Covers the study of (mostly homogeneous) macroscopic systems from a heat/energy/entropy point of view. Maybe combine with [tag:statistical-mechanics].
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 ...
-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 ...
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 ...
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, ...
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 ...
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 ...
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 ...
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 ...
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 ...
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
...
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, ...
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, ...
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?
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:
$$\delta_k = ...
0
votes
1answer
38 views
Deriving work done on a solid
For a homework problem, I've been asked to show that the work done by increasing pressure isothermally on a solid of mass m is: $$W \approx -\frac{m\bar{\kappa}}{2\bar{\rho}}(P_f^2 - P_i^2)$$
where ...
9
votes
3answers
143 views
Surface energy as thermodynamic potential
Consider free energy of sharp interface $\Gamma$
$$ \int_\Gamma \sigma\;\mathrm{d}S $$
or also free energy of diffuse interface of characteristic width $\epsilon$ given by Cahn-Hilliard/Allen-Cahn ...
5
votes
1answer
88 views
What exactly heats a pool of water on a sunny day
On a sunny day an outdoor swimming pool will heat up fairly quickly. My question is, what is the exact mechanism for this and can we put numerical figures on it?
Given that water is clear and ...
-2
votes
0answers
26 views
How to work with photolithography to develop a microprocessor? [closed]
I am trying to learn this but it's too much. How does photolithography play a part in electrical micro-circuit design for a microprocessor?
Also, how do components like diodes and transistors shrink ...
0
votes
0answers
33 views
How can I derive an expression for $du$ as function of $T$ and $P$? [closed]
I have to start with an expression of $dq$ and the gas may be seen as ideal.
-1
votes
1answer
29 views
Which is more efficient cooling? Cooling yourself from cold water from Referigerator or Airconditioning? [closed]
Case a: You chill a glass of water in refrigerator to a certain temperature and drink it.. it lowers your body temperature by X degrees.
Case b: You switch on the a/c for a certain duration.. it ...
1
vote
1answer
58 views
Forms of the first law of thermodynamics
The first law of thermodynamics states that
$$\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 ...
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
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
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
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 ...
4
votes
5answers
217 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 ...
1
vote
2answers
68 views
Can a hybrid vehicle ever be more efficient than a hydrocarbon-only vehicle built with the same parts?
Based on the laws of thermodynamics, shouldn't it be theoretically impossible for a non plug-in hybrid vehicle to ever be more fuel-efficient than a vehicle that connects the same engine directly to ...
0
votes
1answer
23 views
heat spread from hot coil: diffusion, radiation?
When modeling the heat spread from a hot object through a vacuum (or a gas), what is the dominant physical process for heat energy transportation? Thermal diffusion, thermal radiation, a mix of the ...
0
votes
1answer
31 views
Mass fraction of fuel on a fuel droplet surface
Concerning combustion of fuel droplets:
Why is the mass fraction of fuel on a fuel droplet surface slightly less than one?
It is known that the temperature is below the boiling point at the fuel ...
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 ...
1
vote
1answer
44 views
Function for heating of a planet by its star
I'm looking for an algorithmic model which will gives rough estimate of the average temperature of a planet's atmosphere (good enough to say whether there will be liquid water anywhere, or if metals ...
7
votes
1answer
86 views
How does the boiling time depend on the number of eggs
My nephew showed me an exercise from his school-textbook about boiling eggs. Here is the exercise (translated from german):
To make one hard-boiled egg in a pot of water one has to put it for 8 ...
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 ...
1
vote
3answers
54 views
Consistent application of Microscopic evolution with the definition of Macrostates?
How does one use the concept of microstate evolution and definition of macrostate in a consistent manner. As my understanding goes the thermodynamic concepts are defined in equilibrium. How does one ...
-8
votes
1answer
81 views
Dry Ice in Liquid States [closed]
If I melt the dry ice in a pool.
I am thinking that Can I have bath without getting wet by melted dry ice?
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
1answer
35 views
Do metals generally lose more heat by convection than non-metals?
I've been experimenting with radiative cooling lately and in my mind convection is always an enemy (especially forced convection since the modules are outdoors).
Looking at Newton's law of cooling, ...
0
votes
0answers
23 views
Superconducting solenoid [closed]
I'm trying to solve the following exercise:
A superconducting solenoid made of Ta is cooled by liquid Helium. The solenoid is made of a thin wire (diameter 1mm). The diameter of the solenoid is 3 cm, ...
0
votes
1answer
46 views
Second law of thermodynamics
I think this is a simple question. If I have that $E(L)=\tau L$ and we are told that $\tau=BTL$ would this mean that $E=BTL^2$ implies $dE=(2BTL)dL$ or should I sub $\tau$ straight into the second law ...
2
votes
2answers
64 views
Does fire create air resistance?
Does fire create air resistance/drag? So, for example, would it be harder to swing a flaming sword than a normal one?
0
votes
2answers
63 views
Minimization of energy and maximization of entropy
Are maximization of entropy and minimization of energy equivalent? Or are they contrary?
Why should the thermodynamic potentials such as $G$, $A$, etc, be minimum at equilibrium?
I am confused. ...
2
votes
2answers
59 views
Why increase pressure to go from liquid to steam?
Assume the following phase diagram $T-H$ of water
At a nuclear engineering course, I was told that in order to increase the performance of a pressurized water reactor, one has to increase the ...
1
vote
1answer
35 views
Adding heat to a system doesn't equal the work done by gas?
So, I answered a physics question for a class that goes as follows:
A gas in a cylinder is kept at a constant pressure of $250000\: \mathrm{Pa}$ while $300\: \mathrm{kJ}$ of heat are added to it, ...
0
votes
1answer
39 views
How long does the 2nd pot of water take to boil right after the 1st one finishes?
Say I have a pot of water that boils in 20 minutes, at whatever temperature.
If I leave the fire on, take the pot off, pour the hot water into a container, refill the pot with tap water and put it ...
-1
votes
0answers
51 views
Maths manipulations for thermodynamics introduction
I study thermodynamics and statistical physics introduction.
I see statistical physics is considered by states.
$ \Omega \left ( E \right ) $ below is the number of states whose energy lies between ...
0
votes
0answers
28 views
Constant Pressure lines in evaporation diagram
Assume that there is the following $T-H$ phase diagram with the constant pressure lines.
Which is greater preasure? The preasure drops as we move to lower temperatures (i.e. from top to bottom)?
...
0
votes
1answer
45 views
Finding cooling time using an impulse of heat energy
A chip has a thermal resistance of 0.007 K/J s and thermal capacitance
of 1.6 J/K. You give it an impulse of heat equal to 100 J. How long
does it take for the chip to cool to within 10% of the ...
1
vote
0answers
42 views
Calculate heat loss of aquifer with unknown shape
I am trying to calculate the heat loss of an aquifer. An aquifer is basically a hot-cold storage $80\: \mathrm{meters}$ under the ground. It is an open system and utilizes groundwater. By pumping for ...
