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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984 views

Why does the Nusselt Number for a Constant Surface Temperature, Thermally Fully Developed Circular Pipe Converge To a Constant of 3.66?

I have flow in a pipe with a constant surface temperature boundary condition. The velocity profile is fully developed and the temperature profile is also fully developed. In my textbook, it says that ...
1
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1answer
247 views

Pressure Question

I have a question that relates to an arrangement of natural gas pipelines but for simplicity I will be describing a less complex problem. Imagine that I have two large pressure vessels connected by a ...
1
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1answer
187 views

Heat Equation with In-Depth Radiation Exact Solution [closed]

I am looking to solve the heat conduction equation in a semi-infinite solid with in-depth radiation on the domain $-\infty < x < 0$. The governing equation of this problem is: $$\rho c \...
3
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1answer
161 views

What would a normal thermometer read at the Sun's photosphere?

I was reading about the Earth's Thermosphere, and I quote this: "The highly diluted gas in this layer can reach 2,500 °C (4,530 °F) during the day. Even though the temperature is so high, one would ...
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1answer
241 views

Speedy Cooling - Ice based Cooler - Efficient Way

Ice is used in a cooler in order to cool its contents. Which of the following will speed up the process? A) Wrap the ice in a metal foil B) Drain the water from the cooler periodically C) Put the ...
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1answer
140 views

Ideal gas in ensemble

I want to calculate the phase space density for a single ideal gas particle in a microcanonical ensemble. I know that the partition function is given by the well-known expression that you find for ...
2
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1answer
195 views

Why do windows fog quickly?

I'm from Bogotá, Colombia. In my city the temperature is all the year between 12ºC and 25ºC (bus mostly under 19ºC) and when I drive and it's raining, there is accumulation of fog on the windows, but ...
2
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0answers
237 views

Heat Transfer, Water in a pot

I have a question I've been pondering for a while. How can I determine the temperature in a pot filled with water? I know these things: I have a known mass of water, of mass $m_w$ I have a pot of ...
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3answers
946 views

Comparison between entropy and internal energy

Why is entropy change a better way of determining a spontaneous process compared to the change in internal energy?
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1answer
226 views

How does heat travel?

How does heat travel? I have two competing thoughts here!! Firstly some form of atomic/molecular process liberates a photon in the infrared region of the spectrum which is detected as heat by a ...
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2answers
742 views

Why does a substance expands upon freezing? What conditions necessitate this?

Why does a substance expands upon freezing? What conditions necessitate this? For example, how does the slope of sublimation or fusion curve in a P-T diagram affect this?
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5answers
1k views

Is the atmospheric pressure the cause of a planet's surface temperature or is it the temperature the cause of a planet's atmospheric pressure?

I heard a climatologist on a talk show saying that one of the widely known arguments used by climate scientists to exemplify what a runaway greenhouse effect could cause to Earth's temperature and ...
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1answer
271 views

Is the physics of $j = \rho v$ questionable? What are the consequences? [closed]

In electrodynamics you have the quantity $\mathbf{j}$, which represents density of current per surface area. It is often said that $\mathbf{j} = \rho\mathbf{v}$, for geometric reasons. When I make ...
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1answer
4k views

Derivation of the Maxwell-Boltzmann speed distribution

We're currently deriving the Maxwell-Boltzmann speed distribution, but I'm struggling to squeeze out the right answer. For the first exercise we have to derive the fraction of molecules travelling ...
4
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2answers
202 views

What is the connection between the non-reversibility of the decay of unstable nuclei (as Uranium, Plutonium) and the 2nd principle of thermodynamics?

The 2nd principle of the thermodynamics says that if a system (e.g. an ideal gas) is left undisturbed, its number of microscopic states only increases. This is a statement of irreversibility of the ...
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2answers
143 views

Second Law from Statistics

Hi all I hope you can help me with the statistical origins of the Second Law. I cannot find anything that mathematically proves that order from disorder is impossible only improbable Leading me to ...
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1answer
151 views

Is state defined for non-equilibrium systems?

