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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Spin version of Maxwell's demon: Where's the energy?

I have confused myself about the following variant of Maxwell's demon and I can't seem to find out where the energy went. Consider this: You have a chain (one dimension) of spins (up/down) with a ...
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49 views

Heat transfer in a cylinder with two insulators

I'm used to problems which ask me to find the heat transfer when, for example I have a very long cylinder covered with an insulator, each with their respective conductivity coefficient. I'd use the ...
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2answers
312 views

Why does the windshield of my car freeze even if the outside temperature is above freezing? [duplicate]

Under what conditions does the windshield of a car freeze even if the outside temperature is above freezing? It is not clear whether this is related to the question why bridges freeze with ...
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141 views

calculating heating effect of coil submerged in hotwater tank

i would like to estimate the temperature increase of water passing through a heating coil submerged in a hotwater tank, i know the cold flow and temperature and the temperature in the tank, as well ...
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79 views

Relation between a Quasistatic and a reversible process

Why is it that if a process is reversible, it is quasi-static? Does it mean that then the process is also non-dissipative if it is quasistatic?
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259 views

Ultra-Relativistic Fermi Gas - Chemical Potential and Energy

I tried deriving the expression for chemical potential of a Relativistic Fermi Gas using asymptotic expansion (for large z) in : $$ N = \frac{V}{h^3}4\pi (KT)^3g_s \int \frac{p^2 dp}{e^{p-\nu}+1} $$ ...
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73 views

How to do Legendre transformations on a S=S(U,V) P,V,T system?

Problem: For a $P,V,T$ system with constant mass,entropy can be considered a function of $U,V$, i.e. $S=S(U,V)$. The goal is to derive the rest of the thermodynamic functions via Legendre ...
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185 views

Find state equation given cubic expansion and isothermal compressibility coefficients

Context: I'm studying basic thermodynamics. Since I'm not confident that I translate the terms correctly, let me define the following coefficients mathematically: \begin{equation} \beta = ...
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36 views

Transient conduction - Biot number with irradiation?

I understand that the Biot number is essentially the ratio of the convective effects to the conductive effects in transient conduction but what if there is some sort of radiation from a heat source ...
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42 views

When heat is rejected by a system what will be $q$ according to sign convention?

When a system rejects heat it can be assumed that work is done by the system. So as per law the sign convention must be negative. But my physics textbook says the reverse. So is the question.
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56 views

(Iso-baric / iso-choric) Thermodynamic cycle question

A very simply question: Is something that even somewhat resemble a thermodynamic cycle consisting of two iso-baric and two iso-choric cycles possible (such that you get a simple rectangle representing ...
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18 views

Work of an adiabatic/ compresion: sign?

As far as I am concerned the adiabatic work it is given by: $$ W_{adi}=\frac{P_{f}V_{f}-P_{i}V_{i}}{\gamma -1}$$ While I have seen in other examples that it is used with $1-\gamma$ in the ...
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109 views

Is the thermal time constant simply the heat capacity divided by thermal conductivity?

My professor gave me this temperature decay data to make an exponential fit and told me that the time constant tau should be heat capacity over thermal conductivity. I'm looking at the wikipedia page ...
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46 views

What creates black streams in fountain-type fireworks?

For the New Year celebration of 2014, my nephews set off a series of fountain-type fireworks (as shown below). When one looks carefully into the shower of light, one notices that a fair number of ...
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41 views

How does pressurized gas constantly push?

If a gas, such as hydrogen, is pressurized into an air tight container, a force in terms of pascals (or whatever unit you want to use) is exerted, correct? That is what pushes against every surface ...
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67 views

Can I make a cold air trap (thermal equilibrium)

Thermodynamic buoyancy. I have an air intake for combustion air entering into a basement furnace room. During cold weather, the air enters unchecked (no damper on pipe is allowed). I have this 5 ...
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50 views

Equilibrium Condition

In classical thermodynamics, equilibrium conditions means maximum entropy for a closed state. However, people always talk about equilibrium for open systems as well. How can one say that an open ...
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71 views

NPT simulations: what is the inertia of volume fluctuations (using periodic boundary conditions)?

