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location Massachusetts
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visits member for 3 years, 11 months
seen 9 hours ago

May
26
awarded  Revival
May
11
comment Donors/Acceptors in Metal Oxides
Is it a well-known fact that Cr is substituting Ti? (Not substituting O, not interstitial, not defect complex, not a combination of the above?)
May
3
comment Relativistic origin of magnetic field
@ChrisWhite - No one is disputing that "Electromagnetism in its entirety can be derived from Coulomb's Law plus SR, full stop." But there's a difference between "X can be derived from Y" and "Y is a fundamental principle and X is its less-fundamental consequence". For example, you can "derive" Newton's law of gravity from Kepler's laws, even though the former is actually more fundamental. These so-called "derivations" are useful and healthy pedagogical exercises, sure, but sometimes they mislead people about the big-picture fundamental relationships between different physical laws.
May
3
comment Microscopic picture of an inductor
Current creates a magnetic field because current is moving charge, and moving charge creates a magnetic field. Are you asking: "Why does moving charge create a magnetic field?"
May
3
answered Microscopic picture of an inductor
Apr
29
answered How does a force on electrons produce a force on a metal plate
Apr
26
comment What is the reasoning behind hole carriers being able to carry heat?
I wrote a more detailed explanation of what holes are at physics.stackexchange.com/questions/10800/… - that may help answer your question.
Apr
26
answered Is light red shifted in optical tweezers?
Apr
25
answered Integrating factor $1/T$ in 2nd Law of Thermodynamics
Apr
16
answered To which real densities do carrier densities in the semi-classical model of a crystal correspond?
Apr
16
comment To which real densities do carrier densities in the semi-classical model of a crystal correspond?
Why do the density of electrons and holes depend on the corresponding effective masses? Are you assuming that the total mass of all electrons is equal to the total mass of all holes? Because that's not true.
Apr
16
awarded  Nice Answer
Apr
15
revised Physics Equations for Grad School / Physics GRE Prep
added 291 characters in body
Apr
15
comment Physics Equations for Grad School / Physics GRE Prep
An example of why this isn't true: From high school until right before the GRE, I never bothered to remember whether it's B = mu H or H = mu B. Then I memorized it for the GRE, and I've never had to look it up again. It just took a bit of mental effort -- very worthwhile. Knowing things like that is very very useful, because if you're calculating or thinking through something, you don't want to have to keep stopping to look up dumb things like that. Even if it only takes 20 seconds to look it up, that's enough to lose your train of thought.
Apr
15
answered Physics Equations for Grad School / Physics GRE Prep
Apr
12
comment Why isn't temperature measured in Joules?
You can define temperature as "The temperature of a body is the energy of an ideal 1D classical simple harmonic oscillator in thermal equilibrium with that body." Then there is no "per degree of freedom" in the definition -- we're just talking about an energy.
Apr
12
comment Why isn't temperature measured in Joules?
I disagree. You imply that if two things are measured in the same unit, then they have to be exactly the same thing. Therefore, you say, a unit system that measures temperature in J is not "correct". Well, I think that a knowledgeable person can understand that temperature is not exactly the same thing as total energy content, but can still measure both in J. (The advantage of using kelvin IMHO is not that it's more "correct", but to reduce the frequency of stupid confused mistakes and miscommunications -- which is still worthwhile!!)
Apr
10
answered Effects of surface roughness on specularity
Mar
25
answered Indirect band gap semiconductor for LEDs?
Mar
25
comment Fermi and Boltzmann distribution of carriers in semiconductor
For n-type, the criterion is "When any given conduction-band state has probability << 1 of having an electron in it." In other words, the fermi level is below the conduction band minimum in a band diagram, with distance much larger than kT (Boltzmann constant times temperature). How much larger than kT? Well, the larger it is, the more accurate the Boltzmann approximation is! There is no quantitative criterion that I can tell you: It depends on what you're calculating and how accurate you want the answer to be.