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

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2d or 1d conduction in this scenario?

There is a rectangular fin attached to a heat exchanger with a base temperature of 350K. The fin has uniform properties and experiencesa uniform heat generation. It also experiences heat transfer with ...
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3answers
868 views

What does it take to derive the ideal gas law in themodynamics?

How can the ideal gas law be derived from the following assumptions/observations/postulates, and these only ? I'm able to measure pressure $P$ and volume $V$ for gases. I notices that if ...
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1answer
2k views

Why do steam bubbles increase in size as they rise?

In the following video (a customer's review of a glass kettle), we can observe water boiling: http://youtu.be/jByY5I7Xk7w?t=2m55s As the kettle starts to boil at around 2:55, we can see large steam ...
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1answer
1k views

Questions about compressing a liquid in a piston

We were discussing super-critical fluids with a colleague and as were thinking about different thought-experiments, it occurred to me that you cannot really have only liquid in a piston in a real ...
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6k views

Why does hot oil explode when pouring water on it?

I am puzzled to know, What is the reason of hot oil make sound and explode when we pouring water on it?
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3answers
2k views

Prove that negative absolute temperatures are actually hotter than positive absolute temperatures

Could someone provide me with a mathematical proof of why, a system with an absolute negative Kelvin temperature (such that of a spin system) is hotter than any system with a positive temperature (in ...
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4answers
1k views

Why water inside a vessel placed in a body of boiling water does not boil?

Imagine a plastic sac filled with water placed in a pan of boiling water. I wonder why the water inside the sac does not boil. The temperature of the water inside the sac should be the same as that of ...
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1answer
361 views

Energy versus free-energy diagram

Energy versus free energy diagram. I haven't been able to find an adequate definition of these two terms in relation to each other. Could someone point me in the right direction, please? From Borrell ...
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2answers
193 views

Entropy change in a cycle with two isochoric and two adiabatic processes [closed]

Prove that the change of Entropy in a cycle with two isochoric and two adiabatic processes is 0. How can I prove that? Thanks!
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5answers
214 views

Normal distribution of x, xdot

I have some real measurements from a process and I happened to look at the mutual distribution of (x(t), xdot(t)). I found that they seem to follow 2d normal distribution around (mu, 0). See image, ...
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2answers
1k views

Are information conservation and energy conservation related?

as evident from the title, are both, conservation of energy and conservation of information two sides of the same coin?? Is there something more to the hypothesis of hawking's radiation other than ...
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1answer
2k views

How does smoke move in the air and how can I direct it so it will go to a place I want it to go?

Let's assume a close room with 1-2 people who only one of them smoking cigarette. What is the equation describe the smoking spreading? is it diffusion? what are the parameters is so? Is there a ...
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1answer
179 views

In a Sterling Engine, does heat from the hot side transfer to the cold side?

A Sterling Engine is a closed system. The "hot" side oscillates between higher temperature with higher pressure and lower temperature with lower pressure. Does Nature switch back and forth between ...
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2answers
890 views

How long will my fridge magnet stay attached to my fridge?

I have a 500 gram neodymium magnet. It is stuck to my fridge. With the constant pull of gravity trying to pull it off how long would it stay attached to fridge? For arguments sake lets say my fridge ...
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4k views
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392 views

At what point can we assume the tip of a fin is adiabatic?

Let's say there is a fin that is 1mm thick, extends 8mm from the surface, and is 10 mm wide. The fin is exposed to a moving fluid. Can we assume the adiabatic tip condition and use the characteristic, ...
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1answer
1k views

In what situations do I use the characteristic length of a fin to find the surface area?

So I'm learning about fins in heat transfer and it seems that there are two separate formulas for the surface area of a rectangular fin of length L, width w and thickness t. The fin is attached to a ...
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2answers
2k views

What meaning do changes in the absolute value of Gibbs free energy have in a simple expansion process?