NPT molecular dynamics (MD) are most often implemented by adding some kind of piston that regulates the pressure. The inertia of this piston determines the dynamics of volume fluctuations in the ...
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79 views

How does this Stirling engine work?

I'm looking at this Stirling engine, which I believe is a variation of an alpha type. It was developed by Nasa back in the 1980's. The thing is, I can't figure out how the hydrogen cycles through the ...
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51 views

Radiant barrier effect on inside of a hot water tank

I am trying to determine if a radiant barrier on the inside of a hot water tank would increase its insulating value. Normally radiant barriers are used in house construction to reduce heat loss by ...
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61 views

Would a cone shaped Stirling engine work?

A Stirling engine evidently functions by heating and cooling air, thus making the piston move up and down. What if the heated side of the cylinder were shaped as a sort of cone with a gentle slope, ...
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2answers
258 views

Compression and expansion

Why is it that solids on compression [As in striking a hammer etc.] heat up, but liquids and gases on compression [Pressurizing liquids causes them to freeze or gases to liquify] cool down? Can ...
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281 views

How effectively does heat flow through copper wire?

If I have a line of copper wire (lets say 1 meter long, 1mm thick) and one end is a flattened disk of copper about the size of a quarter, and I apply a lot of heat to it (I'm talking 800 Celsius) will ...
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1answer
105 views

How do Stirling engines work?

How do Stirling engines work? I understand the heating and cooling of air, but how much faster (or more force, I'm not sure which to use) does the piston move per degree Celsius that the temperatures ...
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1answer
53 views

How to calculate the heat of a metal?

Lets say I leave 100g of copper outside under a convex lens that is 1m squared in surface area (I've been told that 1m2 of sunlight equals 1Kw). How would I figure out the temperature reached for a ...
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64 views

Rijke tube: Why Sound is maximum when heated 1/4th length from bottom of tube

Why is it that the maximum sound coming from a Rijke tube happens only when heated wire gauze is placed at 1/4th the length of the tube from the bottom. According to Rayleigh: If heat be ...
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17 views

What coefficients may be used for evaluation the heat keeping?

The final goal is to compare two objects (similar to boxes) from the standpoint of keeping the heat inside? May be there is numerical coefficient or any approaches?
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86 views

Heat balance: how to obtain T(t) with energy in the form u(T(t))

I have the heat balance equation for a cooling case in the form: $$\frac{dU(T(t))}{dt}=-J$$ with $U$: energy (J) $T$: temperature (K) $t$: time (s) $J$: heat flux leaving the system (W) I have ...
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41 views

Expression of $\kappa_S$ in photon gas

I'm calculating some properties of the photon gas. That gas is defined by $$U=aVT^4$$ $$P=\dfrac{1}{3}aT^4$$ I calculated photon gas entropy: $$S = \dfrac{4}{3}aT^3V $$ I could calculate $c_V$. ...
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482 views

What is the difference between heat and work

If 700 J of work is applied onto a gas, and therefore the gas contracted 0.005 m^3 (from 0.01m^3), and the temperature stayed at a constant rate of 300 kelvin, then how come when pressure is added to ...
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29 views

Statistical mechanics prerequisite [duplicate]

I`ll be taking senior level and graduate course in statistical mechanics in a month. I was wondering what would be the best statistics and probability textbook to prepare for it? I`m currently ...
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195 views

Does coffee with cream cool slower than black coffee?

At Modernist Cuisine, Nathan Myhrvold states that "Coffee with cream cools about 20% slower than black coffee". He then gives three reasons, two of them related to radiation and the third one to the ...
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61 views

Legendre transformation of Entropy as a natural function of T, V, n

I have come across a question asking to get a Legendre transformation of entropy which is a natural function of 1/T, V, n starting from entropy being a natural function of T, V and n. However, I have ...
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83 views

Calculate thermal conductance of thick walls of a cuboid, given material conductivity and cube size

Note 1: This is probably as much a maths question as physics. Note 2: I am not sure if my use of the the term conductance is correct. I want to make a kiln. The walls will be thick. I can come to ...
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53 views

Linearity of the critical isochore?

