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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30 views

Equations describing a counterflow heat exchanger [closed]

One of my experiments has a water-cooled apparatus. I plan on building a heat exchanger to keep the apparatus cool. However, I am not very good at thermodynamics and could use a hand. Also, from a ...
-3
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1answer
47 views

Convert ambient heat into electricity without violating laws?

This is an idea I've been playing around with for a while, even though many people tell me it's impossible. However, as far as I can tell, it does not actually violate the laws of physics. I am ...
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1answer
43 views

Newton's Law of Cooling [closed]

As shown in Figure 3.3.11, a small metal bar is placed inside container A, and container A then is placed within a much larger container B. As the metal bar cools, the ambient temperature $T_A(t)$ ...
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1answer
66 views

Is $\mathrm d(PV)$ the same with work received OR produced?

When I write energy conservation in thermodynamics problems, I write the internal energy before, the pressure energy before (so these two are the enthalpy) and then I add/substract the work and heat ...
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0answers
11 views

Fuel cell HHV vs LHV

I am calculating the maximum efficiency for a fuel cell, based on both HHV and LHV. For hydrogen and oxygen in a PEMFC I calculated these at STP as 82.9% (based on HHV) and 94.5% (based on LHV). I do ...
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2answers
72 views

Why is heat transfer reversible when temperature difference is infinitesimal?

I don't understand why heat transfer from hot reservoir to the system is considered reversible in this case: $T_{reservoir}$ = $T_{system}$ + dT but it's considered irreversible in this case: ...
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1answer
24 views

What is the temperature of compress air entering the atmosphere?

Hi to all of you enthusiasts, I have a 1 liter compressed air tank with around 207 bar of air pressure inside. It has been sitting out for a while and I am assuming its temperature has almost reached ...
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0answers
19 views

Is there a limit to how hot an object can be? [duplicate]

We know that speed of physical object cant exceed speed of light, a body cant be cooler than 0k, thus does there exist a limit to hotness of an object?
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2answers
49 views

What's the origin of electrical resistance?

i know relation ohm : $$R=\rho\frac{L}{A} $$ i want to know about resistance from view point small particle like atom,dipole: when we have a resistor,with a special voltage it has loss power: whats ...
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2answers
44 views

Why water's temperature is less than air?

I measured the temperature of water and found that it's less than room temperature. I think evaporation is the main reason of it. Are there any reason except evaporation for the less temperature of ...
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1answer
16 views

The difference between Gibbs and grand canonical potentials

To my understanding, Helmholtz free energy is a measure of the amount of work a thermodynamical system can perform. Following the same reasoning Gibbs and Grand-canonical free energies measure the the ...
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2answers
39 views

How to find out the work done by an ideal gas if Pressure and Volume both change? [closed]

The Problem was to find out the phase where the highest work has been done (AB/BC/CA). We know that $W = \int_{V_a}^{V_b} P(V)\text{d}V$. I was confused while finding the work in AB or AC Phase. ...
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3answers
24 views

Heat transfer between water tank and room temperature

I have 20 gallons of water (salt water, but this might be irrelevant). These 20 gallons have a heater that maintains a minimum of 78º F. With the increasing temperatures during summer, the water ...
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0answers
19 views

Is there a temperature drop at the rotor plane of a wind turbine?

A wind turbine rotor produces power from the torque generated by the rotor blades. This torque arises from forces on blade elements which in turn are the consequence of pressure differences on each ...
3
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1answer
124 views

Why don't we observe spontaneous symmetry restoration in nature?

Why do we always observe spontaneous symmetry breaking in nature and not restoration? Does there exist some argument with the 2nd law of thermodynamics and the entropy of the universe increasing? If ...
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1answer
39 views

Will there be any heat transferred between water at 100 degrees Centigrade and steam at 100 degrees Centigrade if they are brought into contact?

