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Dec
20
comment The status of the BICEP2 'discovery' after Planck 2014
Related: physics.stackexchange.com/q/103951
Dec
4
comment Questions concerning some parts of the section on one-particle states in Weinberg's first volume on QFT
@ZhengLiu, you're welcome. Yes, the basis $\beta$ is orthonormal in the sense of eq. 2.5.19. $N(p)$ must be different from 1 because the argument of the delta-function changes from the standard momentum $k$ (in eq. 2.5.5 and 2.5.14) to an arbitrary momentum $p$ in eq. 2.5.19. The text between eqns 2.5.14 and 2.5.19 works out the proportionality factor between these two delta-functions, and $N(p)$ is selected to cancel that factor.
Nov
7
comment Deriving Heaviside-Feynman formula for the electric field of an arbitrarily moving charge from Lienard-Wiechert potential
@guillefix, ok I see. I should have read more closely before piping up. Nice approach! (But how can you not trust something by someone named Field?)
Nov
7
comment Feynman's proof for Liénard-Wiechert's potential of a moving charge
@guillefix, think I got it. Thanks again for the catch.
Nov
6
comment Feynman's proof for Liénard-Wiechert's potential of a moving charge
@guillefix, thanks! I see now I got sloppy with $c$ in a couple places. (I understand better now why people set $c=1$!) I will patch it today or tomorrow. (I think I need more time than I've got right now, to avoid making another goof.)
Nov
5
comment Where's the energy in a boosted capacitor?
@BenCrowell, thank you for this comment (and sorry for my earlier snark). I think there is a way around the divergence-issue for electrostatic systems, which I have added to my answer (in a major re-write). I hope you can take a look. Thanks again.
Nov
5
comment Where's the energy in a boosted capacitor?
OK I see what you're doing. I should have read your post and previous comments more carefully before commenting; I see you've explained all this before. Sorry. Thanks for being patient.
Nov
4
comment Where's the energy in a boosted capacitor?
I think your argument is undermined by an incorrect formulation of the field energy: $\int T^{00} d^3x$ is not covariant. It should be $\int T^{0 \nu} n_\nu (\gamma d^3x)$.
Nov
3
comment What does a relativistically moving capacitor “look” like?
@BenCrowell, you are correct, thanks. The important thing is to get the kinematic factors right.
Oct
31
comment Is there an intuitive explanation for why Lorentz force is perpendicular to a particle's velocity and the magnetic field?
@RobJeffries, Nice! Thanks for the info. "Nulling" set-ups are very attractive; er, well, you know what I mean...
Oct
28
comment Perceived acceleration in an artificially modulated gravitational field
@Hypnosifl, I think the thing about tidal forces is you need (at least) two particles separated in space-time, and the tidal forces decrease with the separation. (That's why I specified a "test particle".) You're right that the proverbial astronaut falling into a black hole will be spaghetti-fied.
Oct
28
comment Perceived acceleration in an artificially modulated gravitational field
@DavidHammen, more's the pity. It seems like one of those things that just ought to be true (imho).
Oct
28
comment Perceived acceleration in an artificially modulated gravitational field
Mach's principle!
Oct
27
comment Reading the Feynman lectures in 2012
@Physikslover, I'm not recommending Mead's monograph; as Ron Maimon mentioned, it wasn't particularly successful. I think the point was to "motivate" Maxwell's equations by showing how they emerge from something simpler. (Ron Maimon may have a different perspective; my default posture is to defer to him!)
Oct
26
comment what could generate a high-pitched whine in electronics where the frequency depends on the current?
@user6972, ok, got it.
Oct
24
comment what could generate a high-pitched whine in electronics where the frequency depends on the current?
@user6972, I concur about power supply switching frequencies in general; it all depends on the supply. However, high voltage supplies tend to run at lower frequencies because of the high effective capacitances presented by the high voltage transformer. Moreover: 1) I claim magnetostriction as part of the power supply, since the transformer is an element in it. 2) a steady sound usually requires a driving signal at the same or a related frequency, so just saying "magnetostriction" without identifying an underlying frequency source is not an answer, imho.
Oct
24
comment Physical explanation for capacitive circuit
@IncnisMrsi, I take your point that my answer could be elaborated. Would you care to add an answer that does so?
Oct
24
comment How does current flow from the emitter, through the base and to the collector in a NPN transistor?
@IncnisMrsi, updated per your recommendation. Thanks!
Oct
24
comment Reading the Feynman lectures in 2012
The potential-based approach to EM you mention may be "Collective Electrodynamics" by Feynman's student Carver Mead: en.wikipedia.org/wiki/Carver_Mead . PS: I recall you writing favorably of Steven Frautschi's S-Matrix book. I saw him recently; in retirement, now 80, he has re-invented himself as a teaching assistant, and won Caltech's Feynman Prize for Excellence in Teaching this year. When I mentioned his S-Matrix work being cited here, he ducked his head and said, "Well, that was a long time ago..."
Oct
23
comment Reverse bias P-N junction
@IncnisMrsi, 1) Thanks, my wording was poor; I've attempted to improve it. (I meant excess in comparison with the unbiased junction.) 2) I concur. Apparently this ideal diode characteristic works fairly well in germanium diodes (because $I_0$ is larger than for silicon?), but in Si, other mechanisms, like carrier generation in the transition region, cause the reverse-bias current to vary with voltage.