Apparently both PV and PdV are used, in different contexts, which is confusing.

If the heart has to pump blood across the body, it applies PV work. However if I said work is PdV, then the work done by the heart is 0 because the volume of blood in the body is constant. But that's definitely wrong cause the heart has to supply work. But I don't get why using PdV is wrong.

But if a gas expands, the work it does is -PdV, where dV is the expansion of the gas. I can't even apply PV because V is not constant.

This brings me back to the first law. dU = Tds - PdV for reversible processes.

dW = -PdV. If we integrate, we get W from dW. If W is the work done, then what is dW? Does dW even have any physical meaning? What's the difference between dW and W?

Similarly, what's the difference between d(PV) = PdV + VdP, and just PV after integrating?

Some of these terms seem to have no physical meaning whatsoever and are just math. I don't understand.

Based on the first law, after receiving external heat, saturated liquid can turn into either compressed liquid at higher temperature or binary mixture at the same temperature. What is the thermodynamic parameter (from the thermo tables), that would determine what will likely happen? Would I always be able to use this parameter to predict it? I thought compressed liquids almost never occur...

The sun warming the earth and the heat make air rise up in the atmosphere. I wonder how strong the airflow needs to be to keep the clouds up there. Maybe it is more of a aerodynamics question but I think it is concerned.

Why constant volume gas thermometer has this name , however when i put it in a hot water the gas expands

I am designing a subsurface thermal mitigation trench for work. Providing a reasonable temperature gradient per distance would also be helpful, as I could back-calculate the conductive heat transfer rate. Sources preferable, but expertise is also highly appreciated!

More info: The trench(es) need to be sized to lose 7degC in a given length.

Initial sizing calcs:

1. Joules needed to be transferred to lose 7deg C from total water vol (specific heat analysis)
2. Joules that a certain vol of gravel (starting at 20degC) has the potential to absorb before reaching 27degC (specific heat analysis) -- result from 2 must be greater than 1 (that's how I got an initial trench geometry)
3. Darcy flow calculation to estimate the hydraulic conductivity that we'll need to pass our flow in a reasonable time (this is how we'll estimate our gravel class size -- hoping to do some field testing if able)

Calc I need an appx heat transfer rate for: 4) First, we split up the trench volume into small volumes: From Darcy, we can estimate detention time per small volume. For the first small volume, we know that Tw=27. To predict the end temperature of that first small volume and use that for the next small volume, we need to know an appx value for the heat transfer rate in Watts (aka the heat that the rock absorbs from the water). If we have that rate (reminder that a Watt is a Joule per second), we can multiply detention time by the rate to get Joules absorbed. From my specific heat analysis in 1), I know how many joules correlate to a degree lost in the water. I can then divide the Joules I lost in the small volume by the Joules/deg C lost in water. Then I subtract that deg C lost from the starting temp of water to be my starting temp for the next small volume. I will do that until I get to the end of the trench. I will then have an appx value for the temp leaving the trench.

5. Final and most challenging calc will be to estimate how long it takes for the gravel to lose heat to the surrounding clay soils. 2D heat conduction/partial derivative fun! Will do my best to simplify, let me know if you have any ideas!

[EDIT] See my post on r/hydrology for replies

Hi guys,

I'm a real-time captioner and have been captioning a Thermodynamics class for a student. The instructor is saying something that sounds like "Cautious Cochran's equation" and I'm not sure if she's really saying "cautious." She is Asian and so her accent might be interfering with what I'm hearing. I've googled and have not come up with Cautious Cochran's. Thanks!

An excerpt from the transcript if it helps: "Let's rewrite that down for the Cautious Cochrans equation which involves a gas in the phase transition."

And another: "[Indiscernible] Cautious Cochran's equation is only appropriate for the solid and vice versa or liquid and vice versa."

Hello,

So I have an in-house software that can calculate crossflow heat exchangers and one of the outputs of that software is the temperature profile (a 2D profile) at the outlet of the heat exchanger. I would want to calculate the profile (5 and 10 cm after the outlet) but by taking into account the diffusion/mixing of the air. How would you do that (without using CFD or heavy solutions) ?

Thank you in advance,

Hi everyone, as stated in the title I am looking for a simple program to sketch accurate td cycles. Thank you in advance

I've been using CoolProp for a python project and it's absolutely amazing! The only thing it lacks is thermodynamic properties (thermal conductivity, specific heat capacity, melting point, etc.) for solid materials like copper, steel, pvc, etc. which I need for my project. If anyone can recommend a database for solid materials that would be awesome!