Below is a simple representation of the thermodynamics of a steam turbine. Stream kinetic and potential energy changes are neglected and no other type of non-PV work is done besides shaft work. ...
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2answers
153 views

In the Niagara Falls, which factors prevent rise of T H2O falling a certain height?

I need some ideas on a problem. The first part says: Whats the posible rise in the temperature of the water falling 49.4 m in the Niagara Falls? That one was easy, with answer 0.112 Kelvin. ($\Delta ...
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4answers
3k views

Which is more efficient, heating water in microwave or electric stove?

So our propane tank in the kitchen ran out again today. Which is more energy efficient, boiling water in a microwave on an electric stove? All things being equal i.e. starting temperature and mass ...
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1answer
113 views

Equipartition theorem for flowing gas

If an ideal gas is flowing with a velocity $v$, how is the equipartition theorem applied. Normally, we can say that $\frac{1}{2}mv_{x,rms}^2=\frac{1}{2}k_BT$. We can do the same thing for $v_y$ ...
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1answer
1k views

What's the temperature distribution of a plane wall insulated on one side with no internal heat gen.?

Let's say we have a plane wall that is insulated on one side, exposed to a fluid on the other side, and has no internal heat generation. Would the shape of the temperature distribution be a constant? ...
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1answer
107 views

How can I calculate the amount of bubbles needed to lift a submerged body?

Our teacher had us come up with our own thermodynamics experiments, so I put a glass of water inside a vacuum-capsule. Inside the glass of water I have placed a non-floating object, a small plastic ...
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1answer
2k views

Confused with stress, strain and linear thermal expansion

Four rods A, B, C, D of same length and material but of different radii r, 2r , 3r and 4r respectively are held between two rigid walls. The temperature of all rods is increased by same ...
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0answers
307 views

Is the $mL_c$ value for triangular and rectangular fins the same value?

I am looking at the solutions that my professor put up and I feel that he did something wrong. Here is the question and I will give my stab at the solution so you can see why I think that it is wrong. ...
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5answers
1k views

Second law of Thermodynamics: Why is it only “almost” always true that entropy is non-decreasing? [duplicate]

Wikipedia - Second law of thermodynamics: ...the entropy of any closed system not in thermal equilibrium almost always increases. I understand that the second law of thermodynamics is based on ...
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2answers
329 views

When do thermal and chemical equilibrium not coincide?

What is an example for a system, which is in chemical equilibrium, but not in thermodynamical equilibrium? And what about the other way around? It seems to me, that as long as Parameters like ...
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0answers
325 views

What's the efficency of a steam jet pump?

Jet pumps or venturi pumps are often stated as having a "terribly low" efficiency, steam jet pumps specifically are usually describes as "only justifiable when there's an abundant steam supply anyway" ...
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1answer
518 views

How do I find average temperature given a temperature distribution?

I was told to find the temperature distribution of a wire with a current going through it. So I found $$T(x)=T_{\infty}-\frac{\dot{q}}{km^{2}}[\frac{cosh(mx)}{cosh(mL)}-1]$$ I need to find the ...
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4answers
1k views

What does third law of thermodynamics tell us?

I just have a question concerning the third law of thermodynamics. The third law describes that the entropy should be a well defined constant if the system reaches the ground state which depends ...
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1answer
547 views

How to calculate the Gibbs free energy out of a given Entropy?

I'm stuck in a small calculation. I try to solve a small exercise. You should calculate for a given entropy the Gibbs free energy, where the entropy is given as $S\left(U,V\right) = ...
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1answer
456 views

How should a heat current be defined for two component system with unequal numbers

I'm trying to calculate a heat current in a linear response regime of a very weakly-interacting two component low temperature Fermi gas with imbalanced population. I think there are two standard ways ...
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3answers
668 views

What is the most efficient way to use hand dryer?

What's the most efficient way to place your hands under the hand dryer? Let's assume that dryer creates simple downward flow of hot air. Here are some examples:
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380 views

How equivalent are heat energy and work energy in connection with a spinning flywheel?

