# What is Thermodaynamics: Definition and 153 Discussions

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1. ### Advice for building a Hilsch Vortex tube

Hi so I wondered how crude can a vortex tube be build? Is high precision needed to build one, or could I build it out of literal cardboard and use a bicycle pump as a pressure source?

6. ### What can I add to a Taekwondo body protector scoring systeme to be more stable?

I am developing a Taekwondo body protector scoring system using a textile piezoelectric sensor. The sensor is designed to detect mechanical force and convert it into an electrical charge. However, it is sensitive to temperature variations, leading to potential inaccuracies and instability in the...
7. ### B Is it possible to boil water by stirring it?

According to the Joule experiment, work can increase the temperature of a liquid. Is it possible to boil water simply by stirring it? How much energy would be required?
8. ### I Statistical ensemble in phase space

Hi, I've a question about the concept of ensemble is statistical physics. Take a conservative system in a given macrostate (e.g. with a given energy): there will be a number of phase space's microstates compatible with the given macrostate. If I understand it correctly, basically the...
9. ### A Temperature and pressure change in an air vessel

Hello All, I am trying to model an air vessel with a constant volume during charging and discharging with compressed air. Are the equations in the attached photo are suitable? where m is the air mass, Gc and Gt are the mass flow rate in and out of the air vessel with a volume V. Tac is the...
10. ### I Confusion about the entropy of mixing

Hello everyone, I am seeking some clarification regarding a question related to thermodynamics and statistical mechanics. My understanding is that when we combine two identical boxes with the same ideal gas by removing the wall between them, the resulting system's entropy stays the same...
11. ### A H-Theorem and Lagrange multipliers

so I was studying H theorem from Richard Fitzpartic's site. https://farside.ph.utexas.edu/teaching/plasma/Plasma/node35.html Given H, they consider the following equation and set the constants as I want to understand how they got these particular values for a, b &c can we consider the...
12. ### Thermodynamic equilibrium for systems only open to particle exchange

I am only interested in the initial equilibrium conditions, and I am struggling to convince myself whether that should correspond to the equality of chemical potentials for H2 or an equality of temperatures as well. My work is as below: We take both gases as simple ideal (this is only relevant...
13. ### Determine the signs of ∆H, ∆G, ∆S for the reaction: N2(g) → 2N(g)

Hello are you able to explain the two incorrect ones. I am not sure how to do this as there is no phase change involved, nor is there a temperature or volume change.
14. ### B Gas cylinder at constant pressure being exposed to atmosphere, how?

im learning thermodynamics and currently in a lesson about thermal processes. one process has constant pressure and before diving into equations or any proof the book provides a figure of a gas cylinder. the cylinder has a movable piston/lid on one side. the book then says "...and the piston end...
15. ### Gas temperature in a constant volume

An insulated container (constant volume, adiabatic) contains an Ideal gas with pressure P1 and temperature T1. We open the container's hatch for a few seconds and let some particles escape from the container, then we close the hatch again. We know container's pressure has reduced by exiting...
16. ### B Filling a bathtub (Hot water to cold water exchange)

Hey there, I honestly don’t know if this is the right place for this, but I figured I would ask. lets say you are going to take a bath, but someone just took a shower and you know your hot water heater will be running low on the goods. Just for arguments sake, let’s say you have just enough...
17. ### I Surface Absorptance Without Radiant Emittance

how can I go about finding the surface absorptance of a material if I'm not given the radiant emittance of the non- black body?
18. ### Unclear differential equation from a thermodynamics textbook

In the thermodynamics textbook there is written: 𝛿𝐴 = 𝑇𝑑𝑆 − 𝑑𝑈 = 𝑑(𝑇𝑆) − 𝑆𝑑𝑇 − 𝑑𝑈 = −𝑑(𝑈 − 𝑇𝑆) − 𝑆𝑑𝑇 = −𝑑𝐹 − 𝑆𝑑𝑇 How did we get the bolded area from TdS? Is that property of derivative, integral, or something else :/
19. ### I Is RMS speed dependent on pressure?

As it is clear from the formula for r.m.s speed ,it is only dependent of temperture for a particular gas,I think the answer should be "No change happens".But I wonder whether there is more insight to this.And is my assumption true?
20. ### I Is this transformation irreversible?