Since heat transfer needs a finite temperature difference, it seems unlikely. However, the enthalpy of steam is greater than water (due to the latent heat of vaporisation) so is there a possibility ...
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1answer
37 views

Helmholtz Free Energy Representation of Ideal Gas

It is asked to find the fundamental equation of a monoatomic ideal gas in Helmholtz Representation, where $$F = U - TS$$ My Attempt :: $S$ of a monoatomic ideal gas is given by: $$S = Ns_0 + ...
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1answer
18 views

Using the central equation for inelastic collisions entropy change

I have what appears to be a simple question but I seem to be missing something. Take a constant temperature system where a ball travelling at $5ms^{-1}$ strikes a stationary ball and sticks to it in ...
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2answers
48 views

Free Expansion Of and Ideal Gas

We know that in free expansion of an ideal gas, no heat enters or leaves the system. We also know that $P_\text{initial}V_\text{initial}=P_\text{final}V_\text{final}$ is valid. If heat exchange ...
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0answers
7 views

Energy Per Dry Ice Mass Required By de Laval Expansion of Air?

There has been a lot of talk lately about CO2 extraction from air. Since there are rocket engines in which ice crystals precipitate in the exhaust when expanding into near vacuum, it seems dry ice ...
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4answers
55 views

Is the speed of sound in air constant?

In Optics lecture we took a formula for the speed of a wave which is: $$ v=\frac{\omega}{k} $$ where $\omega$ is number of complete vibrations per second: $$ \omega=\frac{2\pi}{\tau} $$ and: $$ ...
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1answer
50 views

What is the difference between a white object and a mirror in terms of heat absorption?

Will a white object be cooler than a mirror when exposed to direct sunlight? Or will it be the other way around? If mirrors bounce all visible light and some of the other elements of electromagnetic ...
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0answers
17 views

Can we define a whole body thermodynamic model for human?

we can consider human body as a thermodynamic system and an energy exchange system, so we can investigate efficiency, entropy, exergy, is there any thermodynamic model for whole body?
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0answers
25 views

Reversibility principle for classical mechanic

I'm studying this colloquium about quantum fluctuation relations for nonlinear thermodynamic, but I'm having a problem. Reading about the principle of micro-reversibility of the dynamic of a system i ...
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2answers
37 views

Does the increase of pressure of a gas also increase the energy?

It says that in adiabatic process, the heat is constant but the temperature is not. My question is, if you increase the pressure, the temperature is going to rise as pV = kT. But we know that the ...
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1answer
31 views

Why can't an ocean liner be powered by an engine that takes heat from the ocean water and eject ice cubes?

The following excerpt comes from the physics textbook Understanding Physics, by David Cassidy, Gerald Holton, and James Rutherford: A ball lying on the floor will not somehow gather energy from ...
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2answers
33 views

Maximum Work for A engine

We are asked to find the maximum work obtainable from an engine. Which the engine operates between two reservoir, one at T1 and another at T2. Given T2 > T1 , and the final common temperature is T3. ...
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0answers
22 views

Water heating efficiency - wood powered pool water heater

I have created a wood burning stove that has two compartments. The compartment on the bottom is to have wood inserted into it and lit aflame. The second compartment is welded directly above the ...
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0answers
31 views

Is there any attempt to overcome Carnot efficiency? [closed]

The history of thermodynamics is quite long and matured. And I wonder is there any attempt (success or fail) to overcome the Carnot efficiency? If there is success, what is it? By overcoming Carnot ...
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2answers
74 views

If I spinned a fan in a vacuum it will keep spinning forever. Why can we not make energy out of it? [duplicate]

Suppose we created a vacuum and spinned a turbine inside it with some amount of force. According to newton's second law it will keep spinning as there is no air resistance, so why can we not make ...
2
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4answers
49 views

$ P = \frac {v^2}{R} $ derivation confusion [duplicate]

I have always had a confusion of why we use $ P = V I $ or $ P = I^2 R $ and not $ P = \frac {V^2}{R} $ for relating to power loss due to heat in high tension lines. I know there are a lot of ...
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0answers
24 views

Why doesn't the heat capacity look like this?

Let us say we have a system with two states, energy $0$ and $E>0$. At absolute zero all particles are in the lowest state (of energy $0$). To rise one particle to the state $E$ this would require a ...
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0answers
26 views

Heat capacity and fluctuation-dissipation theorem, meaning of energy fluctuations?

I have read that from the fluctuation-dissipation theorem that the heat capacity is proportional to energy fluctuations (or populations fluctuations). In this context what is the meaning of 'energy ...
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4answers
362 views
+50

Why is entropy an extensive quantity?