Let's say we have two identical spinning flywheels, that have arbitrary geometry, and are made of copper. Now we apply some heat energy at the center point of flywheel A, causing it to slow down a ...
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1answer
90 views

Terminology question about energy

I'm looking for the appropriate term to use for what gets "used up" as potential energy is converted to heat and work. For example, some of the the energy in solar radiation is converted by ...
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4answers
3k views

Thermodynamics of supercooled water

Now that it's been freezing outside for the last few days, I experimented a bit with supercooling. I've left a bottle of clean water outside for a few hours, and behold, when I shook the bottle, the ...
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5answers
2k views

Should I heat my room when I'm not here, energy-efficiently speaking?

I was wondering as it's getting cold : is it better for my electricity bill to shut down completely my (electric) heater during day, and to turn it on again when I come home (then it will have to heat ...
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0answers
298 views

Exam review question on thermodynamics [closed]

A cube has a side length of $20\text{ cm}$. An atom in the gas moves around the cube as shown. It continually bounces off the four lateral walls of the cube. The atom has a mass of $6.6\times ...
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2answers
672 views

Finding coefficients of volumetric expandtion from a know coefficient of linear expandtion

Starting from a homework problem: An aluminum cup of $100 cm^3$ capacity is completely filled with glycerin at $22°C$. How much glycerin, if any, will spill out of the cup if the temperature ...
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1k views

Is a world with constant/decreasing entropy theoretically impossible?

I'm not 110% sure exactly what I mean by this question. It was sparked by a friend who said he wished the law of entropy were reversed, so he wouldn't have to worry about cleaning the bathroom. ...
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1answer
285 views

When to use Heat Diffusivity eqn and when to use Fourier's law to find temperature distribution?

Let's say that there is a circular conical section that has diameter $D=.25x$ without any heat generation and I need to find the temperature distribution. Originially I thought I could use the heat ...
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1answer
337 views

Joining the definitions of entropy

$\int \frac{Q_{rev}}{T} = \Delta(k_B\ln\Omega)=\Delta S$ Could anyone give some definite proof for this? I was able to prove that the two definitions of change in entropy are equivalent for an ...
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1answer
642 views

Deriving an Expression for Entropy

How to derive an expression for entropy in form of $S = \ln \Omega$ from the form $\displaystyle{S = - \sum_i \; p_i \ln p_i}$ ? That is the last formula taken as a definition of entropy. Just a ...
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1answer
234 views

Expansion of Helmholtz energy

To get an expansion of Helmholtz energy of a) an ideal gas b) a Van der waals gas we must integrate $\left ( \frac{\delta A }{\delta V} \right )_{T}=-P$ I saw the solution is : Can you ...
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2answers
138 views

Does it make sense to have a theory of thermodynamics about coldness?

When someone comes in from the cold to a heated room, it sometimes feels like there is coldness radiating away from that person. Is there a sense in which we can say that coldness radiates similarly ...
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6answers
6k 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 ...
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2answers
311 views

How much energy from extreme coldness?

Let's say I have: 1: one mole of extremely cold ideal gas 2: unlimited amount of ideal gas at temperature 300 K 3: one ideal heat engine Can I generate for example 1 MWh of mechanical energy using ...
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1answer
234 views

Energy formula for separating $O_2$ from mixture of $O_2$, $NH_3$ and $H_2O$

I have a physics problem I'd like to make sure I get correct. The practical aspect of this problem is that the photosynthetic efficiency of algae is inhibited with dissolved O2 in the growth medium, ...
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1answer
786 views

How does heat transfer between two atoms in solid material?

Been looking at heat equation and it's derivation, according to Wikipedia it uses 2 mathematical assumptions. My problem is that although it all seems OK, what is the physics of heat transfer in ...
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1answer
2k views

Heat Flux and Spatial Temperature Gradients

The derivation for Heat Equation I am reading starts by stating Net change of heat inside $[x,x+\Delta x]$ = Net flux of heat across boundaries + Total heat generated inside $[x,x+\Delta x]$ and ...