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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Third Law of Thermodynamics

Does Third Law of thermodynamics violates Quantum Mechanics, I mean Why third law of Thermodynamics holds if we think about it in terms of Quantum Mechanics ?
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Mass, energy, and entropy.

I have a seemingly simple question about the relation between these three that for some reason doesn't make sense to me. If entropy is the disorder of a system, then a low entropy state is one of ...
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44 views

Thermodynamics : Work done on gas inside container by the force due to pressure exerted by external atmosphere

Suppose a container is fitted with a massless and frictionless piston lying on a table such that pressure due to gas is greater than pressure due to external atmosphere. Hence piston will move. Now ...
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Average surface temperature of Earth [duplicate]

I had a question in my school exam. Will the average surface temperature of the Earth be lower or higher, if there was no atmosphere? Now, the answer expected is "The avg temp will be lower, because ...
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Numerical Estimates for the fermi Energy [on hold]

The question is to give the numerical estimates for the Fermi Energy of electrons in a typical metal, nucleons in a heavy nucleus and $He^3$ atoms in a liquid $He^3$ and compare the results. I know ...
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What does this extraneous solution to an elementary thermodynamics problem represent?

I recently had the following homework problem (Knight 3e, exercise 16.72, page 467): The cylinder in Figure CP16.72 has a moveable piston attached to a spring. The cylinder's cross-section area ...
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(No need of 2nd law) Adiabatic and isothermal compressibility relation with specific heat

One classical example of thermodynamics is the relation between the Adiabatic and Isothermal Compressibility Relation with the Specific Heats: Compressibilities: ...
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308 views

Existance of gusts of wind, an anamoly?

Enthusiast + Student, not a pro, so pardon my ignorance. How can wind possibly flow in gusts? The way I understand it, a gust is a pocket of air which hits you at slightly higher speed. But how ...
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24 views

How is $P = P_{ext}$ derived, for reversible processes?

I know that for a reversible process the inexact differential of $Q$ must be equal to $TdS$ because the total entropy of the universe must remain constant. Is there a way to formally derive that the ...
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36 views

Why is entropy in a heat flow process always positive? [on hold]

Entropy in a quasistatic heat flow process is given by $$\Delta S=C_1\ln(\frac{T_f}{T_{10}})+C_2\ln(\frac{T_f}{T_{20}})$$ Where $C_1, C_2$ are heat capacities at constant volume, $T_{10}<T_{20}$, ...
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How water anomalies affects the Mpemba effect?

I understand basic explanation of Mpemba effect like this: Distant molecules are easier to realign into new formations. But if we compared two bottles of water like in Mpemba effect and one of them ...
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48 views

How can heaters using the same type of fuel be more or less efficient than each other? [duplicate]

I can understand there being a cost difference if you used gas vs electricity to heat your home. But for heaters using the same energy source, how can they be more or less efficient than each other? ...
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44 views

What is the smallest engine theoretically possible?

Is there some law of physic forbiding small engines that use say petrol? I would like to know of some analogy between electric motors and heat engines. In the latter we can make a motor so small that ...
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13 views

Can super heated air be used to drive a turbine driven engine? [on hold]

Air a mixture of gases is ever trying to expand. By accelerating this property can air be used as a fuel in a turbine driven engine which can give sufficient output to be used in an automobile.
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22 views

Activation energy as a function of defects

Electrical conductivity has a form of Arrhenius equation. It is assumed that activation energy decreases with defect density in the insulating state and becomes zero in the conducting state. Can one ...
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16 views

Calculating/estimating heat transfer losses for hot air balloon (lantern)

I'm trying to build a flying lantern / hot air balloon that flies as close to hovering as possible (as opposed to up-up and awaaay). To see if this is feasible I'm trying to simulate as much as I can ...
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30 views

Is equilibrum condition consequence of second law of thermodynamics?

Equilibrum state is generally stated to be most stable state. Is equilibrium state consequence of second law of thermodynamics at microscopic level. i.e a system tends to reach the equilibrium state ...
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35 views

Is it possible to tell time within a steady state?

(Inspired from this question (A) which unfortunately the link in the selected answer is now 404 broken (which might have contained my answer), thus cannot read any deeper than that) From the above ...
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Energy conservation of permanent magnets engine

How to explain the fact that magnet can attract an object (apply a work W) without losing a (significant) part of its internal energy? How to apply the energy conservation principle? Please think to ...
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28 views

Why does this derivation of the dependence of free energy on pressure not work?

It is well known that the Gibbs Free Energy of a gas depends on the pressure via the following formula: $$G_m(p) = G^\circ_m + RT\ln{\frac{p}{p^{\circ}}}$$ Where $G_m$ is the molar gibbs free energy ...
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11 views

Energy under constant pressure and temp

Conceptual problem: let A + 2B -> C + 2D be a reaction under constant pressure P and temp T, that goes all the way towards the products. A,B,C,D are ideal gases. let $\Delta H$ be the molar enthalpy ...
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4answers
54 views

Fall in temperature due to expansion

The following lines are from my book. The gas is heated and allowed to expand at such a rate that the fall in temperature due to expansion is less than the rise in temperature due to the heat ...
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Are there any equations describing solid, liquid, or plasma containing the gas constant $R$? [closed]

The gas constant $R$ relates so much to Boltzmann constant $k_B$. Boltzmann constant= gas constant divided by the Avogadro constant. For example, $R$ can be used in Nernst equation. So, $R$ is not ...
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Determination of “Heat transfer coefficient” between block of aluminium and air

I need to build a temperature control system for a hot plate of aluminium, which I intend to heat with an electric cartridge, and the temperature I would be measuring with a PT100 to close the loop. ...
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Entropy change of mixing

$dU = Tds - PdV$ definition of internal energy $dU)_{V} = TdS$ internal energy change at constant volume $-C_{v} \Delta T = T_{1} \Delta S$ integrating, assuming that $C_{V}$ is invariant through ...
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What is an expression/physical law that relates high frequency thermal fluctuations to gas pressure?

