Covers the study of (mostly homogeneous) macroscopic systems from a heat/energy/entropy point of view. Maybe combine with [tag:statistical-mechanics].

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330 views

What is the resolution to Gibb's paradox?

This question is essentially a duplicate of Gibbs Paradox - why should the change in entropy be zero?. The question concerns the following situation: I have some gas of identical particles and they ...
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0answers
58 views

How hot is it on the slopes of an erupting volcano?

I was watching The Return of the King and just after Frodo and Sam destroy the One Ring, they run out of the mountain and hide on top of a rock as the volcano erupts and lava flows past them. Is this ...
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0answers
29 views

where i can find thermodynamics properties of octane

where can I find thermodynamics properties for octane at $T_s$? I need this properties or where can i find all ? $$liquid-density=\rho_l=?$$ $$vapor-density=\rho_v=?$$ ...
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0answers
30 views

Experimental results regarding non-extensivity in small systems

From a statistical mechanics point of view, the extensivity of thermodynamics arises from the fact that we can neglect interaction terms in the Hamiltonian. This allows us to infer that when we put ...
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1answer
54 views

What is the equivalent of a eutectic point for a (two-liquid)+solids system?

I deal with complex phase equilibria involving solid and liquid phases at high temperature. Below is a simplified temperature-XH2O diagram for a multicomponent system at constant pressure. (Note: ...
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2answers
684 views

Auto-refrigeration

I'm reading about the auto-refrigeration effect and can't find a really good explanation. Is the idea that when you have your condensed liquid, and then release it into a low pressure environment, ...
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1answer
129 views

Fluid Temperature as a function of Mass and Ambient Temperature

Problem I am trying to create an equation to calculate fuel temperature based on ambient temperature, heat exchange rate, and mass. The idea is very simple; when the mass is heated, depending on how ...
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1answer
50 views

Thermodynamics of zinc chloride to zinc

I am trying to deposit zinc metal from zinc chloride using chemical vapor deposition. But it is thermodynamically feasible to obtain zinc from zinc chloride?
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2answers
93 views

Prove identity of partial derivatives

I can not do the following problem: Prove the identity: $$\left( \frac{\partial x}{\partial y} \right)_{z}\left( \frac{\partial y}{\partial z} \right)_{x}\left( \frac{\partial z}{\partial x} ...
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3answers
1k views

Does evaporation decrease entropy?

I have a container of water in thermal equilibrium (there are no temperature fluctuations inside the container). Some molecules will evaporate out of the container of water thus decreasing its ...
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3answers
630 views

work equals heat in isothermal process only for ideal gas?

Is it only for an ideal gas that the work equals negative the heat in an isothermal process or is this general for all kinds of systems? Or put differently, how do I know that for an ideal gas the ...
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2answers
883 views

Why does my kettle only make a noise when it is turned on

Almost as soon as I turn my kettle on it starts to make the familiar kettle noise, yet very shortly after turning off the power the boiling noise stops and the kettle is totally silent. The ...
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2answers
187 views

How does electric current generate thermal energy?

How does free electrons moving through a wire cause random vibrational motion of the positive ions?
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5answers
2k views

Does entropy increase or decrease as our Universe is expanding?

Scientists say that entropy of our universe is increasing as it is expanding and our universe is cooling down gradually from the time of its birth. If something is getting cooler and cooler, then how ...
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2answers
184 views

Inequality of Clausius

I'm having some trouble understanding the inequality of Clausius. My treatment of the inequality applies to an engine operating in a cycle between two temperatures (i.e. like a Carnot engine). The ...
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2answers
87 views

Coupling constant is turned off adiabatically?

To me, adiabatic processes are idealisation. What do people mean with statements such as: "turning off the coupling constant (in QED say) adiabatically"?
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2answers
568 views

Why does welding produce UV light?