When looking at the pressure-temperature diagram along the critical isochore (up to quite high reduced temperatures) a very linear trend can be observed. From a theoretical point of view, is the ...
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1answer
135 views

Maxwell relations, confused on how the solution got to this answer

I have a problem where, basically, in part (a) I correctly found a fundamental equation $$TdS = dE - 2\sigma l dx$$ Then the problem goes on to say that the only parameters of interest are $x$ and ...
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108 views

Characteristic length of differential element of cylinder surface?

I am trying to find the Nusselt number for a small element of the outside of a cylinder that has a height of $Delta z$. I found the average Grashof number of a surface as $$Gr_{L}=\frac{\beta \rho ...
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70 views

How to relate compression ratio to work input for a compressor in a heat pump

So I need to calculate the COP (Coefficient of Performance) for a 6-ton air conditioner system in terms of the compression ratio. I understand that: $$COP=\frac{Q_L}{W_{net,in}}$$ I cannot find out ...
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51 views

What's the typical material used as a low-k ILD in semiconductors?

Semiconductor companies (namely IBM, Intel, TSMC and etc.) may use different low-k materials as inter-layer dielectric (ILD) in their fabrication process. I'm wondering what is the most typical one. ...
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151 views

Thermodynamics and wall insulation [duplicate]

Australia's history has been littered with the problem of how to manage safety in bushfires. The local fire authority recommend consideration be given to preparing a bushfire management plan, deciding ...
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123 views

Definition and Measurment of Energy and Heat in Thermodynamics

The concept of work is clear from mechanics, where $dW=F\cdot dx$, leading to $dW=PdV$. Also different forms of energy are defined in mechanics, e.g., $E=\frac{1}{2}m v^2$, etc. How are the energy and ...
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34 views

Properties of material affecting magnetic refrigeration

I am reading about magnetic refrigeration at the moment - I see that two factors affecting the applicability of the material for use in magnetic refrigeration is that they must react strongly (i.e the ...
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1answer
289 views

Light bulb filled with air inside volume of helium [closed]

I have a question on my homework I can't really figure out. It's from Reif, problem 3.6, Fundamentals of Statistic and thermal physics Question: A glass bulb contains air at room temperature and at a ...
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2answers
80 views

Why the temperature of gas produced by firework falls rapidly?

When a firework explodes, the powder in the firework burns rapidly to produce a small volume of gas at high temperature and pressure. Can anyone explain to me why the temperature of the gas falls ...
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49 views

Experimental heat equation for a bottle of water

I'm not sure if this question belongs here, because I'm asking a question about experimental data rather than theoretic formula. Anyway, I'm trying to figure out the time-temperature relation of the ...
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97 views

diesel steam engine

Let's imagine a diesel engine's cylinder head would be heated by external means, to some hundred degree celsius, and the fuel would be replaced by water. We've made modifications so that water ...
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114 views

Food kept in a container in my pressure cooker cooks slower

The pressure cooker is heated at the base to cook the food. The pressure cooker releases steam when the force exerted by pressure exceeds the counterweight, at which point the weight rises up allowing ...
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1answer
268 views

Problem involving 1st law of thermo and ideal gas law

Problem: $1.0 \text{ kg}$ of air at pressure $10^6 \text{ Pa}$ and temperature $398 \text{ K}$ expands to a five times greater volume. The expansion occurs such that in every instance the added ...
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230 views

Thermodynamics shell-and-tube heat exchanger

A shell-and-tube heat exchanger is used to cool a flow of steam using a stream of cooling water cooling water. The steam enters the heat exchanger at $0.1 ~bar$ and a quality of $91$% , and exits as ...
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68 views

Is it correct to assume that a stretched rubber-band has negative entropy change?

If so, how could we express it in equations connecting S,T,Q? I was wondering if the net change is heat transfer was positive; Since we could feel the heat when it is stretched.