Entropy change in a system is denoted by ∮𝛿Q/T + S generated. There is entropy change associated with heat transfer. My question is, do we have entropy change associated with work transfer? I know that lost work in a process generated entropy that is always positive, but is there any entropy (positive or negative) due to work transfer? Thank you.

https://preview.redd.it/6i44y8iafvvd1.png?width=1216&format=png&auto=webp&s=58c12efa66f9939e781bad3a4e53b0a7dfb4c32e

OK so I am attempting to cool a space. It is a computer cabinet that was built before they got so hot. I’m installing fans and having intake fans makes sense to me but I ran across a way that middle eastern homes cool themselves which lead to questions about the exhaust fan. Is it more efficient to have a fan blowing air out of the cabinet directly OR is it more efficient to have a fan blowing air across the exhaust port to pull the hot air out? If this is not where to ask this kind of question I’m sorry, I’ve done some research but nothing seems to be addressing this specific issue.

Also, reposted to adjust title per rules

I've been stuck on this homework problem in which initial pressure, initial and final temperatures, and the relationship between p and V (p*V^1.2 = constant) are given for a piston of C02 that undergoes an expansion process. We were told to assume ideal gas law and constant specific heats. I was able (I think) to calculate the final pressure, but now I need to find the work done and I have no idea how to proceed.

The closest I've gotten is finding the specific volumes of the initial and final states, but I can't find an equation to give me V and I don't know of any way to find the work done by a gas without it. I've tried to do systems of equations by mixing and matching pV = mRT, v = V/m, and the pV^1.2 = constant relationship but no solutions presented themselves (as in, the systems returned no solutions or 0 for all variables).

The only things now I can think of are if I miscalculated my final pressure, which if I did idk how I should have done so, or if there's some equation I need that wasn't given in the formula sheet for some reason.

Just in case it helps, here are the relevant values:

Initial pressure = 600 kPa

Initial temperature = 400 K

Final temperature = 298 K

p(V^1.2) = constant

Calculated values:

Final pressure: 102.571 kPa

Initial specific volume: 0.000126 m^3/kg

Final specific volume: 0.000549 m^3/kg

Question for everyone smarter than me (ie. Everyone here). I’m looking to purchase a heat mat for a small enclosed workspace in winter. If the mat is pulling 1500W on the top end would that theoretically be the same heat output to a 1500W space heater (Presuming they both are both a steady draw.) Considering they both are solely meant for heating I figured it would be close?

Both of them are difference between real and ideal fluid at set P and T. At the same time in different books and papers they use departure functions and residual properties. Can someone please explain the difference?

Currently taking thermodynamics, and I’m really unhappy with my textbook. It feels like it lacks the conceptual explanations and understanding, as in it prioritizes deriving equations and then demonstrating procedures that get you the correct answer. I’m doing well in the class in terms of grades, but I feel like if exam questions were to have a “why” appended to them (e.g. “why did the enthalpy increase?”) I’d be doomed.

I want to become a propulsion engineer, so this class is going to be incredibly important for the career I hope to have, and I feel like I’m wasting my time studying thermodynamics with this textbook.

Any books (hopefully cheap!) that you’d recommend?

Current book: Thermodynamics: An Engineering Approach by Yunus Cengel

I understand some values are missing on tables because there are some places where the substance is not vaporized.

However I don't understand how can it be missing in the middle of other values like this.

I'm currently living in the basement of the house I just moved to, it's in northern michigan so it gets a bit cold. There's a furnace that heats the upstairs exceptionally well but there's nothing for the basement.

Now my thermodynamics question is this, would a fan at the top of the stairs sufficiently blow the hot air down stairs to provide extra heat?

From my (extremely) limited understanding, a fan is going to cool the hot air that it pulls through and that air is going to in turn just rise up vs actually making it to the basement

Am I wrong? Am I missing something?

Guys I'm not asking for a complete solution but just a guidance! 1- I've not been able to find T2s and T4s without using the variable heat tables.(That's the condition in the question, we just cannot use the heat tables) 2- how do I calculate W over here , what I'm able to get is work/mass. But I need mass as well to calculate net WORK. But I've not been able to find that as well!! I've found out mass/time but how do I calculate mass by using PV=mart as I don't have a specific volume value! Thanks

Plz guys

I have a thermodynamics 2 (chemistry) exam and I want to learn how to program my calculator to do the math for me

(Calculate constants of equations, sums, etc.)

