Physics Lab Questions
Describe the most probable method by which Sodium-22 decays
Beta-plus decay. It releases a positron, becomes an excited neon-22 isotope, then releases a photon and goes to the ground state. This positron may then annihilate with an electron, creating a photon with a different energy.
Quantum Computers
The usual binary bits are 0 and 1 In quantum computers you have an equation a0|0,0> + a1|0,1> + a2|1,0> + a3|1,1> in which the a's are not completely independent; they are made of complex numbers and they must be normalized (a0^2 + a1^2 + a2^2 + a3^2 = 1) An inherent problem with quantum computing is quantum mechanics is inherently probabilistic, not 100% correct
What is a black body and why is it relevant?
Objects with emissivity of 1 that radiate and absorb energy perfectly.
What is the relationship between the wavelength of electromagnetic radiation and the size of a dipole for the Rayleigh scattering approximation to be appropriate?
Particles have to be much smaller than their wavelengths; particle size < 1/10 wavelength
What is the quantitative relationship between count rate and shielding thickness, and how does the shielding effect photon energy?
Shielding does not affect photon energy. It affects count rates with an exponential decay relationship as shielding increases.
For a 2 kΩ resistor at room temperature and electronics with 30 kHz bandwidth, find the RMS voltage arising from thermal noise.
<v^2> = 4Rk(b)Tf
What are Cooper Pairs and why do they allow a superconductor to have zero resistance?
A pair of electrons (or other fermions) bound together at low temperatures. Because they have a very small attraction between them, they are able to reach an energy level lower than the Fermi energy, which contributes to the material being a superconductor. When many Cooper pairs form in a metal, they open a gap in the spectrum of allowed energy states - meaning that excitations will need a certain minimum energy to occur. This energy gap leads to superconductivity.
What happens at the junction in a p-n diode, and what are the implications for the current-voltage characteristics of the device?
At the junction of a pn diode when there's no current, electrons cross the junction (from n to p) and holes cross the junction (from p to n) naturally. Eventually, Coulomb forces prevent more ions from crossing (depletion region) stopping the current.
Graviton
Boson with spin 2 (Tensor boson) Graviton doesn't fit with quantum field theory Requires renormalization Electrons do, cause you put in me = infinity - infinity + 9.11*10^-31 into QFT and get 9.11*10^-31 but it doesn't work for gravitons... we keep getting infinity Light will self interfere when it goes through materials which makes it slow down The theory of everything/the standard model
What is thermal radiation and what causes it?
Electromagnetic radiation caused by thermal energy
What is the purpose of a four-point resistance measurement?
It is a measuring technique which uses two separate pairs of current-carrying and voltage-sensing electrodes. It gives a much more accurate measurement than the two-point resistance measurement since it neglects the resistance of the contacts and leads.
What is the physical origin of thermal [Johnson] noise?
Johnson-Nyquist noise (thermal noise) is the electronic noise generated by the thermal agitation of the charge carriers (usually the electrons) inside an electrical conductor at equilibrium, which happens regardless of any applied voltage.
Name a real-world application of nuclear magnetic resonance technology and explain how it is applied.
MRI technology, modeling protein folding, drug screening and design
Geoffrey, an amateur quantum mechanic, was looking at the current in a photodiode. When he raised the intensity of the laser, he noted that the current also increased. Geoffrey concluded that energy of the light is dependent on the intensity of the light. Do you agree? Explain why or why not.
No, Geoffrey has no idea what he's talking about. The energy of a photon is dependent only on the frequency of the photon. He notices a higher current because there are more photons of the same energy hitting the metal.
What model is normally used to describe the vibrational energy levels of a molecule for small changes in the interatomic distances?
Quantum harmonic oscillator
What type of bonding exists in semiconductors and how does that contribute to its conductivity?
Semiconductors form a lattice similar to conductors, but their atoms are locked in covalent bonds. In general, this decreases the overall conductivity compared to conductors. The resistivity can be changed by either exciting the electrons or introducing imperfections into the lattice, which is called doping.
General Relativity
Spacetime is smooth Due to the Heisenberg Uncertainty Principle, there is a limit to the smoothness of spacetime Given a very small portion of spacetime, this principle will cause relative unevenness The smallest length is approximately 10^-50 meters Planck length is 10^-35 m (maybe this is the smallest length?) This is the minimum wavelength Since wavelength is inversely proportional to temperature, this gives a theoretical maximum temperature
Give one example of a natural phenomenon that can be explained by Rayleigh Scattering, a form of dipole radiation?
Sunlight scattering on particles in the atmosphere, which causes emission largely of blue light
Why does the Compton Edge occur?
The Compton Edge occurs at the max energy value that is scattered by a photon that does not get absorbed. It is the highest point before the energy spectrum dips before the peak energy value corresponding to an absorbed photon.
Under what conditions does the quantum harmonic oscillator model not accurately describe the vibration motion of the diatomic molecule?
The QHO does not accurately describe the vibration motion of the diatomic molecule when the equilibrium bond distance is large or when the molecule is at a high electronic energy level.
At very low currents, random fluctuations (noise) can become significant. Explain what this can tell us about the magnitude of the electron charge.
The charge of an electron must be quantized, so that at low currents you see noise because there are differences in the number of individual electrons flowing.
What is the flaw in Rayleigh-Jeans' classical model and how did Planck correct it with Quantum Mechanics?
The classical model allows molecules to absorb energy at any frequency, and Planck corrected this by showing how energy is quantized. The classical model predicts that particles would have infinite energy as wavelength decreases, while the quantum model accurately predicts the energy at all wavelengths.
According to the Stefan-Boltzmann law, how is the intensity of thermal radiation related to the temperature?
The temperature to the fourth power, specifically the delta.
What corrections were necessary to get the correct shape of the Blackbody Spectrum from the voltages measured using different filters?
The three corrections were transmission of filter, distribution of filter (half bandwidth and peak wavelength), and responsivity of the photodiode detector.
Explain what T2 is in a pulsed NMR experiment.
The time constant for the exponential loss of xy magnetization of the dephasing of two staggered spins.
Why does the photoelectric effect support the particle description of light?
The wave description of light would suggest that energy of light would be dependent on the intensity (amplitude) of the wave. But we notice that the maximum kinetic energy of electrons struck by light is constant at a given frequency regardless of the intensity. Furthermore, we notice that when hit by low intensity light above a threshold frequency, electrons are ejected immediately. This could only be explained by a particle of light hitting an electron, since a low intensity wave of energy would imply it needs a longer delay of time before any electrons can be excited.
In Laser Induced Fluorescence, will the emitted photons have longer or shorter wavelengths than those of the initial laser beam?
The wavelength will be longer than the wavelength of the initial beam. The emitted photons will have a lower energy and thus a larger wavelength
When doing NMR of a fluid, what is the relationship between the diffusion coefficient of the probed molecules and the output height in a pulsed spin-echo experiment?
There is an exponential relationship that is a factor to the effects of spin-spin relaxation.
What happens to the junction in a p-n junction when you apply a forward voltage? Reverse voltage?
When you apply a forward voltage, the holes on the p side and the electrons on the n side are repelled away from the poles, shrinking the depletion region and allowing the ions to jump across the barrier; this allows a current to flow. When you apply a reverse voltage, the ions are attracted to their poles, widening the depletion region: that makes it harder for ions to cross the junction = no current.