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 does the hot cup of tea cool down? [duplicate]

I am not sure about my answer. I think that the cup of tea cools down by the following modes: Radiation between the surface of the tea and the air molecules. Conduction between the tea and the cup ...
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1answer
113 views

Pressure and Temperature in a few-body system

This question is very much related to this other one, and the introduction is the same. If we want to define intensive properties for few-body systems it is not straightforward. I am referring ...
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100 views

Non-extensivity in a few body system

Thermodynamics finds application in many areas of physics, many of them sharing the feature of fluid-like or many-body like behavior. However, small systems or few body systems have been studied too ...
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61 views

Niagara Falls mist

I am at Niagara Falls and noticed that the mist increased significantly at night and in the early morning. I guess it is acting like a cooling tower. Why is this happening?
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102 views

Is heat supposed to bounce on an insulator or slowly go through it?

I have a simple flat electric heater at home, fixed on a wall. 50% of the energy is absorbed by the wall !! What would happen if I put some isolation material between the heater and the wall ? Would ...
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319 views

How is pressure an intensive coordinate?

Most textbooks explain intensive coordinates by asking us to consider a system and divide it into two parts. The properties which remain the same will be called intensive and the properties that ...
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2k views

Specific heat capacity at constant volume VS constant pressure

For an ideal gas, why is the specific heat capacity at constant volume lower than the specific heat capacity at constant pressure?
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89 views

Traveling Back in Time at the End of the Universe [closed]

I apologize if this question may seem fantastical. Since the heat death of universe is said to be inevitable, is it possible to travel back in time to a younger universe when the time comes? Is that ...
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3answers
100 views

The thermal expansion of material

The question is that: they drill a hole in the middle of a metal. Then when this metal is heated, will the hole become larger or smaller? The hole will get bigger, by experiment, but I think that when ...
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37 views

Singular isothermal density distribution

Does anyone know where the Singular isothermal density distribution $$ \rho(r)=\frac{\sigma_{V}^{2}}{2\pi\,G\,r^{2}} $$ or how it was derived. Are there any good references on how this equation was ...
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49 views

phase equilibrium

For a 2 phase closed system : $$d(nG)=(nV)dP-(nS)dT+\Sigma\mu_idn_i$$ now the question is, when we can use formula, $d(nG)=(nV)dP-(nS)dT$, for the above system, instead of the first formula, and why? ...
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Is heat always associated with Light?

I have found that light always produces heat. The only cases I think heat is absent with light is Fluorescence and Phosphorescence (maybe because they emit low energy but maybe the heat is still ...
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2answers
298 views

Is it possible to measure temperature using sound?

Is it possible to measure the temperature of something using sound, by blasting sound waves at solid objects, liquids and gasses, then measuring how much sound is reflected or absorbed, to give an ...
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107 views

Wanted: simple physical representation of the influence of payload onto car tires [duplicate]

From measurements I can tell that weight, as a function of tire pressure increase, is not linear but more something like this: 50kg = 1 change in pressure 100kg = more than 2 changes in pressure, ...
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93 views

Where do atoms of H and He go when they rise in the atmosphere? [duplicate]

What happens to atoms/molecules of H and He when we let them freely fly in atmosphere do they get into space? Is there any particular way to enumerate the 'force' that drives them upwards? Does ...
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2answers
254 views

Multiplicity vs Partition function

I'm a little confused between all the different notations for the multiplicity and partition function. They're not the same thing, are they? I know that entropy can be expressed as $ S = k \ln\Omega $...
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101 views

Black plastic balls on reservoir to reduce evaporation - why black?

The story is here It states: The balls sit on top of the water and block the sunlight from hitting it, helping to reduce water evaporation as well as natural contamination and certain ...
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88 views

Is thermal and electrical conductivity correlated for all phases of matter?

