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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570 views

Specific heat of aluminium as a function of temperature

I am trying to find the curve describing the specific heat of aluminum as a function of temperature between about 50K to 300K(or room temperature) for an experiment I am doing. Ultimately I need to ...
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4answers
441 views

Can we detect whether food has previously been heated in a microwave oven?

An acquaintance told me that she refuses to eat microwaved food because she read that the microwave changes the molecules of the food somehow. Her statement is vague and her sources are dubious but I ...
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0answers
102 views

Lennard-Jones induced pseudo-molecules

It can be shown that the Lennard-Jones potential - which describes the interaction between particles in non-ideal gases - gives rise to pseudo-molecules: after a triple "collision" of three ...
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2answers
483 views

Can you extinguish a fire by cooling down the fuel?

I know that temperature plays a crucial role in the process of ignition, as most combustible materials will spontaneously start burning in presence of enough oxygen when heated above the kindling ...
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1answer
244 views

How do you explain the fact that when air expands freely into an evacuated chamber from a constant pressure atmosphere, its temperature increases?

I came across this paper: Baker, B. (1999). An easy to perform but often counterintuitive demonstration of gas expansion. American Journal of Physics, 67(8), 712-713. ...
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2answers
509 views

Physical significance of negative temperature

I read some answers regarding negative temperatures but I think my question is new. I want to know that what is the physical significance of negative temperature. Suppose I say a body has ...
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2answers
219 views

Heating effects of microwaves

I understand the heating effect(dielectric heating) of microwaves. This heating is caused by using a frequency of 2.45 GHz and this is the same frequency at which Bluetooth works and the L and S ...
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1answer
104 views

Definition of entropy

In my textbook there is written $$\Delta S = \int_R \frac{\delta Q}{T}$$ where the $R$ means calculated along a reversible transformation. The variation of entropy only depends upon the ...
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2answers
653 views

Why is the direction of a flame is always upward? [duplicate]

why a candle's flame direction is always upward? What is the coposition of fire? Don't say it's energy. There is something else.
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3answers
334 views

Why does ice melts, waits for 100 degrees and THEN vaporises? Why is not the process of expansion of things continuous?

What I am asking is this: Why can't a body be solid, then solid-ish, then solid-like, then liquid-like, then liquid-ish, then liquid, then vapor-like and then vapor? Why is there a rigid temperature ...
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1answer
550 views

Meaning of heat sink in engine analysis [closed]

I am confused with the meaning of sink. What part of vehicle engine acts as reservoir and what does as sink?
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1answer
150 views

Comparing unknown temperature scales [closed]

Source: Principles of Physics by Resnick, Halliday, Walker. $9^{th}$ edition. Chapter 18. Problem 6. On a linear $X$ temperature scale, water freezes at $-125.0^0X$ and boils at $360.0^0X$. On a ...
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5answers
12k views

Why does a candle blow out when we blow on it? Our breath is 16% oxygen and only 4% CO2

Don't say that a layer of carbon dioxide covers the flame, because our breath has more oxygen than carbon dioxide. Also, our breath does not cool the flame as it is itself warm. So what is happening ...
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1answer
143 views

Doubts about the ideal gas law

EDITED AND SOLUTION: In fact my stupid mistake was to take the wrong value for my $P_1$ and I was getting an 1.2998 that finally is a 1,9178 within just a difference about 7%. So at the end they are ...
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1answer
360 views

Understanding collision terms in Boltzmann equation

I am reading a paper that deals with the Boltzmann equation. They add a collision which is supposed to account for collisions which happen when particles are within a radius of $d$ from each other. ...
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0answers
40 views

Explain Heat Transfer

I would like to know what are these formulas used for. There is no intro about it in my book at all, and I am reading Heat Transfer book. If needed Q. can be edited.
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1answer
158 views

Does thermal energy include the kinetic energy of the electrons?

Or is the kinetic energy of the electrons not counted towards the thermal energy? (In other words only the energy of the lattice structure is counted)
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0answers
58 views

What's the connection between being homogeneous in energy and having a proper time scale?

Towards the end of page 8 of this scientific paper, I have found the following sentence: ...given the fact that the system is homogeneous in energy, or equivalently, that it has no proper time ...
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0answers
254 views

Determine pipe outlet temperature without length?

Let's say I am flowing a fluid through a pipe that has a constant surface temperature. Is it possible to determine the heat flux on the inner surface without knowing the pipe's length when the only ...
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2answers
114 views

Container divided in two parts with ideal gas

I wanted to ask your help with the following problem: Consider a closed container, that is, with insulated walls impervious, and rigid. The container is divided into two parts by a wall having the ...
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0answers
59 views

Applied / environmental question: direction of exhaust fumes

I'm not sure the Physics StackExchange is the perfect place for this environmental/applied physics question, but as I found no forum more fitting I ask my question here. Otherwise please move my ...
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3answers
195 views

Turbofan: Efficiency wrt. the bypass ratio: “a lot of slow air > a little faster air”?

Reading / viewing up on how jet engines work, this video explains at the 9:02 mark that, for turbofan engines, ".. it is more aerodynamically efficient to have a lot of air moving relatively slowly ...
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1answer
86 views

Why is $\left.\frac{\partial C_V}{\partial V}\right|_T$ different in these derivations?

I want to show that $\delta q$ is not an exact differential. Starting from $dE = \delta q - pdV$ and because $E := E(V, T)$ is a state function, which allows to express the exact differential as $$ ...
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1answer
66 views

Density as a function of Temperature?

I know that volume = mass/density....and density is the function of temperature. Is there any standard equation that describes density as a function of temperature? The system I am interested in is ...
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2answers
315 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
57 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
51 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
644 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
126 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
49 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?
3
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2answers
92 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
586 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
848 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
172 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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3answers
2k views

What is happening as our Universe is expanding? Is entropy increasing or decreasing?

Scientists say that entropy of our universe is increasing as it is expanding and our universe is cooling down gradually from the time of it's birth.If something is getting cooler and cooler then how ...
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2answers
179 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
86 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
521 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
116 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
72 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
24 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
84 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
64 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
46 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
679 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
40 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 ...