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seen Apr 8 at 7:26

Jan
17
revised Do amorphous metals undergo conchoidal fracture?
added 100 characters in body
Jan
17
comment How to “read” the temperature of an abstract system?
Systems flow towards probable states, that is - states that have a large number of micro-states. The (log of the) number of micro states is the entropy. So systems "want" to be in (="go to") states with high entropy, but that usually means states with high energy. The trade-off between entropy and energy is exactly the temperature. In evaporating liquid, for example, the vapor state has a higher entropy, but also a higher energy. Therefore, at low $T$ the system is liquid, but when $T$ is high enough the system prefers to be at a higher energetic state (="pay") because its entropy is higher.
Jan
16
answered Do amorphous metals undergo conchoidal fracture?
Jan
16
comment How to “read” the temperature of an abstract system?
You wrote the answer. This is the definition of $T$. In the Ising model. for example, there is no sense in talking about "average kinetic energy of the spins" or anything of that sort. I'd say that a good way to think about the temperature in this case is "how far above the ground state can I go", which is roughly equivalent to "how much energy can I pay in order to buy some entropy"?
Jan
16
answered Hydrostatic equilibrium of a star derivation
Jan
12
revised Do all closed systems, only considering kinematic/mechanical principles, exhibit time reversal symmetry?
external magnetic field (thanks to Ron maimon)
Jan
12
comment Do all closed systems, only considering kinematic/mechanical principles, exhibit time reversal symmetry?
@Ron Maimon. You are, of course, right about that, but this is a minor point in my answer. I edited it now.
Jan
11
comment Why doesn't phase space contain acceleration/forces?
This is wrong - it's only good for an infinitesimal $\delta t$ later (i.e. to first order in $\delta t$). If you want to know the trajectory you need to know the accelerations. But these can be calculated from the position and velocity.
Jan
11
comment Do all closed systems, only considering kinematic/mechanical principles, exhibit time reversal symmetry?
Strictly speaking, in classical mechanics the answer is that the dynamics are fully reversible. However, In the real world you are bounded, even theoretically, by the uncertainty principle, not to mention the outrageous impossibleness of reconstructing the the system with reversed velocities. Also, many-body dynamics are generically chaotic, and infinitesimal deviations of initial conditions will result in a significantly different evolution.
Jan
11
answered Do all closed systems, only considering kinematic/mechanical principles, exhibit time reversal symmetry?
Jan
10
answered Why doesn't phase space contain acceleration/forces?
Jan
9
awarded  Commentator
Jan
9
comment Calculating Fraunhofer diffraction patterns
The homework tag was not there, if I recall correctly, but even if it was, I was not aware of this policy. I will abide in the future...
Jan
8
answered Calculating Fraunhofer diffraction patterns
Jan
4
comment Which symmetry is associated with conservation of flux?
Could you give a different example? The example you gave is not a conservation law, but merely an integral formulation of the fact that $\nabla^2 \phi=\rho$. Also, when introducing relativistic effects, it is not true anymore.
Jan
3
awarded  Analytical
Jan
2
answered What is the rationale behind representing a state function by a complex valued function in QM?
Jan
2
comment Conjugate Variables and Fourier Transforms in Classical Physics
Why do you say that this is the case in QM? Very loosely speaking, the momentum is the Fourier transform of $q$, not of $V$.
Jan
2
revised What does a unitary transformation mean in the context of an evolution equation?
forgot a kronecker delta
Dec
28
answered Determine the tensor of contraint and deformation of a cube under compression