Covers the study of (mostly homogeneous) macroscopic systems from a heat/energy/entropy point of view. Maybe combine with [tag:statistical-mechanics].
5
votes
4answers
594 views
Is energy the ability to do work?
Here was my argument against this, the second law of thermodynamics, in effect says that, there is no heat engine that can take all of some energy that was transferred to it by heat and do work on ...
1
vote
1answer
201 views
Does a negative line tension in a 3-fluid immiscible interface make sense?
This question is inspired by this question/answer pair: Is this formula for the energy of a configuration of 3 fluids physically reasonable?
Consider three immiscible fluids forming contact surfaces, ...
2
votes
1answer
20 views
Solid in Liquid Heat Transfer
If there is a solid immersed in a large (but finite) pool of water, where the solid has temperature $T_s$ and the water has temperature $T_w$, with $T_w>T_s$, how can I calculate $T_s(t)$ and ...
1
vote
0answers
12 views
Heat equation with Neumann boundary conditions
I would like to explore some ideas with different geometries exchaging energy mainly by radiation.
This means solving Laplace equation for the steady state.
However the boundary conditions are always ...
4
votes
2answers
85 views
Is there a general form for heat capacity?
Can you derive a general form (not keeping either volume or pressure constant) for heat capacity from the first law of thermodynamics? Do you have to make any assumptions to do so?
It sounds trival, ...
-1
votes
0answers
43 views
Thermodynamics - Volume expansion [closed]
A glass flask (not completely filled) contains a volume of mercury $V_m$ at a certain temperature $T_0$. If the maximum volume that the glass flask can hold at this temperature is $V_g$, what should ...
6
votes
3answers
118 views
What is the deal with heat capacity?
For some time now I've been confused about heat capacity. The way I understand it, if I put in an amount of heat energy into the system, $dQ$, its temperature will change by $CdT$. But now, ...
10
votes
3answers
519 views
Best way to chill a cup of coffee with cold water and 5 minutes [duplicate]
Initial data
1 x 3/4 full cup of hot coffee / tea / your favorite morning beverage
cold water
5 minutes
Considering that it's starting to get hot outside, and we all want to drink reasonably cold ...
1
vote
0answers
57 views
Chemical Equilibrium - Why does changing the temperature shift the equilibrium? [migrated]
I know that raising the temperature in a reversible chemical reaction causes the equilibrium to shift to the endothermic side.
I know that $\Delta G = \Delta H - T \Delta S$ but I don't know how to ...
1
vote
2answers
51 views
on Brownian motors
From this review on Brownian motors, there is such a statement without detailed explanation:
(I think this statement is general enough so that one does not need to read the article)
"Apart from ...
2
votes
2answers
40 views
A box with cooler and heater on opposite faces
Suppose there's a box with one face cold, and the opposite face hot. So when the air molecules hit the cooler face, it will transfer its momentum and energy to the wall, bouncing back with less ...
0
votes
1answer
73 views
What is the difference of work $W$ and thermal energy $Q$ in thermodynamic Stirling-process for ideal gas?
What is the difference of work $W$ and thermal energy $Q$ in thermodynamic Stirling-process (in simple form) for ideal gas?
I think that you need work to preserve this process and you bring thermal ...
6
votes
1answer
1k views
LED Thermal Modeling (How to solve heat equation with constant heat source)
I have a mechanical design with LEDs that generate heat. I want to estimate the temperature at the LED junction vs. time, but especially at steady state.
Knowing the LED voltage drop and current, I ...
0
votes
1answer
232 views
How much energy Maxwell's demon will earn?
Suppose we have one mole of one-atom ideal gas at temperature $T$.
Suppose Maxwell's daemon has separated molecules into two sections, one with speed below mean and another with speed above mean.
...
2
votes
1answer
26 views
Gas in movement
Suppose I have a gas contained in a solid box and I drop it from a certain height, is the temperature of the gas going to change because of the velocity that it acquires during the fall? If so, by how ...
4
votes
1answer
48 views
Fluids in thermodynamic equlibrium
I am reading about the Euler Equations of Fluid dynamics from
Leveque's numerical methods for conservation laws.
After introducing the mass, momentum and energy equations, some thermodynamic
...
1
vote
3answers
68 views
Office Air Conditioning and Open Windows
Today is nice and sunny and the air con is on in the office. It's a fairly large office seating 20 people, 30x30x15ft with 20 small windows and a single large AC unit on the back wall where there are ...
