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Questions tagged [thermodynamics]

Covers the study of (mostly homogeneous) macroscopic systems from a heat/energy/entropy point of view. Consider also using tag: [statistical-mechanics].

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How does thermal energy in a system do mechanical work?

Explain that a system with thermal energy has the capacity to do mechanical work. Also, in addition to this what are the differences between work and mechanical work?
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25 views

How can one the find normal force of a block resting on a pool floor underwater?

There is the force of gravity on the block, the atmosphere, the water pressure o top of the block, but how about between the block and the floor? Would we calculate water pressure pushing up from the ...
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How does thermodynamics affect our lives? [on hold]

How does the second law of thermodynamics affect our lives? Could we live without the second law of thermodynamics? What is the whole purpose of thermodynamics affecting our lives? I know this is a ...
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27 views

Perpetual Motion and Thermodynamics [duplicate]

How does the second law of thermodynamics disprove all perpetual motion machines?
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Can the internal energy of an ideal gas ever be zero? Help me understand HW feedback

HW Q: Can the internal energy of an ideal gas ever be zero? Explain. My Response: (implied Yes) wrote ΔU=0 describes a system where *Case a: |Q|= |W|, Q= -W * means heat added to system is ...
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1answer
20 views

Vrms for monotomic, diatomic, and polyatomic molecules

in my notes from class I have that Vrms= sqrt(3kT/m) for a pt molecule Vrms= sqrt(5kT/m) for a diatomic molecule and that Vrms= sqrt(6kT/m) for a triatomic-> higher order molecule but RMS Speed of ...
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37 views

Gibbs free energy the 3 equation confusion

Hello can you please help me with thermochemistry? I have been taking chem classes since highschool and they ALL talked about Gibbs free energy, enthalpy, and entropy... even in college I got away ...
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1answer
59 views

What physical law(s) are violated by two masses occupying the same space at the same time?

The two masses have equal mass and equal volume. I think the 2nd law of thermo is violated (because there are less ways to arrange a single mass in space than two), but what other laws could be ...
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22 views

Convergent Flow [on hold]

Need some help with this fluid dynamics question please. A high-speed drill required cooling to ensure that it does not overheat while in use. To achieve this, high-pressure air is directed through ...
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1answer
28 views

Thermal equilibrium of objects in space

I've read somewhere (not a reliable source though) that an object in space and far enough away from any heat source other than the CMB would eventually cool to 3K, the CMB's temperature, but no colder....
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2answers
47 views

Is spark from spark plug in SI engine (IC engine) not counted as a hot reservoir in Heat Engine?

Are the Heat Engine (HE) and Internal Combustion Engine (ICE) not the same because ICE only exchanges the heat to the surrounding[only one reservoir]? And I have a question what if it's an SI engine ...
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4answers
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Water mixture at different temperatures

Let's say that we mix homegeneously and instantly cold water at $t^\circ $ C and hot water at $T^\circ$ C (like in a water tap) in ratio $p:1$. My question is the following: What is the instant ...
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3answers
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Thermodynamic Entropy seems to be contradictory

For an ideal gas the entropy change with energy is inversely proportional to temperature: This must yield: $$S=\frac 3 2 k_B \ N ln(T)$$ For various reasons, this equation is hard to find. ...
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2answers
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Question on the temperature dependence of the partition function

Let's just say we're looking at the classical continuous canonical ensemble of a harmonic oscillator, where: $$H = \frac{p^2}{2m} + \frac{1}{2} m \omega^2 x^2$$ and the partition function (...
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2answers
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Is the *measured* temperature inside a hollow black body the same if we place a thermometer inside it or let it make contact with the body itself?