This is a cyclic transformation. Is it safe to say thay it's irreversible because if you reverse it, it means I could extract an amount of heat from a cold reservoir and move it into a hotter reservoir with no other effect?
21. ### I Understanding Heat Capacity and Work: Clarifying Misconceptions

In defining the heat capacity of a subatance as the constant relating change in temperature to change in heat, is it assumed that the system does no work? Does it really say (heat capacity is the constant relating heat change to temperature change when the system does no work)?
22. ### A Free expansion of an ideal gas and changes in entropy

For a freely expanding ideal gas(irreversible transformation), the change in entropy is the same as in a reversible transformation with the same initial and final states. I don't quite understand why this is true, since Clausius' theorm only has this corrolary when the two transformations are...
23. ### I Expelling warm air from voids - what is left afterwards?

A cylinder contains a powder made up two substances - spherical grains of metal and spherical grains of plastic. In between the grains are voids, so that the cylinder is made up of 75% powder grains and 25% air. The cylinder is oriented vertically. It is sealed at the bottom and open at the top...
24. ### Where can I find a PVT table for water for a 3-D webapp?

I'm looking to create a little webapp where the user can see the 3-D PVT phase diagram, giving the user functionality like orbiting the surface and moving a point along the surface. (I attached an image of the surface I'm referring to). To do that, though, I would need the data defining the PVT...
25. ### Heat Flow in a Pipe with integrated Sensor

Hello, I have the following problem and hope you can help me. I have the setup as shown in the drawing. A pipe that is flowed through, and in the middle of it another pipe, but at the end of this branch is a sensor. Initially the fluid would flow, with a temperature of 25°C. If my thought is...
26. ### Question about the rules of thermodynamics

Hello , we learned in thermodynamics that to calculate ΔHr °(of the reaction ) using ΔHf °(standard heat of formation ), we have to respect that ΔHf °=ΔHr ° of a reaction forming 1 mol of compound from pure elements in their most stable form at standard state . the problem is when we want to...
27. ### Spectral Intensity Formula by Using Frequency

I found there is kind of solution in Pointon's book: An Introduction to Statistical Physics for Students. But I don't know how to find intensity by using frequency.
28. ### I Extremely strange consequence of the speed of heat value

By chance, I have read a paper that left me in shambles. I would like you to help me figure out if it makes sense or not. I tried to follow Bridgman's logic, without a complete success. The extraordinary claim is that, if you start with a system consisting of a 1cm^3 copper cube where 2 faces...
29. ### Microstates for a gas of hard spheres in a box

Hi, I have to found the number of microstates for a gas of N spheres of radius r and volume v in box taking into account the reduced volume after each sphere. V sphere << V box. I'm struggling to find the microstates in general. I don't see how to find the number of microstates without knowing...
30. ### Quick question about the heat Q transferred between water and skin

My work Q= 0.025 * 4190 * (34-100) = -6910 but on chegg but did they do Q= 0.025 * 4190 * (100-34) = 6910. I thought the initial is 100C and final is 34 because it goes from 100 to 34
31. ### Confusion about whether to use the specific heat of water or ice

My thought process of how i do the ice melting part: (note I just ignore the copper/lead part cause I already know how to do that part) Q_ice + Q_melt + Q_liquid so, it 0.018(2100)T+0.16(4190)T+0.018(334*10^3) but on chegg they didn't use 2100 but they just use 4190 instead and I am confused...
32. ### How to determine mdot for choked airflow through a valve, given T,P

Hello, I am working on a project that involves air at high pressure and temperature flowing through an orifice (valve) from one container to another. For each container, the volume, temperatures and pressures are known. The valve diameter is also known, and the fluid is air (which can be...
33. ### I Entropy change due to heat transfer between sources

Hi, starting for this thread Question about entropy change in a reservoir consider the spontaneous irreversible process of heat transfer from a source ##A## at temperature ##T_h## to another source ##B## at temperature ##T_c## (##T_h > T_c##). The thermodynamic 'system' is defined from sources...
34. ### I Theoretical maximum efficiency of a heat engine without Carnot

Through an intriguing fictitious dialog between Sadi Carnot and Robert Sterling, Prof. Israel Urieli of the Ohio University shows that it is not required to invoke entropy, the second law of thermodynamics, and the Carnot cycle with the [ideal] adiabatic processes in order to find out the...
35. ### Thermodynamics of chemical reactions

In chemical reactions generally ΔG < 0 , but if we were to consider a reversible path between pure reactants and products at 1 bar pressure , shouldn't the ΔG = 0 for every reaction ? and if it is due to non-pv work , I don't see any non pv work being done in reactions happing in a closed...
36. ### Engineering Thermodynamics: Pass-out Turbine Question

Hello Physics Forums, I have been struggling with this question for some time now and I'm not sure my method is correct - please see attached. Any help you can give to check I'm on the right lines would be very much appreciated! Cheers, C
37. ### Engineering Entropy of Otto Cycle: Deriving s1=s2