After reading this interesting post about why copper is a great heat conductor, I wondered if a plasma would conduct heat better than a gas. Or, are some properties like density more important for ...
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178 views

Do photons have six degrees of freedom?

Calculations involving pressure and volume relationships of photon gas during the cosmologic expansion of the universe posit an adiabatic cooling process with a heat capacity ration of 4/3. This ratio ...
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112 views

The formula for calculating work transfer for a reversible adiabatic compression

Should we not be using Pdv for calculating the work done during a reversible adiabatic process ? Why are we using Vdp ? Can some one please explain this ? (P.S. I'm taking an introductory course on ...
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3answers
172 views

How much effect does painting a domestic radiator have?

Yes, I know it should be called a convector :-) The obvious effect is that a coat of paint, which is generally a thermal insulator, can limit the rate of heat transfer from the metal to the air. But ...
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86 views

Isothermal versus Isometric/Isochoric

My impression is that all three have something do with change in internal energy, in thermodynamics. But I cannot tell exactly their relations. First isothermal, which literally means zero ...
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124 views

Temperature of a Diamond in a Boiling Pot of Water

Suppose I have a boiling pot of water (100 degrees C) and drop a diamond in. Does the diamond eventually reach 100 degrees C? Since the diamond is a rigid structure, its molecules do not vibrate ...
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3k views

Does fire have an influence on the weight of a burning object?

I know that fire is a chemical reaction, and for that reason it has no mass, but I'm not sure that this fact is enough to conclude that it has no effect on the weight of a burning object. Consider the ...
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174 views

How to compute this change of Gibbs energy for a Van der Waals gas?

The change of Gibbs energy at constant temperature and species numbers, $\Delta G$, is given by an integral $\int_{p_1}^{p_2}V\,{\mathrm d}p$. For the ideal gas law $$p\,V=n\,RT,$$ this comes down to ...
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127 views

Can you huddle next to a fridge in sub-zero temperatures and keep warm?

There's a saying I've heard in so many places.. "It was so cold that we used to huddle next to our refrigerator to keep warm..." I had heard this phrase uttered some 30 or so years ago, and it's stuck ...
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201 views

What is the purpose of the linear approximation $\Delta L = \alpha L_0 \Delta T$?

What is the purpose of the linear approximation $\Delta L = \alpha L_0 \Delta T$? When using this, we run in all kinds of problems. For example, when a material heates up twice by 1K we get $L=L_1+\...
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1answer
101 views

Reversible cycle approximated by Carnot cycles

My textbook, W.E. Gettys, F.J. Keller, M.J. Skove, Physics 1, gives the definition of a reversible transformation as a transformation that can be inverted by effectuating only infinitesimal changes in ...
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24 views

Is this 'combined statement' really equivalent to both the 1st and 2nd laws of thermodynamics?

Quoting from Prausnitz' Molecular Thermodynamics of Fluid Phase Equilibria, For a homogeneous, closed system ... taking into account interactions of the system with its surroundings in the form of ...
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104 views

Is lost work always positive? How to approve?

I am dealing with a question asking for comment on the sign of lost work. The case is to consider an irreversible process and a reversible process which share the same starting and ending state. Is ...
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29 views

Lens-Mirror systems and conservation of specific intensity

This came out of a discussion I started yesterday and a related discussion I found. I'll recap the problem quickly: Consider two blackbodies, with surface areas $A_1$ and $A_2$ and temperatures $T_1$ ...
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1answer
48 views

Adiabatic Process: Direct Formula for new Temperature and Pressure [closed]

Assume we have an ideal gas in a piston that has pressure $p_0$ volume $V_0$ and temperature $T_0$. Then we assume, that the gas is compressed to a new volume $V_1=\alpha V_0$, where $\alpha \in (0,1)$...
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259 views

Paradox of potential energy of gas molecule

We know that when water is heated at its boiling point, it will become vapors. Then because there is no change in temperature, the heat supplied will be transferred to the potential energy of the ...
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131 views

Second Law of Thermodynamics and heating a blackbody with another blackbody

Given a large blackbody with surface area $A_1$ and temperature $T_1$, let's assume I can use some mirror and lens system to capture all the emitted radiation and transfer this energy to a smaller ...
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3k views

Why does friction produce heat?