Looking directly at a welder is dangerous because large amounts of UV light is produced. What makes this light? Is it electrons from the current that excites metal atoms, and these atoms sends out UV ...
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0answers
121 views

Questions about the second law of thermodynamics

I can not understand the second law of thermodynamics and have trouble explaining the following questions: 1) In the tropics the water near the ocean surface is warmer than the deep water. Does the ...
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0answers
73 views

Derivation of cylindrical line heat source problem?

I have a line heat source embedded inside a cylinder and I am trying to find the temperature distribution T(r,t). By using the similarity variable, the solution to the differential equation is ...
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1answer
25 views

Delayed Phase Transition at Condensation of a Gas?

It is known that super heated liquids do exist, eg. when purified water is carefully heated above its boiling point. However I've never heard of subcooled gas having a temperature below its saturation ...
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2answers
2k views

How does heat pass through glass?

So infra-red and ultra-violet waves from the sun heat up the glass by conduction then it then radiates this heat to heat up a room or a car? Is that right? How would one calculate how much heat a ...
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1answer
85 views

Thermal Dynamics Conduction Comparison

I didn't have to take much physics for my advanced college degree, so I apologize if my question is painful. From reading and experience it seems that water is a better thermal conductor than air. I ...
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0answers
66 views

Medical/Physics Question - Cut Off Finger Preservation

This question might require some knowledge of medical/tissue, but physicists seem to know everything, so figured I've give it a shot here ;) When people put cut off fingers they are told to put the ...
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0answers
47 views

Vacancy Generation / Annihilation Time (Relaxation Time)

Vacancy Generation/Annihilation Time, Recombination Time and Relaxation Time ($\tau$) are all synonymously used in atomic physics literatures. They're defined as the time that it takes for vacancies ...
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1answer
711 views

What do we mean when we talk about Gibbs Free Energy?

Before I start, I'm aware that this question may be better suited on the Chemistry or Biology site, but it's my belief that physicists are more likely to have a clear understanding on what certain ...
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2answers
41 views

Infinite heat from gas?

Does any type of gas keep the same temperature if is constantly exposed to same force? In other words, must we change the applied force in some moment to get heat again, or is the same constant force ...
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1answer
101 views

How do we know that the Virial Expansion exists?

How do we know that the Virial Expansion exists? How do we know that we may always write $\frac{p}{kT}$ as a power series in $\frac{N}{V}$? That is, how do we know that there exists $B_{i}$ so that ...
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1answer
84 views

Why is $B(T)\approx b(T-T_C)$ near critical point $T_C$ in Landau theory?

In Peskin&Schroeder page $270$ equation $(8.4)$ you see that they approximate the function $B(T)$ near the Curie temperature as $$B(T)\approx b(T-T_C)$$ i.e. they omit $B(T_C)$ in the Taylor ...
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2answers
3k views

How to calculate fluid's temperature change in a pump?

To calculate the pressure at the outlet of a pump we use pump performance characteristics i.e. charts giving pump head as a function of volumetric flow. When the fluid flows through a pump, it's ...
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2answers
117 views

Modelling multiple heat sources?

I am working on a project to model heat transfer in a data center. I have basic models for a one dimensional heat transfer using conduction and convection. Such as, taking a server and modelling the ...
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2answers
130 views

Work in Newtonian Mechanics and Thermodynamics

Well, is the basic difference between the work that we learn in Mechanics and that in Thermodynamics? This is because in Mechanics, whenever work of magnitude $W$ is done on a system $S$, then the ...
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3answers
336 views

How do greenhouse gases trap heat?

I am looking for a molecular-level understanding of the greenhouse effect. What is it about the carbon-dioxide molecule (and methane, and water, etc) that is different from other gasses ...
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0answers
269 views

Partition Function of a three state particle system

I've just finished studying the partition function of a two-state particle system, where particles can have a 0 energy value or E energy value . That is: Where $t_j$ is a variable of value ...
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4answers
652 views

Is vapour pressure based on partial pressures or just total pressure on the liquid?

