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location Baltimore, MD
age 29
visits member for 3 years, 10 months
seen 5 hours ago

I'm a physics graduate student.


2d
comment Thermodynamics only deals with homogenous systems?
Would you like to give reasons to support your assertions?
Sep
5
comment How come the atmosphere moves with Earth?
possible duplicate of Why does the atmosphere rotate along with the earth?
Jun
30
comment How quickly should a fluid come to hydrostatic equilibrium?
Hi Graviton, thanks for your comments. I don't think I understand very well, yet, though. Where does the 4-6 microseconds come from, and why do viscosity and surface tension come into play?
May
11
comment Intuitively, why does removing solutes cost $k_B T$ of free energy per molecule?
These are equivalent way of saying the same thing. If we believe that entropy is extensive, then the entropy per ion must be intensive. That means it can only be a function of the concentration, not the volume. That is why it is not necessary to go through an argument about the volume of the tank and N!, etc. You just need to believe in extensivity of entropy.
May
11
comment Intuitively, why does removing solutes cost $k_B T$ of free energy per molecule?
momentum is unimportant because it doesn't change between the two reservoirs. I'm expecting the reader to use some discretion. This lets me avoid unnecessary lists of caveats and assumptions.
May
11
comment Intuitively, why does removing solutes cost $k_B T$ of free energy per molecule?
$e$ is the base of the natural logarithm. My expression for the entropy is obvious from extensivity. The problem with what you have is that the ions are indistinguisable, so you're missing a factor 1/N!. You can confirm that modifying your expression in this way is equivalent to what I said.
Feb
22
comment Entropy and probability
Pressure is a macroscopic concept. If we are considering the motion of the individual molecules in this way, pressure is not relevant.
Feb
9
comment Why does time reversibility imply equilibrium in a thermodynamic system?
possible duplicate of Intuitively, why is a reversible process one in which the system is always at equilibrium?
Feb
2
comment Why am I not burned by a strong wind?
@Anixx As the answer states, I am referring to energy density, not velocity.
Feb
1
comment Why am I not burned by a strong wind?
I was in the process of writing an answer when this question was closed. I would like the question re-opened because I find it extremely unlikely that the OP will understand the linked question to be equivalent, and also because my answer takes a considerably different tack than it would when answering the linked question.
Feb
1
comment Is thermodynamic reversibility a function of path?
@joshphysics good link. I updated the answer.
Feb
1
comment Is thermodynamic reversibility a function of path?
@Christoph Good point. I updated the answer.
Jan
27
comment Why and how is the speed of light in vacuum constant, i.e., independent of reference frame?
@JerrySchirmer To be honest I was describing physics that was beyond me. I simply remembered reading this in Feynman's QED. Looking it up, on pp 89 it says "The major contribution occurs at the conventional speed of light... but there is also an amplitude for light to go faster (or slower) than the conventional speed of light. You found out that in the last lecture that light doesn't go only in straight lines; now, you find out that it doesn't go only at the speed of light!" Maybe I misunderstand just what this means, though. I don't know quantum field theory.
Jan
19
comment Noether Theorem and Energy conservation in classical mechanics
pppqqq Is correct. Your procedure begins by assuming that for a given trajectory, the value of the Lagrangian is constant. I.E., you throw a ball through the air and over the course of its flight kinetic-potential energy stays constant. This is wrong, so of course the result you get is wrong.
Jan
14
comment Solids: is energy expended in some fashion by, say, a table to keep the top from sinking down against gravity?
That is very difficult to calculate. All I am saying is that things decay. Old buildings fall down, old statues rust, etc. Presumably a table will decay a bit more quickly with gravity than without it. There is not necessarily a constant energy input required. It is just the case that some non-zero amount of energy is required over long times in order to counter the table's gradual decay.
Jan
13
comment Solids: is energy expended in some fashion by, say, a table to keep the top from sinking down against gravity?
The energy of the table changes whenever its state changes. If the table breaks and collapses all at once, then that's how it happens. If it slowly sags with age, then that's how it happens. I don't understand the second question.
Jan
13
comment is action integral Lorentz invariant?
I'm not sure what the "first postulate" is, sorry. I'm saying that there is some $\mathbf{x}(\tau)$ which is the trajectory. It's a four-vector position as a function of proper time and is invariant. If you find it in two different frames, you'll get two different sets of coordinates describing the same $\mathbf{x}(\tau)$.
Nov
30
comment Do heavier objects fall faster?
No. The claim that heavier objects fall faster is not a claim that they have greater acceleration at a given distance from the Earth when viewed in an inertial frame. If you read the answers to the question you already linked, this is made clear.
Nov
30
comment Do heavier objects fall faster?
What about it? It's a correct statement. What do you want to know? Be more specific.
Nov
6
comment Does measuring the operator of a wave function collapse the wave function to the measured eigenstate?
If the $\phi$ are normalized, then your $\psi$ is not normalized, making this question difficult to understand.