1,067 reputation
314
bio website
location
age
visits member for 2 years, 10 months
seen Oct 6 at 23:55

Jan
22
answered What are conditions for the existence of a critical value (for a phase transition)?
Jan
19
comment how to represent the effect of linking rigid-bodies together?
Why don't you just treat them as a single, large rigid body?
Jan
18
comment How to “read” the temperature of an abstract system?
When you couple your system to a bath, you actually allow it to exchange heat with it. The total system (your system and the bath) will be in the most probable state - the one with the highest entropy. Since the total entropy is the sum of the bath's and the system's entropy, the total entropy is maximal when $$\frac{\partial S_{bath}}{\partial E}=\frac{\partial S_{system}}{\partial E}\ .$$ This quantity is defined to be $1/T$, and the probability to find your system in an energy $E$ goes like $e^{-E/kT}$, as can be seen by taking a first order Taylor series around the maximum.
Jan
17
comment How to “read” the temperature of an abstract system?
forget what I wrote about f. It is confusing and wrong. In this formulation the free energy is given by $f=-k_bT\log Z=k_bT\log \sum_i e^{-\beta E_i}$, and the mean energy is calculated by averaging $\langle E\rangle=Z^{-1}\sum E_i e^{-\beta E_i}=-\partial_\beta \log(Z)$
Jan
17
revised How to “read” the temperature of an abstract system?
erased wrong statement
Jan
17
comment How to “read” the temperature of an abstract system?
The point is showing that coupling your system to an external bath results in a probability that goes like $e^{-\beta E}$, for some $\beta$. This is te definition of $T$, and is crux of the matter. Until you've shown this you don't know that "T will just take its value" from the bath, because $T$ is not even defined.
Jan
17
revised How to “read” the temperature of an abstract system?
book reference
Jan
17
comment How to “read” the temperature of an abstract system?
I posted a real, elaborate answer. Hope that gets the job done.
Jan
17
answered How to “read” the temperature of an abstract system?
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?