What causes two objects sliding against each other to produce heat? Why don't they generate visible light or something else?
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87 views

Newtonian heating

Suppose a fluid over a heated surface which is being stretched and the flow starts. Now the boundary condition at the surface is assumed as $q_w$ is proportional to the surface temperature. It is ...
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111 views

Usage of the formula Pdv for irreversible thermodynamic processes

Why do we use the formula Pdv for calculating work done in a irreversible process ? As per my knowledge , the term P indicates the pressure of the surrounding , which infinitesimally differs for the ...
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224 views

What is the physical fundamentals of Pascal's law

Pascal's law or the principle of transmission of fluid-pressure (also Pascal's Principle) is a principle in fluid mechanics that states that pressure exerted anywhere in a confined incompressible ...
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111 views

Paradox about the efficiency of a thermal engine

Based on that, the algebra calculation is correct. But it seems confused to me. Even though the overall efficiency reduces, it is still useful, isn't it? From the same amount of energy we can produce ...
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91 views

What are the main differences between 'Physics machine learning work' and Mainstream Machine Learning?

I have read both works from Thermodynamics of Prediction, Optimal Causal Inference and taken basic machine learning classes from computer science on prediction models, but I have yet to seen any ...
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2answers
170 views

Confusion between thermal energy and heat

In the past, I usually misunderstood that thermal energy and heat are the same. However, some materials say that thermal energy is the intrinsic value of the system, including potential energy and ...
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90 views

When can we assume a body to behave like a black body?

In an experiment I estimated the temperature of a Tungsten bulb filament measuring the resistance of it, and tried to verify whether the power of the filament is proportional to the fourth power of ...
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2answers
250 views

Why does temperature have no uncertainity?

Background My understanding is that temperature is not a quantum mechanical operator. Hence, thereby it should not have any uncertainty. However, any instrument that tries to take the measurement of ...
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52 views

Can a pure vacuum with 0% gas be created? [duplicate]

If we were to create a vacuum room with no air in it to avoid air resistance can it be done?
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1answer
110 views

Calorimetry - Emitted Joules [closed]

How can one calculate the total amount of emitted joules from an object with a temperature that isn't constant? A great start is this formula: ...
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2answers
46 views

Work done of the gas

My homework is that : a container contains two half parts X and Y separated by a plate P. Part X contains ideal gas, while part Y is vacuum. Then the plate P is removed, so gas from X can spread out ...
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232 views

Vacuum pressure equivalent in a siphon hose filled with water?

So, I was thinking about how siphons worked today, and no matter how high or low the tube goes, as long as the end is lower than the start, water will flow. So, if you took a hose, ran it up 50 feet,...
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2answers
66 views

The quickest way to reduce an isolated system's temperature

If you are in an isolated room, having an air conditioner and a ceiling fan. What is the quickest way to cool the room. Using only the air conditioner or both of them?
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47 views

Why classical open system and Bose-Einstein condensate are not fundamentally the same?

The classical partition function for an open system is given as $$ Z_{\text{max}} = \sum_{N=0}^{\infty} \dfrac{h^{-N}}{N! } \prod_{j=1}^{N} \left( \sum_{i=0}^{\infty} e^{-\beta (E_{ij}-\mu)} g_{i} \...
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What does the particle to volume density physically mean for Bose-Eisenstein condensate?

The average number of particles $\langle N\rangle$ for a Bose-Eisenstein condensate in 3D is given as $$ \dfrac{\langle N\rangle}{V} = \dfrac{V^{-1}}{e^{\beta (0-\mu)}-1} + \int_{0}^{\infty} \dfrac{V^{...