Imagine the inside of a black body shell with non-zero thickness. Inside the shell, there is photon gas produced by the shell [at a temperature much higher than zero, say $1000(K)$]. Is there is a ...
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1answer
36 views

Real gas heat calculation [on hold]

For the Van der Waals gas we get a cycle consisting of 2 isobaric and 2 isenthalpic processes. The state equations are given as: $p = \frac {RT}{v-b} - \frac {a}{v^2}$ $\;$ and $\;$ $u = cRT ...
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2answers
53 views

Time for black body to cool to a given temperature

I'm trying to figure out the time required for a blackbody to cool assuming it only looses heat via radiation. I can estimate the mass, specific heat, surface area, emissivity, initial temperature, ...
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Maximum achievable mechanical work when heating water

I am doing a question about finding maximum achievable mechanical work when 1L of liquid water is heated from T₁ to T₂ at atmospheric pressure. The solution is $$W = \int_{T_1}^{T_2} \frac{T - T_1}{T} ...
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2answers
71 views

Work extracted from a pressure cooker

How much work can you extract from a turbine connected to a pressure cooker? The situation is modeled as a rigid adiabatic tank of volume $V_{tank}$, which initially contains water at a high ...
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0answers
16 views

Determine Fuel Flow for the piston engine [on hold]

A piston engine produces 400 kW of power. Its sfc is 75 μg/J. Determine the fuel flow in kg/hr.
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0answers
14 views

How to determine amount of heat absorbed by evaporator, power consumed by the compressor? [on hold]

A refrigerator having NH3 as refrigerant .Superheated vapor at 0c and 300 kpa enters the commpressor and compress isentropically till 2000kpa .water cooled condenser rejects heat from vapor till it is ...
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0answers
25 views

Advanced and Retarded Coordinates for Hawking Radiation

I am trying to solve the Klein Gordon equation near the horizon of a black hole. For a more accurate result I am using the Vaidya Metric in which the mass is a function of retarded time u. When ...
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Calculating the Variance of the Thermal state

We have a Harmonic Oscillator in the Thermal state $\tau(\beta)$ which is defined $$\tau(\beta) = \frac{e^{-\beta H}}{\mathrm{Tr}(e^{-\beta H})}$$ where $Z = \mathrm{Tr}(e^{-\beta H})$ is known as ...
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1answer
37 views

Calculate temperature decrease of an evaporating droplet

I have a small droplet of Cyclohexane which evaporates at room temperature and ambient pressure. I measured the volume at two different times ($\Delta V=1.5\times10^{-10}$ m$^3$). I would like to ...
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21 views

Conceptual meaning of Thermal States

Thermal states are generally defined as $$\tau(\beta)= \frac{e^{-\beta H}}{\mathrm{Tr}(e^{-\beta H})}$$ What are some physical statements one can make about them? A system in thermal equilibrium is ...
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Mean squared velocity in Poiseuille flow

For fully developed laminar Poiseuille flow in a tube, I came across this relation: $$ \bar{u^2} = 2 u_b^2 $$ where $u_b$ is the bulk or mean velocity, defined by: $$ u_b = \frac{1}{\rho A} \int_A \...
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1answer
36 views

Is a Radiation Structured BH a Possible Alternative to a Singularity? [on hold]

A BH singularity implies an undefinable region of infinite density that cannot be explained by current scientific theory. There's got to be other explanations for the formation of a BH that do not ...
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1answer
30 views

Fire -Thermodynamics

Could a steel box lined with Rockwool (interior only) be an adequate shelter during a fire? How can I determine the temperature inside of the box at peak fire temp? How long could someone withstand ...
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0answers
24 views

How can I plan my physics learning? [on hold]

i would like to start learning physics on my own, but the problem is i dont know where to start, and what topics i should do after, Im a 16 year old learning at a high school with high level phycics, ...
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2answers
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How to see Planck's radiation law as a consequence of Bose Einstein statistics?

Planck's law comes about from the following ingredients. 1) The mode density per unit volume in a cavity is $8\pi\nu^2/c^3$. 2) Within each mode, assume Boltzmann statistics i.e the probability of ...
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2answers
34 views

Calculating initial average energy of a thermal state

We are given a system with the Hamiltionian $$H = \sum_i \omega_i a^{\dagger}_ia_i \tag{1}$$ where $a^{\dagger}_i, a_i$ are creation and anihilation operators. I did the calculations and got the ...
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1answer
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Understanding conditional Entropy between two physical states

From the paper: https://arxiv.org/abs/1303.4686 Let us consider an ensemble of N identical quantum systems with Hamiltonian H and density matrix $$\rho^{(N)}$$ which is diagonal in the energy ...
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What about the information related entropy? [on hold]

In the thought experiment Maxwell's demon what about the entropy related to sorting the molecules and also the entropy of Opening and closing the gates?
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How is the graph of density-temperature in an isochoric process [on hold]

What will the graph between density and Temperature be like in an isochoric process?
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Minimal energy required for distilling salt from water

Suppose we dissolve some salt into some water at room temperature. Then we want to get the salt back. How much energy is needed? There must be some minimal value, right?
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4answers
269 views

What are the differences between specific latent heat and latent heat?