Anyone out there who can guide me?

Hello! I am working on my first technical paper, and need to measure Heat Blocking Efficiency (Heat Blocking Efficiency (HBE, %) = [1—(transmitted heat flux)/(incident heat flux)] × 100 (%)). I am having a hard time tracking down a solid method for doing this, and am looking for some advice. Open to outside labs with defined methods, but ideally would like to test this in house. I would be able to purchase equipment to make it happen, and I think I have everything needed but a heat flux sensor. However, I am not sure exactly how to start. I am feeling a bit lost and overwhelmed and looking for guidance. Thanks in advance!

Hi! I have been designing a multi effect distillator using Excel and water 97 add in. I have derived all the H&MB equations but they are non linear so I am planning to do Newton Raphson method to solve. An issue here are the enthalpies given by the addins.

My assumption is that if I partially differentiate h wrt to T (for finding the jacobian matrix) in an environment of constant pressure (the evaporator), I must get Cp.

But if I try to multiply Cp and T and compare it with h, I get significant error in the values given by the addin. They are almost similar until 300 K but diverge past 310, 320 K

My question: is my substitution correct and is followed in the industry?

Is there any factor which I should multiply to make Cp*T and h to be equal in this addin?

In the industry how do they solve these simultaneous non linear equations coz if differentiating is not a possibility, then most of the times these equations don't seem to be converging.

I am considering a fridge (radiator position etc. not considered, let it be a spherical fridge in which heat is removed uniformly from every point)
The fridge is in equilibrium at some Teq with its environment which is at some Tenv.
If I introduce to the system a container at Tenv and of non negligible size that is:
In case A: filled with water
In case B: filled 50% with water and 50% with regular air
In which of the two cases will the fridge cool down the container and return to the equilibrium temp faster, and why?
Thanks.

If you have a cooler that is opened every 30 seconds (in order to remove something frozen out of it, or to restock with more frozen things)--how would you keep these items frozen? Does it even matter that it is in a cooler at this point, if the cooler is being opened so much? Does it matter if there is dry ice in the bottom of the cooler (with the items sitting on top for accessibility)? Or would it be more effective to have the items in a plastic container with regular ice surrounding the plastic container (but not inside it)?

I am a junior undergraduate chemistry major and process chemistry seems like a career that would be of interest to me. Would anyone be willing to talk about it sometime who might know about it?

Sorry if this is a silly question, I'm just wondering why the paths for compression/expansion work look the way they do. Please see the picture for more context - the first one shows compression work, where w_compress = the shaded area, and the second is expansion work when the mass M is removed from the piston, giving w_expansion = shaded area.

Compression work

Expansion work

From my understanding, pressure and volume change at the same time in reality, so it would really look like a sloped line between the point (V1, P1) and (V2, P2), but we use ideal models where we pretend that one of the variables stays constant, allowing us to do work calculations. However, if that's the case, why is it that the path of compression moves from (V1, P1) to (V1, P2) then (V2, P2), but can't go from (V1, P1) to (V2, P1) then (V2, P2)? I can see from the graphs of the shaded area that we'd then get w_compress = w_expansion which doesn't make sense, but I can't wrap my head around the real-world explanation for why the paths look this way.

Any help understanding this would be really appreciated - if anything in this question doesn't make sense please let me know and I'll update it or something. Thanks!

Edit: I believe temperature is fixed in this example!

I’m wondering if it would make any difference in room temperature if a window is covered with a black metal plate/foil. On one hand, my test plate gets much hotter than the other objects in the room that are exposed to the sun but, on the other hand, the same rays enter the room and I guess will get absorbed by walls/furniture eventually. So does it make any difference? Does the material make any difference? Also..maybe the placement in front of the windows is not ideal because some IR heat will be radiated back outside?

Anyone know of ways to calculate the increase of temperature of a steel beam (M1)which is supporting another steel beam (M2)? Also, how to calculate the increase of temperature when the M2 beam is insulated along a portion of it's length? My question is fire engineering related (and I asked in other communities) but I'm hoping for more of a detailed process/ explanation. Thanks in advance!