-1
votes
1answer
90 views
The effects of heat on gravitational fields
In boiling soapy water, globs of soap coalesce as the temperature increases to boiling. Does this mean that temperature increases the gravitational pull of bodies?
2
votes
0answers
25 views
Is there a source that provides data for the temperature coefficient of resistivity at different temperatures?
I'm looking for a source where I can find the temperature coefficient of resistivity at different temperatures for pure metals. Tables are everywhere for 20$\,^{\circ}$C, but I'm having difficulties ...
50
votes
15answers
72k views
How Does Mass Leave the Body When you Lose Weight?
When your body burns calories and you lose weight, obviously mass is leaving your body. In what form does it leave? In other words, what is the physical process by which the body loses weight when ...
2
votes
1answer
69 views
Gibbs Paradox - why should the change in entropy be zero?
The Gibbs paradox deals with the fact that for an ideal gas with $N$ molecules in a volume $V$ seperated by a diaphragm into two subvolumes $V_1,V_2$ with $N_1,N_2$ particles in each subvolume, ...
1
vote
2answers
134 views
How long will a water drop exist until it evaporates based on ambient conditions? [closed]
How long will a water drop exist until it evaporates based on ambient conditions? I am looking for a simple equation or table that would tell me how long a water drop may exist until it evaporates ...
7
votes
1answer
633 views
Why is compressible flow near the choke point so efficient?
Imagine a steady state, one-dimensional, compressible flow in a horizontal pipe of constant cross sectional area. This flow can be isothermal, adiabatic (Fanno), or diabatic (Rayleigh). As an ...
7
votes
4answers
365 views
Intuitively, why is a reversible process one in which the system is always at equilibrium?
A process is reversible if and only if it's always at equilibrium during the process. Why?
I have heard several specific example of this, such as adding weight gradually to a piston to compress the ...
2
votes
0answers
32 views
What's the domain of usability of these two expressions for entropy
$$dS=\frac{dQ}{T}$$
$$S=k\ln \Omega$$
What assumptions are being made about the system/process that allow for using those expressions?
-2
votes
0answers
31 views
Thermal physics question for calculating temperature [closed]
If iron rod and copper rod are heated up to 400 C° and 500 C° respectively, and their ends are joined, what will be the temperature of the resulting system?
Would would be the formula to solve this?
6
votes
4answers
2k views
Why does maximal entropy imply equilibrium?
From a purely thermodynamical point of view, why does that entropy have to be a maximum at equilibrium? Say there is equilibrium, i.e. no net heat flow, why can the entropy not be sitting at a ...
8
votes
4answers
193 views
Is “equilibrium state” equivalent to “well-defined state variables”?
Follow up to
Intuitively, why is a reversible process one in which the system is always at equilibrium?
and
How slow is a reversible adiabatic expansion of an ideal gas?
Suppose you have a ...
1
vote
0answers
24 views
Thermal Penetration Depth discrepancy
I've been working on a project that involves thermoacoustics, and one of the commonly-used values in this field is know as the thermal penetration depth. It is calculated as follows:
$$\delta_k = ...
5
votes
2answers
2k views
Minimal temperature achievable by vanilla Peltier element?
I wonder, are there any fundamental issues leading to reduced performance of Peltier elements at cryogenic temperatures (-100C and lower)?
What is theoretical/practical minimum temperature ...
1
vote
1answer
127 views
Joule-Thomson effect of Van der Waals gas
I'm supposed to calculate the inversion pressure $p_i$ of a Van der Waals gas. The state equation of the Van der Waals gas is: $$(p + \frac{a}{V^2})(V-b) = RT.$$ To get a hold of the inversion ...
9
votes
3answers
143 views
Surface energy as thermodynamic potential
Consider free energy of sharp interface $\Gamma$
$$ \int_\Gamma \sigma\;\mathrm{d}S $$
or also free energy of diffuse interface of characteristic width $\epsilon$ given by Cahn-Hilliard/Allen-Cahn ...
0
votes
1answer
38 views
Deriving work done on a solid
For a homework problem, I've been asked to show that the work done by increasing pressure isothermally on a solid of mass m is: $$W \approx -\frac{m\bar{\kappa}}{2\bar{\rho}}(P_f^2 - P_i^2)$$
where ...
0
votes
3answers
140 views
How do you determine the heat transfer from a P-V diagram?
