Recent content by Mayan Fung

Yes, I also found the idea of a diode, which is a passive component, showing a voltage reading strange. However, normal diagrams discussing the pn junctions usually shows a potential difference, just like the one here. It seems that from these diagrams, it is reasonable to measure the potential...

I mean I thought we can measure the built-in potential by directly connecting the diode with a voltmeter

I am thinking about the reason why we cannot probe the built-in potential across a diode with a voltmeter. Obviously, a diode is not an energy source, so it is impossible for it to show a voltage reading. After doing some research, I found some explanations and some questions about them. 1. The...

Ya, I also just thought of that. Thank you for your help.

you are right, that’s 7C2 not 7C3. So that’s 21

i guess you are saying N! However, only if the number of balls in every pile, then I will be counted for N! times. For example, (321) (312) (213) (231) (123) (132). But, for example, (300) is only counted for 3 times instead of 3!. Simply dividing by N! doesn’t work. I am afraid that I didn’t...

I shall rephrase it. Whether which pile is which is not important, therefore, (3,0,0) (0,3,0) and (0,0,3) are considered as one case. The number of balls in each pile is not fixed. The only variable here is N and M

If there are N distinguishable boxes and M indistinguishable balls, the answer is easy as it is equivalent to the combinations of arranging N 0s and (M-1) 1s in a queue. $$\binom{M+N-1}{N}$$ However, if the boxes are themselves indistinguishable (which I name them "piles" instead), how should...

When we learn about the scattering of a particle, the context is usually a charge shooting towards a Coulomb potential. With some assumptions, we can derive the scattering cross-section pretty reasonably. Therefore, the scattering cross-section of different elements in X-ray spectroscopy is...

@atyy , I see your point. But how about when we perform a measurement so that the wavefunction collapse? The electrons will be like a delta function at a particular position

In classical physics, we treat an electron as a point charge with a Coulomb potential ## V = \frac{q}{4\pi\epsilon_o r}##. However, in quantum mechanics, we treat it as an electron cloud. In this situation, how shall we describe the Coulomb potential? Shall we treat the electron as a charge...

Thanks! This makes me recall the fact that ##G=\mu N## and ##G=U-TS+\mu N##

It looks very easy at first glance. However, the variable S is a variable in the given expression. I have no clue to relate the partial derivatives to entropy and the number of particles.

Thank you all for the comments. Some of them are out of the scope of my original problem, yet they are very educational. I didn't intend to put much attention to the isotropy of a dielectric under relativistic conditions. My original attempt is just to study a light beam with a normal incidence...

I once naively think that the speed of light is also a constant in a medium in all inertial frames which is not the case. I tried to derive the result yet there is a discrepancy from the results I read in some articles. For example, from [Link to unpublished paper redacted by the Mentors], the...