If we have two identical isolated macroscopic systems both with energy $E$. The number of accessible states of each of them is $\Omega(E)$ and hence the entropy is $\ln\Omega(E)$. Now if we put them ...
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0answers
48 views

Why different substances have different specific heat capacities? [closed]

Specific heat capacity depends upon the nature of the substance. But i want to know, in what way? Temperature is defined as the average kinetic energy of the molecules. So,for a given mass, a sample ...
3
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1answer
39 views

Relationship between degrees of freedom and heat capacity and absolute zero?

The Wikipedia article on heat capacity indicates that there is a relationship between the number of degrees of freedom and the heat capacity. I understand this in terms of the equipartition theorem ...
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2answers
56 views

Why do color of metals change when they are heated?

I am a 10th grade student.I have seen that when a metal is heated it's color changes.But I cannot understand how heating a metal changes it's color.
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0answers
25 views

What is melting / boiling from the statistical viewpoint?

Microscopically, solids are usually described as "completely ordered" and "strongly bound", liquids "somewhat ordered", and gases "unbound" and "disordered". Thermodynamics predicts that the ...
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1answer
41 views

Why does a magnet loose magnetic strength when heated?

Magnet manufacturing companies, for example Dexter Magnetic Technologies do not hesitate to explain the effects of heat upon magnets - citing how it degenerates the magnetic properties with levels of ...
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2answers
52 views

Gravothermal catastrophe: looking for simple explanation

I am beginning to try to understand the gravothermal catastrophe. I was hoping someone could provide an explanation to help me understand what the gravothermal catastrophe is and why it is important, ...
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1answer
13 views

What are the assumptions made when analysing a stretched elastic string from a thermodynamic aspect?

Consider a stretched elastic string. Then conducting a thermodynamic analysis of the elastic string. The approach is very similar to that used for $(P, V, T)$ systems, with the pressure and volume ...
3
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1answer
53 views

Radiative equilibrium of a relativistic black body

A black body in empty space in the CMB rest frame should (discounting starlight) come to an equilibrium temperature of around 2.7 K. However, if the body is moving relative to the CMB frame, one side ...
3
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1answer
35 views

Balloon of ideal gas suddenly pops in the vacuum of space - final average energy per atom?

My question is: is the following thinking pretty much correct? I'm interested hearing about anything here that is fundamentally wrong. Of course there is a bit of hand waving and approximation. If a ...
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0answers
11 views

Temperature equalization by radiation between two coaxial cylinders - independent of diameters?

Ignoring end effects, assume I have a cylinder of diameter d0 at controlled temperature k0 and then introduce a smaller diameter cylinder of diameter d1 and temperature k1 to the interior. Does the ...
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2answers
35 views

Mixture of different gases filling their container

If I have a closed container with a mixture of two or more gases $\rho_1$ and $\rho_2$ for instance at thermal equilibrium where $\rho_1>\rho_2$ how would I determine what percentage of the ...
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0answers
32 views

How to deduce the modified Flory-Huggins equation in this form?

In the paper of "Solution Properties of Poly(N-isopropylacrylamide)" (M. Heskins and J. E. Guillet, Journal of Macromolecular Science: Part A - Chemistry, Vol. 2, Issue 8, pages 1441-1455, 1968), they ...
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1answer
34 views

Voltage homogeneity across cell membrane

During respiration, individual cells produce a relatively large potential difference ($\sim 100$ mV) between the inside and outside, using energy to pump $H^+$ out of the cell to the liquid ...
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1answer
37 views

Calculation of internal energy and entropy of ideal gas

I am having some trouble with calculation of entropy and internal energy for ideal gas. All sources which provide such derivations which I managed to find either assume a priori that $U$ is some ...
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1answer
32 views

Derivation for the most probable macrostate for distinguishable particles using lagrange's method of undetermined multipliers

We have an expression for $\Omega$ (occupation of each macrostate) in terms of $n_i$ (occupation numbers) . We want to find the $n_i$ which maximises $\Omega$. We now that ...
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0answers
26 views

temperature in adiabatic expansion

First, adiabatic means no heat transfer. Second, isothermal means temperature is constant. Is an adiabatic process also isothermal always? I know that an isothermal process is not always adiabatic. ...