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

learn more… | top users | synonyms (1)

6
votes
2answers
2k 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, ...
4
votes
4answers
415 views

Dissolving oxygen into water

I was wondering how one would actually calculate how much oxygen would dissolve into water given the necessary initial conditions, and what those initial conditions would need to be. I assume they ...
2
votes
0answers
55 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?
1
vote
1answer
677 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
299 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
267 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
561 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 ...
0
votes
1answer
4k views

How to calculate incident solar energy on earth in a given day with a spectral filter

The daily solar radiation irradiation is often quoted as being between $3.2 \text{ kW hours}/\text{m}^2$ and $7 \text{kW hours}/\text{m}^2$ per day . If you filter out a range of wavelengths, how can ...
4
votes
1answer
2k views

Is heat flux density and heat flux the same thing?

Heat flux and heat flux density is the same thing, while electric flux density and electric flux is not the same thing? It makes me confused since we compare Fourier's law with Ohm's law. Here is a ...
-1
votes
1answer
70 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
291 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 ...
1
vote
1answer
191 views

Optimal way to heat liquid from the sun?

I understand that one way to tell how much a liquid will be heated up by a remote heat source is by looking at its color. I assume clear and colorless heats up least, then white, other colors and last ...
2
votes
0answers
114 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 ...
0
votes
1answer
305 views

A really simple question about heat transfer? [closed]

In rectangular coordinates, the heat conduction vector can be expressed in terms of its components $$\vec{\dot{Q_n}}=\dot{Q_x}\vec{i}+\dot{Q_y}\vec{j}+\dot{Q_z}\vec{k}$$ Please explain the meaning of ...
5
votes
2answers
3k views

Do Microwave oven cook times grow linearly with Wattage? Calculating optimal cook time

So this is a completely random and trivial question that was prompted by looking at my microwave oven and the back of a TV dinner and my google searching failed to produce a meaningful answer so ...
3
votes
3answers
4k views

Why the heat flux vector at a point must be perpendicular to the temperature isothermal surface? Is it a definition or a deduction?

Before the question: I am working on numerical calculation of three dimension parabolic equation that based on Fourier's Law of which I am a little confused. Here comes the law in modern mathematics ...
3
votes
2answers
486 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
108 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
847 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 ...
3
votes
2answers
786 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
75 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 ...
18
votes
2answers
973 views

How hot does the tip of a pencil get while writing?

When writing with a pencil, there seems to be quite a lot of friction - which seems like it would induce heat. How hot would the tip of a #2 pencil get writing on normal copy paper?
1
vote
1answer
100 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
912 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
64 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
2k 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 ...
6
votes
1answer
1k 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 ...
3
votes
1answer
378 views

Heat transfer in a pipe

I have a gas (assuming air) at T = 500 K that enters a cylindrical pipe. The outlet target temperature is 330K. There will be heat transfer via: Forced convection from the gas to the inside of the ...
1
vote
3answers
123 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 ...
-9
votes
1answer
289 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?
1
vote
3answers
365 views

Conceptual difficulty in understanding Fourier's Law of heat conduction

I am having a bit of difficulty understanding the statement of Fourier's law of heat conduction. As mentioned in Georg Joos , Theoretical Physics The quantity of heat passing per second through ...
2
votes
1answer
131 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
1k 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
1answer
121 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 ...
7
votes
3answers
377 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
950 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
267 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
280 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
514 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 ...
6
votes
1answer
3k views

Physical interpretation of different boundary conditions for heat equation

When solving the heat equation, $$ \partial_t u -\Delta u = f \text{ on } \Omega $$ what physical situations are represented by the following boundary conditions (on $\partial \Omega$)? $u=g$ ...
0
votes
0answers
279 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)? ...
6
votes
2answers
714 views

Why isn't the Earth's core temperature the average of its surface temperatures?

Assuming that the earth is spherical, that its temperature is continuous, and that some other more or less realistic conditions hold, we might think that the Earth's core temperature should be about ...
2
votes
1answer
99 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
89 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 ...
1
vote
1answer
431 views

Triple point temperature and freezing point

Why does the triple point temperature have very similar values to the freezing point, in most substances?
0
votes
2answers
129 views

Help me check units [closed]

I need to check units in this equation, I know, that there's Pa in the numerator, but what about the denominator? $p_1$ - pressure in [Pa] $T_1$, $T_2$ - temperature in [K] $\rho_w$ - density $g$ - ...
3
votes
2answers
4k views

Why is a degree Celsius exactly the same as a Kelvin?

How on earth is it possible that the difference between two temperatures in Celsius and Kelvin is exactly the same. Given the historical definition of Celsius, I find it hard to believe that this is ...
6
votes
4answers
1k views

Can a single molecule have a temperature?

A show on the weather channel said that as a water molecule ascends in the atmosphere it cools. Does it make sense to talk about the temperature of a single molecule?
1
vote
1answer
120 views

Entropy of Black Hole

What is the relation between the entropy of rotating and non rotating Black hole? Which one's entropy is greater?
2
votes
0answers
157 views

How long would it take for a container in vacuum to leak half of its air? [closed]

Let's say I know the size of the container, size of the hole the air leaks through, pressure the air is under and temperature of the air if that helps anything. Is it possible to calculate this only ...