Recent content by Cathr

Summary:: <mentor moved to general discussion> I mean of the discovery of the Graviton or of any other unification theory that would explain gravity and would be compatible with quantum mechanics. And by impact I don't necessarily mean just the understanding of the world - this is implicit -...

The universe expansion is not constant, it is actually accelerating. So there is no constant value of the Hubble constant, and it's rate of change is not known neither. A lot more data is needed. Yes. Actually, when making his theory of general relativity, Einstein had to introduce a...

Recently I was thinking about the Hubble's constant (which, actually, is not Hubble's and not constant...) and wondering: if the universe is expanding at 70 km/s each Mpc, then there's possible to calculate some expansion of space, say from me to a person 1 meter away from me (theoretically)...

I apologize, the formula is actually ##Q_{ij}= \frac{m}{2} \dot x_i \dot x_j + \frac{k}{2} x_i x_j## .

Let ##Q_ik## be a symetric tensor, so that ##Q_ik= \frac{m}{2} \dot x_i \dot x_j + \frac{k}{2} x_i x_j## (here k is also a sub, couldn't do it better with LaTeX). How do we derive such a tensor, with respect to time? And what could such a tensor mean in a physical sense? It really looks like the...

Thank you! I found a solution, and, if you don't mind, I have a question that is rather out of personal interest. How can I calculate the amplitude of a sound wave that gets out of the tube, comparing to the one that enters? Direct application: Suppose I don't like the sound intensity of my...

Then dS is zero, so I guess it would be the same to write partial derivatives as dT/dE, but I guess you were looking for another answer. I don't really know.

Please help me with this problem I am facing, I am lacking notions of acoustics and I would be very grateful if someone could clarify them: A tube has a revolution symmetry arounf the ##x## axis and has a section dependent of the value of the abscissa (x), so the profile ##S(x)## is known. The...

I see, but why is it dU=TdS+AdE and not -AdE, like in -pdV? For dF, dH, dG I get the right answers, thank you! I think dS(T, E)=(dS/dT)dT+(dS/dE)dE.

I already found a solution, thank you! I might as well delete the thread. But, if you don't mind, please look at my other post on thermodynamics. I have an exam tomorrow and I would be really grateful if someone could help!

Homework Statement I am always confused about how to correctly write the functions U, H, F, G when they're not depending on the usual variables p, V, T, S - same question for Q and W. For example, we have to calculate the temperature variation of a small surface of water when we isentropically...

Homework Statement We have an incident electric field, and there are two cases: 1) the field is polasised perpendicularly to the incidence plane (TE) 2) polarised in the plane (TM) Here I must be able to correctly apply the limit conditions, to find the Fresnel formulas that give the...

I'm studying for an exam (so this is existential) and I never really got how to write the limit conditions for an interface. In my problem, there is an incident electric field, and there are two cases: 1) the field is polasised perpendicularly to the incidence plane (TE) 2) polarised in the...

Thank you! Now it's crystal clear :)

I see. I think the problem is I don't really understand what we do with the ##\frac {\partial f}{\partial y}dy## part when we integrate with respect to x. Even if it is a dy differential, it still depends of x, but we say it's a constant. I guess the right way to do it is to ignore the dy part...