# Tagged Questions

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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### How can there be entropy change in this system?

How can there be an entropy change in this system? Suppose if I have a system consisting of liquid water, $1\, \mathrm{kg}$ at $290\,\mathrm{K}$, I stir it, and do say, $10\, \mathrm{J}$ of work on ...
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### Bernoulli Equation for isentropic ideal gas flow

If one derives the Bernoulli Equation for the isentropic ideal gas flow you get: $$\frac{1}{2}v^2 + g z + \left(\frac{\kappa}{\kappa -1} \right) \frac{p}{\rho} = const.$$ Two questions: Is ...
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### Highest temperature possible to achieve using magnifying glass and sunlight [duplicate]

Temperature of the surface of the sun is about 5750K. Can you heat an object to more than 6000K using magnifying glass and sunlight? According to second law heat cannot be transferred from colder to ...
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### Is it safe and theorotically correct to heat a bar magnet using an open fire/furnace, provided conditions given below?

For a project, I will have to heat a bar magnet to any temperature above 40 degree celsius. For this, can I place the magnet over an open fire --like one in a furnace-- for 20 minutes or so to achieve ...
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### Finding the equation of state from a thermodynamical potential

Given a themrmodynamical potential eg, the helmholtz free energy, $$F=-Nk_BT \ln(V-bN)+aN\ln V + k_BTN\left( \ln N! -\frac{3}{2}\ln T\right)$$ $a,b$ positive constants and $V \geq Nb$ and $N>>1$....
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### Area under temperature time graph

I have an open desiccant heating system which has a flow rate of $0.7$ litres/sec, I have recorded the temperature profile after the air has been heated and plotted it against time in seconds. My ...
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### Does it mean the molecules of all matter above absolute zero temperature are moving? [duplicate]

According to my knowledge, heat is the energy that is stored in form of kinetic energy of molecules in Brownian motion. However, in a macroscopic view, a rigid body seem to be "stable" but still store ...
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### Could you make a system that exchanges matter but no thermodynamic energy?

Normally we distinguish between open, closed and isolated systems, assuming that the 4th possibility does not exist, as an exchange of matter presupposes energy exchange. Could it be nevertheless ...
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### To find average speed of gas molecule when the temperature is incresed

We are given moles of gas let say $n$. Temperature is increased from $T_1$ to $T_2$. Volume is held constant at $V_0$. We need to find the average speed of gas molecule when the temperature $T_2$ is ...
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### Rate of evaporation of water with mass loss at set temperature and pressure

I have a cylindrical tank with an 'm' amount of water in it. The tank is sealed except through an exhaust with a fan that keeps a lower than atmospheric pressure (but not vacuum) inside the tank. The ...
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### Blackbody radiation and emissive power

According to blackbody radiation theory, and thanks to Planck, we now know that there is a energy density, $u(\lambda,T)$ [$J/m^3$], associated with a certain wavelength at a particular temperature. ...
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### Is the Gibbs-Duhem equation always valid?

The derivation of the Gibbs-Duhem equation from Wikpedia uses: As shown in the Gibbs free energy article, the chemical potential is just another name for the partial molar (or just partial, ...
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### Thermodynamic Internal Energy Reference States

The internal energy, $U$, of a given thermodynamic state, $\boldsymbol{R}$, as introduced in thermodynamics textbooks like Callen seems to be defined as: \$U (\boldsymbol{R}) = \Delta U (\boldsymbol{R}...
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### Why does Air Pressure not Matter in Phase Equilibrium?

So long as we are in the ideal gas range, the presence of air does not affect the liquid/solid-vapour equilibrium in any way (such as equilibrium vapour pressure, melting point etc.). Is there a ...
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### Is the amount of radiation you receive in space constant regardless of velocity?

I'm only in high school, so this will probably have fatal flaws. So basically in space, there is bound to be stray radiation, whether from the stars, or cosmic background, floating around right. And ...
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### Does air conduct heat better than saturated steam?

This engineering toolbox table shows thermal conductivity of steam at 0.016. I understand that water is better in conducting heat than air, but if I read this correctly, steam is worse in conducting ...
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### What's wrong with this temperature-in-space calculation?

I'm trying to calculate the steady-state temperature of a body in space, but my numbers are coming up much too small. For example, for a 1-meter cube, I'm getting a temperature of 194 K (or -81 C). ...
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### Problem regarding heat engines [closed]

We have two different set ups: A 200K brick in a 300K environment A 300K brick in a 200K environment From which one of these can you extract more work? Assume equal mass and heat capacity. Using ...
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### Why is thermodynamic equilibrium not reached by the system during a thermodynamic irreversible process?

In reversible process,due to infinitesimal steps,the system remains in thermal equilibrium(why?) . But in irreversible process, thermal equilibrium does not exist. What is the cause for this ...
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### What happens at the subatomic level when energy is added to an atom?

For the sake of simplicity I shall use the hydrogen atom. I know that adding energy to the hydrogen atom means bombarding it with photons. I am curious about what happens to the quarks and electron of ...
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### How can heat turn into light

I am confused about how hot surfaces can radiate light to their surroundings. When I shine a light on a surface the light turns to heat spontaneously, and when I leave that hot surface it radiates ...