The explanation for the boiling point of water is that at 100C, the vapour pressure becomes greater than atmospheric pressure. But say you had a a jar of water sealed in argon at 1atm, which is larger ...
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0answers
216 views

Finding internal energy using equation of state

I have an equation of state as a function of $p$ (pressure), $T$ (temperature) and $V$ (volume) for a real gas and need to find an expression for the internal energy $U$. I know this is given by the ...
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1answer
52 views

Can I apply symmetry to this boundary value problem (BVP)?

Let's say I have a hollow cylindrical shell with inner radius $a$ and outer radius $b$ and length $L$. The temperature at at z=L is $T_{2}$ and the temperature at z=0 is $T_{1}$. There is also an ...
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4answers
105 views

Bulk flow of air in a long tube between Antarctica and Australia?

I have a 5km diameter clear plastic tube which is open at each end and runs from the center of Antarctica to Lake Eyre in Australia. The tube is on the ground where it can be and at sea level on the ...
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3answers
361 views

(Why) would unmaintained water heater use more electrical energy?

I'm specifically thinking about lime/sedimentation at the bottom of water heater, and calcification of heating elements (and not possible thermal insulation deterioration). It is very often claimed ...
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1answer
6k views

Enthalpy Change in Reversible, Isothermal Expansion of Ideal Gas

For the reversible isothermal expansion of an ideal gas: $${∆H}={∆U}=0 \tag1$$ This is obvious for the case of internal energy because $${∆U} = \frac {3}{2} n R {∆T} = 0 \tag2$$ and $${∆U} = -C_P n ...
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2answers
1k views

Difference between pressure and stress tensor

What is the difference between hydrostatic pressure and stress tensor?
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2answers
709 views

Is cooling really exponential?

I did the following experiment: I took boiling water, put a thermomenter (from Vernier hooked up to a TInspire calculator) into it and pulled it out. Then we started measuring the temperature it read ...
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1answer
82 views

Law of equipartition

Law of equipartition predicts the heat capacity of gases correctly. It assumes that inter-molecular attraction in gases is negligible (which is true). But for solids, inter-molecular attraction is not ...
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5answers
2k views

Does it take infinite energy to create a perfect vacuum?

Question is inspired by a recent burst of perpetuum mobile-type questions. It would be nice if one could simply discard them all by an argument that shows it's impossible to create a perfect vacuum. ...
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3answers
160 views

Mechanism of Hawking's radiation and entropy of the black hole

During Hawking's radiation, a virtual particle with negative energy and mass (from pair of particle and antiparticle) fall into black hole and its real partner having positive energy escape from ...
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6answers
946 views

Why does the bathroom become hot after a bath?

I have noticed many times that whenever I enter into the bathroom just after someone else bathed the temperature inside it would be high. Is it that the body heat comes out when we pour a lot of ...
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2answers
68 views

Temperature limit on entropy of a paramagnet

We have $$S=Nk_B[\ln(2 \cosh(x)) - x \tanh(x)]$$ where $$x = \frac{\mu B}{k_BT}$$ In need to show that at low temperatures entropy $$S \approx Nk_B2xe^{-2x}$$ I wrote out the $\cosh(x)$ in terms of ...
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1answer
675 views

Otto cycle efficiency described in pressure ratio (instead of compression ratio)

The Otto cycle's thermodynamic efficiency is often described using the compression ratio: $$\eta_{th} = 1 - \left( \frac{1}{CR} \right ) ^ {k-1},$$ (where CR is the compression ratio, and k the ...
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1answer
90 views

Radiation and absorption

Can thermal radiation from a cooler object (B, which emits longer wavelength radiation) ever ADD to the overall thermal energy level of a warmer object (A, which emits shorter wavelength radiation)? ...
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2answers
67 views

Molecules of a solid [closed]

Question: Molecules of a solid : (a) are always in a state of motion (b) move only when heated (c) move because they are loosely bound (d) do not move at all My attempt: I ...
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3answers
2k views

Surface Area: Volume and its relation with heat

In biology, it is stated that more surface area and less volume helps for keeping bodies cool. How can one explain this phenomena in terms of physics?