All Questions
Tagged with conservation-laws thermodynamics
50
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Which kinds of systems are described by a heat equation?
Every extensive thermodynamic variable has a continuity equation associated to it:
$$\frac{\partial \rho}{\partial t}+\vec \nabla \cdot \vec J=0$$
where $\rho$ is the density of said variable and $\...
- 1,616
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Why is the volume of an isolated system conserved? [closed]
Consider a system(S) + reservoir(R) that surrounds the system(S). The system(S) is not in thermodynamic equilibrium but the combined system (system + reservoir or say SR) is isolated i.e. the walls of ...
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Conservation of Extensive Quantities
Thermodynamic quantities are usually divided into two categories: extensive and intensive. The extensive category is sometimes modified to be an extensive density measured relative to unit mass or ...
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Physical interpretation regarding heat equation [closed]
I have included image of a problem from Oxford ( from a Math course).
I was able to do the question. However, i am stuck at the last part which asks about the physical interpretation. I don't have a ...
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Could gravity be used to cool down matter?
Thermal energy being the movements of particles, could we have a system that could use gravity to reduce the thermal energy of particles?
For example, if we imagine:
A box containing Argon in its ...
- 101
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Conservation of entropy in cosmology
I've been trying to follow the procedure that some books give in order to prove that the entropy of the universe is conserved (S is constant).
It usually goes like this:
Consider the second law of ...
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1
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How does the density of neutrinos travelling at approximately light speed depend on the scale factor?
I am working through Concepts in Thermal physics by Blundell & Blundell. In chapter 25.1, they derive that in the case of a relativistic gas, the density scales not with $a^{-3}$ but with $a^{-4}$ ...
- 546
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How can the entropy of an isolated system possibly change (if we consider the fundamental equation)?
By the definition of an isolated system, $dE = dN = dV = 0$ (energy, no. of particles, and volume).
If we assume that a macrostate has a unique ($E$, $V$, $N$) then we can write
$$dS = \frac1T\cdot dE ...
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Does all the electrical power I put into my motor turn into thermal energy in the end?
I have a closed area and I need to calculate the heat load for HVAC purposes.
In this system I have lots of electrical motors that move a mass back and forth on linear rails.
It's my understanding via ...
- 113
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Problem with the continuity equation for an electron gas
Consider the continuity equation for an electron gas:
$$\tag{1}
\nabla \cdot\left[n(\boldsymbol{r}, t) \frac{\partial}{\partial t} \tilde{\boldsymbol{r}}(\boldsymbol{r}, t)\right]=-\frac{\partial}{\...
- 261
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Differences in using Clausius-Duhem inequality vs Principle of Virtual Work/Power in derriving constitutive equations?
I am a novice getting my toes wet in continuum mechanics and nonlinear elasticity. I have seen papers that use both approaches in developing constitutive connections to compliment balance equations of ...
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Steam pressure and constant fuel temperature in steam turbines [closed]
I've heard that leaving a steam turbine unsupervised can lead to you losing your life.
The reason they state is that the heating medium (or whatever it's called) such as coal or gas, produces variable ...
- 340
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2
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Shouldn't change in entropy always be 0?
We know that entropy is defined as $$dS=\frac{dQ}T$$ Now in any isolated system, by definition $dQ=0$. So should entropy change in any isolated process or system always be 0?
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Does hero's engine have an upper limit for the rotational speed under ideal conditions?
This is a simple hero's engine. The source of the gas and the rotating part is seperated by a frictionless bearing. So only the upper part rotates. My question is this:
Given ideal conditions ( no ...
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1
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Are heat emissions as bad as greenhouse gasses?
Most of the technology we use generates excessive heat. Like cars, computers, smartphones, batteries etc... All of these needs to be cooled down in some way. Is all this extra heat adding to rising ...
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