Equilibrium thermodynamic states follow the state principle and Gibb's phase rule when it comes to fixing a state when given a minimum of (typically) 2 state properties. For non-equilibrium states, I ...
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1answer
70 views

In statistical mechanics, what does integrating with respect to the position of a molecule mean?

So, this is probably a dumb question, but I cannot visualize or make sense of integrating over the position of a molecule in space. Okay, so an example in my thermodynamics textbook: we have N = 5 ...
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0answers
50 views

hydrodynamic entrance length [developing flow]. Non-native speaker needs some help [closed]

I am currently writing my thesis in English as an non-native speaker. From time to time, I struggle quite a bit with the correct expression of the sentences. Could you have a look on the highlighted ...
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1answer
228 views

Heat and the Temperature? [closed]

I have two chambers, one with oil and another with water. They are 1 kg each. When I supply an energy of 33600 J. Find the temperature that oil and water get heated to ? Specific heat capacity of oil ...
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2answers
2k views

Does a cooling mass lose mass as it radiates?

Assume we have a red hot cannonball in space. It starts off with mass M at 1000K. Later it has cooled by radiation to 100K. Has the mass decreased?
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2answers
262 views

Why is there a limited range of possible soap bubble size?

Soap bubbles are never "too small" or "too large". What defines the range of possible diameters of a soap bubble? Related questions: Why do steam bubbles increase in size as they rise, Why is the ...
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0answers
38 views

Where did the energy of the universe come from? [duplicate]

Last week my Physics teacher asked the class this question: If the internal energy of the universe is constant, then where did all the energy come from originally?
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1answer
148 views

Net work and the definitions of enthalpy and entropy and their implications in the Gibbs free energy

From the first law of thermodynamics: $\Delta U = q + w$ By definition: at constante pressure: $q_P = \Delta H = \Delta U + P \Delta V \quad Eq.1$ By definition, in the reversible process: $q_{rev} =...
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1answer
113 views

What is the formula to find the temperature at a specific location of a material as a function of time that is gaining or losing heat?

Imagine a scenario where there is a homogenous material and at a any location in that object I know the temperature at time 0. Given some properties of that material, how can I find out at any other ...
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0answers
172 views

Solution to a kinetic temperature problem

I have a particular equation that needs to be solved for the kinetic temperature of a stream of gas particles. If we let the kinetic temperature be designated by the letter x, the equation becomes, $$...
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2answers
66 views

Is collision in ideal gas because of physical collision or due to repulsion force

Is collision in ideal gas because of physical collision or due to repulsion force when they approach close by?
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1answer
114 views

A question about Memristors

A question about memristors: These semiconductor entities have been defined in terms of magnetic flux leakage, and a non-linear relationship to the electric charge that has flowed. In essence, the ...
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1answer
78 views

Boundary value problem

Consider the boundary value problem \begin{align} \frac{du}{dt}&= \frac{d^2u}{dx^2} , \\ u(0,t)&=0 \\ u(L,t)&=0 \\ u(x,0)&=f(x) \end{align} I know how to solve it using ...
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1answer
273 views

Equation of State and Neutron star

I'm searching for equation of state books, with references to neutron star and ultra-dense matter.
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1answer
105 views

Change in Shannon entropy of a quantum circuit of Hadamard gate and a loop

The following Q&A about reversible computing is available here. It has listed a number of practical scenarios where a reversible circuit can still be dissipating heat. Let's assume that none of ...
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0answers
43 views

Entropy of this system

We have a system with two energy states $E_0$< $E_1=0$. We also know that state $E_0$ can only take at most $m$ particles. Curently, there are $n<m$ particles in $E_0$. Now, I am supposed to ...
0
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1answer
59 views

Entropy change at varying volumes

I have two questions both related to each other: 1.What is the effect of change in volume (whether increasing or decreasing), of a system, under isothermal conditions? How does this change in ...
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3answers
201 views

