experiment (gold foil experiment). Fired alpha particles (helium nuclei) at thin gold foil and observed their scattering. Rutherford scattering.
Niels Bohr 1913
proposed the existence of atomic energy levels.
Rutherford-Bohr Model
positively charged nucleus around which the negatively charged electrons exist, most of the mass is in the nucleus, the size of the nucleus is 104 - 105 times smaller than the atom.
Limitations of the Rutherford-Bohr Model
- couldn’t explain the composition of the nucleus
- didn’t know how the electrons were arranged around the nucleus
- couldn’t explain how electrons didn’t collapse into the nucleus, thought that the centripetal acceleration of electrons would release EM radiation (as it was known that accelerating charges produce EM radiation)
- Zeeman Effect: spectral lines split in the presence of a magnetic field. Showed that the splitting was due to the electron’s intrinsic magnetic field (spin) and the magnetic field due to its orbital motion interacting with the applied magnetic fields. Solved by De Broglie’s matter waves: orbit is a standing wave which retains energy.
Nucleon
proton or neutron, particle in the nucleus.
James Chadwick discovery of neutrons
Beryllium metal bombarded with alpha particles emits neutrons as radiation, then fired into paraffin wax (high in protons) which emitted protons whose energy and velocity could be determined. 4(2)He + 9(4)Be → 12(6)C + 1(0)n. Proved neutrons were neutral as they weren’t deflected by magnetic or electric field.
Continuous spectra
produced by hot metal (lightbulb) or core of star.
Line spectra of gases
Gases produce line spectra with spectral features at the same discrete wavelengths in an emissions and absorption spectra. Thus, a gas emits and absorbs light with the same characteristics.
Joseph on Fraunhofer
assigned the seven most prominent dark lines letters: A at longest wavelength.
Gustav Kirchhoff
the dark lines in the spectrum of sunlight coincided with the position of the bright lines.
Discrete
The emission spectra of gases are evidence for the discrete nature of the energy of atoms; meaning that an atom can exist in well-defined energy levels but not have any intermediate energies.
De Broglie’s (1924) Matter Waves
hypothesis states that all particles have a wave nature. E.g. Electron single slit experiment - behaves like a particle, single line. Electron double slit experiment - behaves like a wave, interference pattern.
Bohr’s Postulates
- There exists atomic states in which no EM radiation occurs even though electrons experience acceleration.
- Each stationary state corresponds to a different energy of the atomic system and the atom may suddenly transition between state by either absorbing or emitting EM radiation. hf = EInitial - EFinal. Ionisation at n = infinity, E = 0.
Rydberg’s Equation
mathematical description of the line spectra of hydrogen only. For Balmer Series n=2 up to n=6. 1/lambda = R(1/nf2 - 1/ni2) ni is initial electron shell, nf is final.
Balmer Series
only red, light blue, dark blue and purple, de-excitations to n=2.
De Broglie’s Equation
lambda = h/p = h/mv. h is planck’s constant.
Davisson-Germer Experiment
firing electrons at a nickel target (acts as a diffraction grating) and measuring how they were deflected using a moveable detector. The experiment measured peaks and troughs in the deflections of the electrons; behaving as waves and interfering with each other.
Kinetic Energy of an electron
EK = q(E)V = mv2/2 (when electron is accelerated through potential difference).
Orbit of electrons
2 x pi x r = n x lambda.
circumference of orbit must be a complete number of wavelengths.
Angular momentum
= mvr = nh/2
fully dependent on electron shell n.
Quantum Probability
electron waves are waves of probability. Responsible for phenomena including fusion in the Sun, alpha decay and 3D touch screen technology.
Wave function
weird w symbol
Observing the electron a certain way (e.g. one slit in double slit experiment) changes its behaviour. Observe particle nature at one slit, overall wave nature. Amplitude of probability waves. Probability is square of amplitude of wave function.
Intensity
proportional to the square of the amplitude.
Probability of finding electron at distance r from its source
P(r) = |wave function|^2 x delta(v)
Schrodinger’s Model
states that the square of the wave function of a particle at any point determines the probability of finding it there.
Schrodinger’s Cat
thought experiment. If a cat were to be put in a box with a 50% chance of killing the cat. Then at the moment of opening the box, the cat is both alive and dead.
Duality of Light
Light as a wave, supported by Young’s Double Slit Experiment. Light as a particles, supported by photoelectric effect.
Duality of Matter
Matter as a particle, supported by life. Matter as a wave, lambda = h/p, Davisson-Germer experiment, electrons diffract in double slit experiment.
Electron double slit interference pattern
lambda x n= 2dsin(theta)
lambda = h/p.
Doubling up
Light and matter can be waves and particles, but only one at a time.
Matter as a wave
Matter only shows wave properties if it interacts with something on a similar scale to its wavelength.
Schrodinger’s collapsing of the wave function
Electron double slit. The act of observing forces the electron to pick a location, so it is no longer a wave of probability. Hence, the observer causes the collapse of the wave function.
Evidence for proton and neutron not fundamental particles
beta negative and positive particles in radioactive decay. n → p + B- + anti v(e) (neutron decays to proton, electron and antineutrino). p → B+ + n + v(e) (proton decays to positron, neutron, neutrino), antimatter. Use of cloud chamber to study cosmic rays. Particle accelerators.
Quarks
Flavours: Up (u), down (d) - Gen 1. Charm (c) Strange (s) - Gen 2. Top (t), bottom (b) - Gen 3. Charged particles. Up, charm and top charge ⅔ e. Down, strange and bottom charge -⅓ e. Can only exist in pairs or threes.
Leptons
Electron (e), Muon (mu), Tau (T), Electron neutrino (ve), Muon neutrino (vmu), Tau neutrino.
