# 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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### Entropy change in mixing two gases

The entropy change of an ideal gas is calculated as $$\Delta S = C_V n\ln(\frac{T}{T_0}) + nR\ln{\frac{V}{V_0}}$$ derived by integrating $dU = TdS - pdV$. It is only true ...
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### What could be the effects of accidentally spilling supercooled water/liquid on an unprotected body surface?

Will the body rapidly start losing heat or will the water find a seed and solidify around it ?
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### Classifying regions of Van der Waal like gas

Given the equation of state $$p+a\left(\frac{N}{V}\right)=\frac{Nk_BT}{V-bN} \tag 1$$ Taking into account of the fact that a realistic model requires $p \geq 0, V \geq Nb, N>0$ classify the ...
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### what is the supercritical state?

is the supercritical state of matter a separate state of matter or is it a combination of a liquid and a gas? could someone give me an overview of this state and its properties. i have seen some ...
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### Heat capacity of solids vs liquids $s_{liquid} = 2\ s_{solid}$

I heard that specific heat capacity if water $s_{\text{water}} = 1\ \frac{\text{cal}}{\text{g}\cdot \text{K}}$ while for ice, $s_{\text{ice}} = \frac{1}{2} \frac{\text{cal}}{\text{g} \cdot \text{K}}$ (...
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### How much heating Earth inner core provide to the surface?

Compare to the energy that the Earth surface receives from the sun, how much power comes from the inner melted core ? How important is this contribution to the surface temperature ?
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### What is characteristic time in Fourier number exactly?

What is characteristic time in Fourier number? How can I calculate characteristic time? Suppose I started heating water in a closed container by immersion rod and temperature increases continuously....
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### Why is there steam in a hot shower even though the water is not boiling? [duplicate]

When I take a hot shower, there is normally a fair amount of steam. However, I don't think the water coming out of the shower-head is actually boiling (unless I am mistaken). Why is there water ...
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### Meaning of solutions of an equation of state [closed]

Question Let $p \geq 0, V \geq Nb, N > 0$. Now we are given the equation of state $$p+a\left(\frac{N}{V}\right)=\frac{Nk_BT}{V-b} \tag 1$$ Classify the solutions of the equation of state ...
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### Problem with indistinguishability in partition function

Consider an ideal gas of classical particles of mass $m$ in uniform potential $\xi$ in 3d. The gas $N$ molecules, volume $V$ and is at temperature $T$. I believe that the Hamiltonian of this system is ...
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### Planck's postulate for oscillators or for light?

I know that Planck originally postulated that the energy of an oscillator in a black body was quantised to $E=nh\nu$ but did he know at the time that this meant the energy of light was also quantised (...
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### Why does aluminium-on-glass mirror work without distortion?

I have read an article about glass (zerodur) with low thermal expansion coefficient. It is mentioned that large casts of such glass are covered with reflective layer of Aluminium and used as mirrors ...
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### Why is $c_p$ higher than $c_v$?

why is heat capacity at constant pressure higher than heat capacity at constant volume? It is supposed to be this way, since if you increase $C_p$ in volume work is being done, in other case not. This ...
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### Does a closed system with fixed entropy imply an isolated system?

Just first to clarify, my university notation is $$\text{change in entropy} = \text{entropy flow} + \text{internal production of entropy}$$ I am confused with the term fixed entropy. Does that mean ...
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### Electrical conductivity of metals on heating

What effect does heating have on metals (to be specific, on the electrical conductivity) ? And why does it have that effect?
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### Paramagnetic/ferromagnetic transition under a magnetic field

The paramagnetic/ferromagnetic phase transition is an archetypal example of a continuous (or second-order) phase transition. When the temperature $T$ approaches the Curie temperature $T_c$, the ...
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### Why is the cloth cooled to a temperature below that of the hot wind? [duplicate]

When a wet cloth is hung up in a hot wind in the desert, it is cooled by evaporation to a temperature that may be or so below that of the air. Discuss this process in light of the second law of ...
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### Why is it impossible to build an air filter that lowers the average velocity of the air molecules in a house?

Imagine a special air ﬁlter placed in a window of a house. The tiny holes in the ﬁlter allow only air molecules moving faster than a certain speed to exit the house, and allow only air molecules ...
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### Speed of heat / quantification of heat and other magnitudes

Is the speed of heat infinite? When solving the heat equation in a semi-infinite bar, we can see that a pulse in the finite end draws an immediate change in every point of the bar. So, at any given ...
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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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### How to convert the molar attenuation coefficient, ie. molar absorptivity, to heat capacity?

In spectroscopy techniques, such as infrared spectroscopy, light of a certain wavelength is absorbed. I see parallels to the concept of heat capacity, i.e. the heat necessary to change the temperature ...
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### Double Pipe Heat Exchanger Flow Hot Cold

Based on sources I referred, I got to know that a double pipe heat exchanger operates in a way where hot water flows in a pipe and comes out as cold water and cold water flows into another pipe and ...
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### when a steam is being liquified, does its temperature change or not?

Here is a question: When ice is being melted, its temperature doesn't change. So, when a steam is being liquified, does its temperature change or not? And is the heat transferred here calculated by Q=...
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### Why is heat transfer better from within circular tubes versus ex. square tubes?

It is claimed that heat exchange, through the tube-suface, is best from within circular tubes versus either (not sure which is correct): from outside of the tube itself other shapes like for example ...
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### How to relate internal energy to atomic motion?

I am trying to conceptualize how atomic motion leads to the thermodynamically-defined internal energy (denoted as $U$ below) through some broad mathematical relationships. I get that the internal ...
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### What causes the black shimmering bands on a sun-lit surface?

I can't figure it out. I thought it was the window causing it, but even without it it happened. It seems to be the motion of the air that causes it, but why shimmering black bands? Sometimes this ...
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### How complex is an energy balance for this problem?

Let's say I have an enclosed room in the shape of a box with forced airflow on the outer walls at an ambient temperature. Each end of the room (forward and rear) has a vent that ducts in warm air to ...
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### Measured temperature (thermometre set to emissivity of 1) of smooth surface too high or to low?

We want to measure the temperature of a smooth flat surface with a thermometre based on absorption of thermal rays. The emissivity setting of the thermometre is set to 1 (same as a black surface), ...
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### Thermo - Plotting constant pressure and temperature on a Pv and Ts diagram

I was just curious what the saturation curve for water (plotting pressure versus specific volume) would look like if you drew both a line of constant pressure as well as constant temperature. I am ...
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### Newton's law of cooling for the heat equation boundary condition

Newton's law of cooling says the temperature of an object satisfies $$\frac{dT}{dt} = -k(T(t) - T_0),\tag{1}$$ where $T_0$ is the surrounding temperature. See these HTML notes for example. Now if ...
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### Second law of thermodynamics (in terms of entropy)

Is the second law of thermodynamics (in terms of entropy) for closed systems or isolated systems? I thought it must be valid for isolated systems, such as the Universe. But the book Fundamentals of ...
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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 ...