Clausius statement of the 2nd Law

I'm slightly messed up with the Clausius statement of the 2nd Law. I've seen at least two versions, which seem to be conceptually different. a) It is impossible to transfer heat from a colder body ...
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0answers
97 views

Robin boundary conditions for the heat equation

Consider a 3D channel with fluid or gas with walls $\Gamma_1$, inflow part $\Gamma_2$ and outflow part $\Gamma_3$. The temperature is described by the heat equation: $$ \frac{\partial T}{\partial t} -...
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1answer
77 views

Problem Boltzmann distribution [closed]

I am trying to solve a problem about boltzmann distribution. If(A)=0.74 Uf(V)=0.037 Ia(A)=0.130 I have to find ...
0
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2answers
146 views

Only differences in chemical potential have physical meaning?

Is the statement in the title true? Does the chemical potential of a single pure substance that does not undergo any chemical reactions or phase changes have any physical meaning?
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1answer
221 views

Johnson-Nyquist noise for resistive element with temperature gradient

In Nyquist's 1928 paper, Thermal agitation of electric charge in conductors, the voltage noise for a resistor is derived assuming a circuit in thermal equilibrium and one temperature. How does the ...
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1answer
59 views

Energy without temperature

If you know the entropy $S$ of your system. Is there a general way to calculate the internal energy $U$ of your system? So the entropy $S$ is the only thing I know of my system. I have no information ...
19
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2answers
3k views

Why would the lack of air in a mine in an asteroid prevent you from flooding it?

In Isaac Asimov's I, Robot novel, two scientists stationed in an asteroid where mining activities are performed say "We could flood the mines, if this weren't an airless asteroid" when considering ...
0
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1answer
180 views

What is the overall rate of heat conduction through two pipes of different dimensions if the thermal conductivity is not constant?

I am trying to work out how I would find the rate of heat transfer through two pipes made of the same material that are joined at their ends but which have different cross-sectional areas and lengths. ...
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0answers
38 views

What does negative absolute temperature mean? [duplicate]

I always understood that the concept of absolute temperature stems from the third law of thermodynamics and that the lowest theoretical possible temperature is zero degrees Kelvin. However, ...
0
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0answers
61 views

Why can thermal shocks damage/break materials?

When an object is heated a lot and then brutally cooled down or vice-versa, it can crack or even break spectacularly. What happens inside that makes it do that? Is it only because the hot core swells ...
7
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1answer
189 views

Why doesn't a Brownian ratchet provide free energy?

A Brownian ratchet is described here at Wikipedia. The "why it fails" section reads: Feynman demonstrated that if the entire device is at the same temperature, the ratchet will not rotate ...
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1answer
33 views

Make a circuit so that it heats water as rapidly as possible

To do this I understand that we need to create the greatest power. What I don't understand is why would we need to use a parallel circuit instead of a series circuit to create the greater power?
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1answer
81 views

Does Nucleation Occur during Phase Equilibrium?

I understand that during phase transition nucleation must occur. I'm wondering, once phase equilibrium is established, does nucleation still occur? For instance, in liquid-vapour equilibrium, does ...
0
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1answer
98 views

Confusion about the Definition of Boiling Point

From the description of boiling here: Boiling is the rapid vaporization of a liquid, which occurs when a liquid is heated to its boiling point, the temperature at which the vapor pressure of the ...
28
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6answers
3k views

Is a suit that hides a soldier's heat signature fundamentally possible?

I recently played "Crysis", a game where the protagonist wears a suit that allows the player to hide both himself and his heat signature. Then I watched Iron Man 3, where a kid suggests that Tony ...
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0answers
74 views

Trying to calculate the length of a tube to heat Ar gas

If one were trying to heat Argon gas from ambient (~20C) through a mild steel tube of some diameter 'd' how long would it need to be? Additional unknown includes flow rate of Ar gas so some general ...
3
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1answer
2k views

Why do vapour cones form around jet fighters?

Apparently this phenomenon has nothing to do with jets breaking the sound barrier and has something to do with the Prandtl-Glauert singularity as described on Wikipedia. But, the Wikipedia article isn'...