What are the differences between specific latent heat and latent heat? As far as I know, latent heat is the heat required or released during the change of state, without change of temperature. So ...
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Why do we take mean occupation number of particles? [on hold]

In statistical mechanics, when we find occupation number of particles using Bose-Einstein distribution or Fermi-Dirac distribution, why do we take the mean value of the occupation number? What does it ...
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1answer
71 views

Is it true that only living things and man-made objects can make an endothermic process that decreases entropy?

I'm not trying to apply the 2LOT to only part of a system. I'm asking if there is a related law or kind of a sub-law. My highschool physics teacher said that if I take an ice cube and shine a bright ...
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Compression to Maintain Temperature

Can a liquid in a pressurized container maintain a very cold temperature merely because it is unable to expand and become a gas?
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1answer
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Energy recovery from refinery cooling tower

Is there any possibility of recovering exhaust energy from a cooling tower by putting a wind turbine above it? Would this affect the performance of the cooling tower?
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4answers
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Why do we use heat engines if they are so inefficient?

Let's say I wish to build an engine which will launch a box vertically upwards. I propose two such engine designs: Engine 1 (finger, spring, box) Step 1: Orient uncompressed spring vertically. ...
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0answers
25 views

Conceptual understanding of Harmonic oscillators in Thermal states

my professor in the lecture defined a general thermal 'state' (which actually is a density matrix) in the following way $$\tau(\beta) = \frac{e^{-\beta H}}{Z}\tag{1}$$ where Z is the partition ...
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2answers
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How to write equation of state in terms of partition function?

While studying quantum gases (fermions, bosons ), equation of state written were $PV = k_B T Z_{gr}$, where $Z_{gr}$ is the partition function of grand canonical ensemble. P and V are pressure and ...
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What formulas do I need to calculate the time it takes for a room with opened window to become the same temperature as outside?

I want to calculate how long I have to open my window to become the same temperature as outside. To make things more easy let’s assume there’s almost no wind helping the exchange of temperature.
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3answers
31 views

Will the particles from 2 objects in thermal equilibirum still collide?

Suppose that we have 2 objects and both of them reached thermal equilibrium. Do the particles of the two objects still collide with each other? If so, do any of the collisions result in the transfer ...
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2answers
43 views

Does heat transfer actually from high temperature to low temperature?

Consider Object A which has a temperature of 70°C and Object B which has a temperature of 20°C. The objects are placed right next to each other. Now, as far as i know, the temperature is defined as ...
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1answer
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Determine the entropy $S(U,V,N)$ from the Helmholtz free energy $F(T,V,N)$ for a Van der Waals fluid

The entropy $S$ of a Van der Waals fluid is: $$ S(U,V,N) = S_0 + NR\ln\left[\left(\frac{V}{N}-b\right)\left(\frac{U}{N}+a\frac{N}{V}\right)^c\ \right] $$ I understand how to get from the equation of ...
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1answer
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Boltzmann distribution derivation from maximum entropy principle

I'm stuck halfway through a derivation of the Boltzmann distribution using the principle of maximum entropy. Let us consider a particle that may occupy any discrete energy level $\mathcal{E}_i$. The ...
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1answer
53 views

Why is work done by the system in a Carnot engine?

In a Carnot engine, represented above, it is said that work is done by the system, but in reality , work is done by the system only in the isothermal and adiabatic expansions (from 1 to 3 in the ...
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1answer
28 views

Caloric equation of state for the Van der Waals gas

I'm currently trying to reproduce a specific derivation of the caloric equation of state for the Van der Waals gas, which I saw a couple of months ago in a Thermodynamics lecture. I'm well aware of ...