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visits member for 2 years, 8 months
seen 4 hours ago

May
19
comment About field gradient
Hi @victorbg, "gradient" can mean many different things. If someone is reading about a "gradient in the magnetic field", they probably mean "gradient" in the more general/english sense of the word, not the "turns a scalar field to a vector field" sense of the word. So, for example, if someone had an experiment with a "large magnetic field gradient", I would think something like the magnets involved in the Stern-Gerlach experiment en.wikipedia.org/wiki/Stern%E2%80%93Gerlach_experiment . Also relevant: en.wikipedia.org/wiki/…
May
18
comment Can Lagrangian mechanics be justified without referring to Newtonian mechanics?
It might also be worth mentioning what L&L do in the first book. IIRC they get pretty far just assuming the eqs of motion stem from an action principle & respect galilean invariance.
May
17
comment Relativistic rigid motion
@facenian It's true that you'd have to take care to avoid letting particles travel faster than the speed of light. Other than that, there is no contradiction.
May
11
comment Took a picture of my laptop screen with my iPhone. The blue pattern in the image seems to magnetic lines. How is this possible?
Was your camera flash on? Also, can you check if the same pattern appears if you take a picture of a piece of paper or some low light/contrast thing?
May
7
comment Simultaneity and quantum indeterminism
Hi @MoziburUllah, in the terminology anyone is using if they say "relativity of simultaneity", events are defined to be simultaneous if events $(t_1,x_1,y_1,z_1)$ and $(t_2,x_2,y_2,z_2)$ have $t_1=t_2$, which is of course not a lorentz-invariant concept, hence "relativity of simultaneity".
May
6
comment Simultaneity and quantum indeterminism
@JohnDuffield All the articles I get are about the constancy of the speed of light? Yes, if you don't assume the length of a meter (SI convention) is constant, you run into all sorts of problems.
May
6
comment Simultaneity and quantum indeterminism
@Abc2000ro Exactly - because the notion of simultaneity of two spacelike separated events has no inherent physical meaning. The physical meaning comes about when you establish real, physical clocks, and then collect the data at some point.
May
2
comment Do the same equations of motion imply the same Lagrangians?
I feel like this doesn't really answer the question. It gives a sufficient condition for generating the same equations of motion, but not (as AV23's answer shows) a necessary one.
Apr
26
comment RC Circuit Analysis
In its current phrasing this sounds an awful lot like you want us to answer a multiple choice homework problem. Could you expand with what parts confuse you?
Apr
24
comment How to simulate pendulum movement with high amplitude
khanacademy.org/computer-programming/forphys/6505289796943872 the "degrees" function turns radians into degrees, (multiplication by 180/Pi) which the trig functions need. no further explanation needed
Apr
23
comment Parabolic slide
@Shadock also, since $\dot{x}(x_0)=0$ you'll run into an integral with a singularity at one bound of integration, but it has to be an integrable singularity. If it puts you off too much just take $\dot{x}(x_0)=\varepsilon\to 0$.
Apr
16
comment Period of a simple pendulum accounting for friction
Hi, there are surely lots of answers to this question on the website: physics.stackexchange.com/q/140943 physics.stackexchange.com/q/20478 etc. The keyword is "damped harmonic oscillator".
Apr
12
comment How did Planck derive his formula $E=hf$?
Just as a lead: what I heard was that Planck (or maybe someone else) discovered an exact form of the blackbody radiation formula which goes like $\nu^3 (e^{C \nu}-1)^{-1}$. He then spent months working backwards from this formula to try to find what physical assumptions he needed to get it, and he found quantization of energy levels in an oscillator $E=n \nu h$ did the trick. (C depends on $h$, $T$ and $k_b$). This is consistent with what is on en.wikipedia.org/wiki/Planck_postulate
Apr
12
comment How does the magnetic field generated from a rectangular cross-sectional current-carrying conductor differ from a circular cross-sectional conductor?
@AdamM-W Oh, you want to know how to numerically calculate it? That might belong to a separate question. High precision would probably be totally useless for you, for engineering/practical calculations, because only the highest order term $\mu_0 I/(2 \pi r)$ will be significant. At most, you'd go to a second order term (in a "multipole expansion"). Anyways, the formula in the paper you linked is just the integral of the vector potential of a wire (the infinitesimal wire can be taken as an infinitesimal "current element").
Apr
6
comment Would a spinning, evenly charged sphere generate a magnetic field?
@HolgerFiedler the magnetic field of a charged spinning spherical shell is exactly like a point dipole outside the surface: hep.princeton.edu/~mcdonald/examples/rotatingshell.pdf and is not "along the z axis", and Marcel's answer does not state that. The equations you are using are confused and are used without meaning. The equation $\nabla \times B=\mu_0 J$ contains different physics than the Lorentz force law.
Apr
6
comment Would a spinning, evenly charged sphere generate a magnetic field?
@HolgerFiedler it DOES mean rigid body, but all the FORCES involved are supplied by the structural integrity of the field. Your equation has nothing to do with the field actually generated by the charges. The Lorentz force law does not give you the field generated by the charges.
Apr
6
comment Would a spinning, evenly charged sphere generate a magnetic field?
@HolgerFiedler your formula would give the needed $\vec{B}$ field to support the charges moving in circles, as if the charged sphere itself had no structural integrity. Which is not relevant to the question that was asked. (That's why people are saying you're wrong -- "F" is irrelevant to the question asked so there's no reason why "$F\cdot qv$" should mean anything, let alone be zero).
Apr
4
comment Translation Operators
Well in one $\delta x$ is an infinitesimal scalar, and in the other $\hat{x}$ is an operator with finite eigenvalues. So the first expansion is incorrect.
Mar
30
comment Physics without time
"obviously"? I don't agree that a photon must have a rest frame, nor that if it did it would have the properties you describe...
Mar
29
comment Starter's book on Quantum Mechanics (more general and simple books than full QM textbooks)
@LandosAdam it's possible I haven't read his quantum mechanics book closely enough and that it's much worse than his mechanics book, I'll take another look over it tomorrow at a library.