Sankaran
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What is $v \, dp$ work and when do I use it?
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15 votes

$PdV$ is boundary work. $VdP$ is isentropic shaft work in pumps (as you have identified above), gas turbines, etc. Now you must realize that even in a pump or turbine the mechanism of work is still $...

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How can one say that if we subtract "TS" from "U" then what we get is free energy?
11 votes

Lets start with the definition of free energy. Free energy is the maximum work you can get out of a system subject to thermodynamic equilibration under some constraints. These constraints could be ...

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For an isolated system, can the entropy decrease or increase?
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5 votes

Isolated system: Since the matter, energy, and momentum is fixed, the total number of microstates available that satisfy these constraints is fixed/constant. So is the entropy constant? Yes, if the ...

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Why do hot objects prefer to emit photons over electrons ? Is there electron-positron annihilation?
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5 votes

The energy levels in any object are quantized. The ground state of an electron is the lowest energy. From there electrons can have many higher energy levels, the highest being that which allows them ...

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Why aren't two systems in thermal equilibrium the same as one system?
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5 votes

In this example both the systems are of the same type of particles (with two energy states) and same number of particles. Therefore thermal equilibrium is defined when energy is equally shared between ...

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How do the molecules of reacting compounds proceed to form "Most Stable" molecule?
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4 votes

Molecules don't know. Consider the following reaction as a template for some reaction that is favored to go in the direction indicated. \begin{align*} A-B +C \rightarrow A-C +B \end{align*} In a large ...

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Does electric potential have a temperature?
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4 votes

The $3/2kT$ is the expectation value of kinetic energy of non-interacting particles in thermodynamic equilibrium, i.e., particles with random velocities in an equilibrium probability distribution. It ...

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Energy equation for an open system
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3 votes

The key to explaining this lies in two pieces that you have correctly written in the equation but have to make the student appreciate: 1) The difference between a $d$ and a $\delta$ $d$ means a ...

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In mechanics, is shock really better expressed as jerk instead of acceleration?
3 votes

Shocks by definitions are discontinuous therefore non-differentiable jumps in the relevant quantities (e.g., pressure jump across a shock mechanical shock in gas). So strictly speaking I think one ...

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When driving uphill why can't I reach a velocity that I would have been able to maintain if I started with it?
3 votes

Here is my take on it. It is somewhat similar to what many others have said but I will try to explain in greater detail, so bear with me in terms of length. Power from engine First let us understand ...

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Entropy exchange of a free fall
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3 votes

In reality the sandbag is going transfer energy via drag to air, work on the ground during impact (by denting the ground), and heat transfer to the ground etc., but I am going to assume your textbook ...

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How can electrons be confined in Quantum dots?
3 votes

My answer is not very different from John's and maybe a little tangential, but just offering my take. Electrons are confined in any system,e.g., a packet of liquid, solid etc. Just the fact that there ...

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Is there an alternative metric for isentropic efficiency that remains valid when broken up into multiple segments?
2 votes

I think the answer to your question is: Polytropic efficiency, it is differential isentropic efficiency and widely used in gas turbine literature. Let me know if you have trouble finding details.

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Why is electrical energy so difficult to store?
2 votes

First, electricity is the flow of electric charges. That is, by definition it is not a stored form of energy but a flux. What you store is always internal energy: energy in the nucleus, electronic ...

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Why do we need different ensembles in statistical mechanics?
2 votes

Another way to say this is as follows: 1) Microcanonical Ensemble: Isolated system -- No transfer energy in any form (as heat, as work, with matter, or as radiation) 2) Canonnical Ensemble: Closed ...

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What is evidence for an irreversible change?
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2 votes

naI think what you are essentially asking is that, "Can we violate or circumvent the second law of thermodyanmics?" The answer is no, based on all the physics we know so far and observations we have ...

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Running a stirling engine in reverse?
2 votes

Any heat engine can be reversed to make a heat pump. Going further, you can use any engine to create any gradient (thermal gradient, chemical gradient). But essentially what you are doing by turning ...

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Irreversible process
2 votes

For systems with fixed amount of gas (closed systems) you always need two variables to define any state: thermal and mechanical variables. Remember a thermodynamic state is usually defined by $n+2$ ...

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Is it wrong to associate non-isotropic flow high with Reynolds-Number and is there a better metric?
2 votes

All flows (high Re flows or not) are non-isotropic. Flow is a response to a non-equilibrium and occurs because of a gradient of some scalar quantity which has a preferred direction (because a ...

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With ideal gases, varying quantity of moles, and having a constant volume how do temperature and pressure behave?
2 votes

To define the instantaneous state of the gas (assuming its homogeneous and at equilibrium at any instant) you need two thermodynamic state variables besides the number of moles (or composition in ...

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Meaning of chemical equilibrium between two phases
1 votes

The answer to this starts from Gibbs-Duhem relationships that define the number of independent variables you have to completely characterize a thermodynamic systems. You can find this in textbooks or ...

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Adiabatic expansion of steam through a valve
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1 votes

Yes. Expansion valve has no work. So you have the final enthalpy (same as initial enthalpy) and pressure. That will give you $S_2$ from the steam tables :).

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What does net mechanical efficiency mean?
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1 votes

Mechanical efficiency is usually used as a metric to account for frictional losses in systems. For example, the transmission of a car transmits mechanical work from the engine to the wheels so the ...

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How can two seas not mix?
1 votes

The mixed state is a thermodynamic equilibrium state and unmixed a non-equilibrium state. A non-equilibrium state can only be maintained if there is energy flux into and out of the system. In this ...

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Are there still 'everyday' phenomena unexplained by Physics?
0 votes

Any problem with multi-body interactions has not really been explained. By that I mean anything which cannot be explained as an ideal gas, ideal of excitations (e.g., photon, phonons), or has well-...

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converting power spectrum to photon flux density
0 votes

It is not clear to me why you are converting one to the other. Here is my understanding, see if it helps: The power spectrum is like a histogram. The x axis is wavelength and the y axis has $\mu m^{-...

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Can the vibrational energy of a engine be used to increase efficiency?
0 votes

Physics allows four ways to transfer energy from any system (such as an engine). These are work,heat,matter,and radiation. Energy is lost in any of these forms. Normally when we talk of work from an ...

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