I doubt this question has been addressed properly before, but if there are similar answers, do direct them to me.
I am currently studying the First Law of Thermodynamics, which includes the p-V ...
0
votes
1answer
34 views
Newton's law of cooling: changing temperature of environment
A metal ball having temperature of $80^\circ C$ is placed into $m$ grams of water at $0^\circ C$. After ten minutes, it was found that the temperature of ball and water are $60^\circ C$ and ...
0
votes
1answer
58 views
Problem evaluating moles in a an isochor transformation
I have a problem with an isochor transformation.
Me and my group of study made an experiment that want to check Gay-Lussac’s law.
We registered the equilibrium states and fitted the $P = nRT / V$, ...
5
votes
1answer
88 views
What exactly heats a pool of water on a sunny day
On a sunny day an outdoor swimming pool will heat up fairly quickly. My question is, what is the exact mechanism for this and can we put numerical figures on it?
Given that water is clear and ...
2
votes
1answer
116 views
Clear up confusion about the meaning of entropy
So I though, and was told, that entropy is the amount of disorder in a system. Specifically the example of heat flow and it flows to maximize entropy. To me this seemed odd. This seemed more ordered ...
0
votes
1answer
253 views
Expressions for canonical partition function and probabilities $p(E_i)$
Given an atom with 4 allowed states corresponding to the energy levels
$E_1 = 0$, $E_2 = E$, and $E_3 = 2E$ with degeneracies 1, 1, and 2 respectively.
How do I find the expressions for the ...
-2
votes
0answers
26 views
How to work with photolithography to develop a microprocessor? [closed]
I am trying to learn this but it's too much. How does photolithography play a part in electrical micro-circuit design for a microprocessor?
Also, how do components like diodes and transistors shrink ...
4
votes
5answers
219 views
Having a problem about entropy, thermodynamics
I am a high school student. So, while studying about thermodynamics, I got a little curious about entropy. As I read, entropy is the rate of change of chaos. So, if the entropy change of a system is ...
4
votes
1answer
68 views
How can I understand a Vortex Tube and its efficiency?
A Vortex Tube takes a pressurized input stream, most typically of a gas, and creates two output streams with a temperature differential. Apparently, it has been described as a Maxwell's Demon.
Both ...
0
votes
0answers
33 views
How can I derive an expression for $du$ as function of $T$ and $P$? [closed]
I have to start with an expression of $dq$ and the gas may be seen as ideal.
1
vote
1answer
58 views
Forms of the first law of thermodynamics
The first law of thermodynamics states that
$$\frac{D}{Dt}(K+U)=W+H,$$
where K is the kinetic energy, U is the internal energy, W is the power of the external forces and H is the heat flux. I have ...
3
votes
2answers
193 views
Ideal gas concentration under temperature gradient
I'm trying to calculate the concentration of an ideal gas in an adiabatic container as a function of position where the top and bottom plates of the container are fixed at temperatures $T_1$ and ...
-1
votes
1answer
29 views
Which is more efficient cooling? Cooling yourself from cold water from Referigerator or Airconditioning? [closed]
Case a: You chill a glass of water in refrigerator to a certain temperature and drink it.. it lowers your body temperature by X degrees.
Case b: You switch on the a/c for a certain duration.. it ...
0
votes
0answers
51 views
heat conductivity experiment [closed]
We did an experiment at University which I feel very confused about:
The image should explain it well. One end of a metal bar insulated in a cover is left in touch with a $160\,\mathrm{W}$ heat ...
2
votes
2answers
111 views
How to reconcile the two definitions of work? (mechanical and thermodynamical)
When studying classical mechanics, work is defined as: $W_M=\int F_{tot} \hspace{2 mm} dx$.
However, for thermodynamics, work is defined as: $W_T=\int -F_{ext} \hspace{2 mm} dx$.
I'm having trouble ...
2
votes
0answers
51 views
Why does the water diffuse at right angles in my coffee? [closed]
I purchase an espresso shot each morning at my local coffee shop, and I usually water it down slightly to take the edge off as well as to cool it. Recently, I have noticed that when I pour the cold ...
2
votes
1answer
40 views
How to homegrow large, momocrystalline water ice crystals
This is a follow up to this question:Can one get clear ice crystals from a dirty suspension?.
How could one grow a large - meaning visible with the naked eye - water ice crstal with common household ...