Hello, just wanted to ask regarding the otto cycle; if we were to find the entropy at the phase of isentropic compression and I was already able to derive the temperature 1 and 2 and the pressures at 1 and 2 and I also have the compression ratio of 10. How do I derive the entropy (s1 and s2)...
38. ### Calculating work and heat transfer in this Carnot process

Hey guys! This is problem from Callens Thermodynamics textbook and I'm stuck with it. My goal was to get a expression for the entropy ##S## which is dependent on ##T## so I can move into the ##T-S##-plane to do my calculations: I startet by expressing the fundamental equation as a function of...
39. ### DeltaG and DeltaA calculation for heating a gas at constant volume

Summary:: Gibbs and Helmholtz energies calculations for heating an ideal gas at constant volume I am solving a problem involving an ideal gas that undergoes several chained changes of state. One of the steps asks to calculate the change in Gibbs Energy (DeltaG) and Helmholtz energy (Delta A)...
40. ### How to calculate the electrical power output using the Organic Rankine Cycle?

Basically this would be a closed loop geothermal system for electrical power generation. The system would consist of 2 Horizontal Wells connected creating a U-shaped closed loop cycle using thermosiphon effect, with constant recirculation. It's not really a conventional geothermal power...
41. ### Isothermal and adiabatic air compression -- different expressions

Hi all, For an Isothermal compression process of air in a vessel with constant volume, I found the following expressions and and The first two give the same result, meanwhile the third gives another solution and I don't know why. For adiabatic compression I found these two expression which...
42. ### Understanding Enthalpy: Confusion Cleared Up | Thermodynamics Course"

Hello everyone! I have a course in thermodynamics this year, and there is a question about enthalpy that I cannot answer: given the definition of enthalpy H=U+PV and the integral form of the internal energy U=TS-PV we conclude that H=TS. We normally say that enthalpy equals the heat exchanged in...
43. ### Engineering Spinodal and Isotherms graph on P-V diagram

Hey guys, During my work I have to show on a P-V diagram certain points on the spinodal for water. How do I draw the spinodel and isothermal lines on a diagram (for example in Excel)? Is it something that I need to calculate? I'm lost on this, I did not find anything relevant on the internet...
44. ### A special case of the grand canonical ensemble

In addition to the homework statement and considering only the case where ##U= constant## and ##N = large## : Can we also consider the definition of chemical potential ##\mu## and temperature ##T## as in equations ##(1)## and ##(2)##, and use them in the grand partition function? More...
45. ### Simulate the temperature inside an aquarium tank

there's a website to size an aquarium heater or chiller. This is just for learning purposes (not for fish) but I have modified these parameters from default to perform a load calculation on a 100 gallon glass tank... For the tank temperature range I set to be between 150-160 degree F. and a...
46. ### How do molecules radiate photons and contribute to thermal radiation?

If a substance gains energy, it's molecules vibrate faster and thus the material increases temperature. My question is if heat lost through any other means but radiation is zero, how exactly does these molecules radiate photons?
47. ### Engineering Efficiency of Heat Engines & Refrigerators: Is Impossible Possible?

For the heat engine: First I converted all the temperatures to Kelvin, ηmax=1-(333)/(1000)=0.667 ηclaim=(1*10^3)/(1.75*10^3)=0.5714 So the heat engine seems to be less efficient than a Carnot heat engine which means it can exist. For the refrigerator: COPmax=(253)/(363-253)=2.3...
48. ### Integration of inexact differentials in Thermodynamics

In Thermodynamics, I have seen that some equations are expressed in terms of inexact differentials, ##\delta##, instead of ##d##. I understand that this concept is introduced to point out that these differential forms are path-dependent, although I am not clear how they can be handled. So, are...
49. ### How Do You Determine Temperature in Series Carnot Engines by Equating Work Done?

I know i have to use the efficiency formula and everything is fine but i don't know how to find T its the only unknown in my equation can someone please tell me how to find T . In the solution they got the value of T by equating the work done by the two engines , but why is their work done equal ?
50. ### Time to Equilibrate 0C Copper & 75C Water

I have a copper cylinder that is at 0C, how much time will be needed for it to equilibrate with 10kg of water at 75C? Copper cylinder: Mass: 105.7 g Specific heat: 0.385 J/gC Initial Temp: 0C Final Temp: 75C ish SA: 25 cm^2 Water: Mass: 10000g Specific heat: 4.184 J/gC Initial Temp: 75C...