When a gas is compressed the 'ideal gas law' can predict what the increase in gas temperature will be. But that's just a mean temperature, right? At a quantum level the frequency of molecular ...
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66 views

Does Expanding Universe conflict with the Thermodynamics 3rd Law? [duplicate]

I know that thermodynamics 3rd law states that if a system in equilibrium is at zero temperature, its entropy is minimum. Expansion of the universe made a transition from radiation dominated era ...
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Entropy - does Heat Death occur in a closed system

Does heat death occur in a closed system? (Assuming you can theoretically have some sort of "closed system".
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38 views

Efficiency of a real engine

Can anyone tell me what is wrong with the following argument? Consider a Carnot engine and an engine $X$ whose efficiencies are $\eta_c$ and $\eta_x$ respectively. Let us assume that ...
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An interesting question on the mass of a droplet of liquid experiencing the Leidenfrost Effect

Background and Inspiration: I have seen numerous demonstrations of the leidenfrost effect, both on the internet and in my mother's kitchen. But I started wondering about a general case after having a ...
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Refrigeration Apparatus flow meter location [closed]

What would be the reasoning for locating the flow meter in it's location in the following refrigeration apparatus (from a thermodynamics perspective):
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3answers
38 views

in which part heat travels faster [on hold]

A room has one wall made of concrete, one wall made of copper, and one wall made of steel. All of the walls are the same size and at the same temperature of 20°C. Which wall feels coldest to the ...
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Why is my breath hot when i open my mouth all the way, but cold if I blow like a whistle? [duplicate]

I observed this as when I want to cool my soup I blow like a whistle and when I want to warm my hands I open my mouth more as my breath is warmer now.
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Thermodynamics - Efficiency of an engine [closed]

I am taking a mathematical physics course and got stuck on this rather simple problem Consider a thermodynamic cycle of monoatomic gas () I want to calculate efficiency of this cycle I know that ...
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47 views

Is the gas constant really a constant?

For ideal gases we have $$ C_p-C_v=R $$ now the left hand side of this equation is temperature dependent, so how is it possible that gas constant $R$ is temperature independent?
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Why does my fireplace shoot flames out the front until I open my backdoor?

I have a natural gas fireplace with fake logs in it. I open the flue, turn on the gas for a couple seconds, and throw a match in. The flames kind of go up and out of the flue, but most of the flames ...
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Is using molar terms with maxwells relations misguided?

My professor wants me to do a lot of derivations using maxwells relations. One of the things he wants me to prove is that from the Van der Waals equation of state one can say: $\Big(\frac{\partial ...
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62 views

An equation to describe a process of heating one room using hotter air from another room

There are two rooms completly isolated from the outer world (a closed system, no heat losses). They are connected with each other through a ventilation tube. Temperature in room A is 32 degrees ...
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The 1st law of thermodynamics

If a gas expands adiabatically will the work done be positive or negative? I think it will be positive as $\delta W=p dV$ and we have a positive sign due to the work done by the gas is positive as it ...
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Are black holes hot?

If no light can escape black holes, in my mind they would act as a greenhouse, collecting radiation from the stars or CMB or anything. Or is all this energy just absorbed by the black hole?
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How can Maxwell's Demon erase information while a black hole can not?

It is stated, that information can't disappear even in black holes. On the other hand, it is stated, that mystic beings, like Maxwell demons, can easily erase any information if they just pay with ...
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Where does information go if thermodynamic death?

Often read that until Hawking the black holes were introducing a problem with information. Allegedly information should be unable disappear, while this was happening in classic black holes. What ...
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Kettle barbecue thermodynamics question

I have an old kettle barbecue which lost its legs to rust. I then placed the dome on top of a truck's brake drum and throttled the airflow by mounting the brake drum on a plate with holes drilled in ...
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31 views

When will internal energy change?

If a gas expands irreversibly and adiabatically from $(V_i, p_f+const)$ to $(V_f,p_f)$ will the internal energy change? I have two thoughts: 1) Yes because $\Delta E= \Delta Q + \Delta W$ always ...
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1answer
55 views

The fly and the sphere of iron

When I was a child, I read something like the following in a book whose name I have now forgotten (I tried to find the original quote on the web, but although there are some similar passages the ...
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What is the cause of the flash of light and debris immediately above this explosion?

First - the footage of the explosion in question: http://www.discovery.com/tv-shows/mythbusters/videos/homemade-diamond-high-speed/ More specifically, about 5 seconds in, I'm speaking of the ...
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Can an object appropriately isolated from its surroundings become colder than its surroundings?

Consider a sealed box, well-insulated on all sides, except for the lid which is transparent to infrared. An object is placed inside the box and the box is evacuated (purpose being to thermally isolate ...
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Melting icicles

If I break off the bottom half of an icicle, will the top half melt faster?     ...
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When heat is applied to the top of a stack of pennies, why does the bottom penny melt first?

I just watched this video where a blow torch is used on the top of a stack of pennies. I'd like to know why the bottom penny melted first.
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Is it possible to disprove water memory with an entropy argument?

Water memory was a controversial experiment claiming to provide an explanation supporting homeopathy. The results were largely dismissed as being tainted by experimental error. One possible mechanism ...