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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1answer
232 views

Vapor pressure higher than equilibrium vapor pressure

Is there any mechanism by which the vapor pressure of a liquid substance (lets say in a vacuum) is measured to be greater than the expected saturated vapor pressure at that temperature? Specifically, ...
8
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1answer
2k 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 ...
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0answers
945 views

Inflating a balloon (expansion resistance)

I am doing a quick calculation on how to calculate the pressure needed to inflate a perfectly spherical balloon to a certain volume, however I have difficulties with the fact that the balloon (rubber) ...
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1answer
147 views

thermal energy while calculating Langevin Forces

I have a quick question from thermodynamics. I remember that we take kT/2 as the kinetic energy per degree of freedom in kinetic theory of gases. But when we do langevin forces (for example in ...
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7answers
7k views

Explanation of “thermite vs ice” explosion

There are several videos of the reaction, where some amount of burning thermite explodes on a contact with ice. An "original" video: http://www.youtube.com/watch?v=IuPjlYxUWc8 A Mythbusters ...
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2answers
624 views

What happens if you connect a hot resistor to a cold resistor?

Kind of an extension to this question: If you heat up an object, and put it in contact with a colder object, in an ideal insulated box, the heat from one will transfer to the other through thermal ...
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3answers
771 views

Imaginary time in quantum and thermodynamics

The following question is about chapter 2 of Sakurai's Modern Quantum Mechanics. I wish I could link to the Google book, but it doesn't seem to have a satisfactory preview to be able to read the ...
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3answers
3k views

Pressure inside a light bulb

It is known that nowadays most light bulbs are filled with gas in order to minimise the evaporation rate of tungsten. Is e.g. argon put into the bulb with reduced pressure, so that the glass doesn't ...
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1answer
378 views

proving that the free energy is extensive

If I have two system of an Ideal gas A and B Each of these system has a partition function: $Z_{A,B} = \left ( \frac{V_{A,B}}{\lambda_T} \right )^{N_{A,B}}$ Where: $\lambda_T = \left ( ...
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1answer
2k views

What energy is transformed to heat when a candle is burned?

What energy is being transformed to heat when you burn a candle?
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1answer
418 views

Uses and interpretation of the 'Bowen Ratio' ($B_o=SH/LE$)

The Bowen Ratio is the ratio of sensible heat flux to latent heat flux, so presumably it gives some information about the relative importance of these processes. But it is not clear how this ...
2
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1answer
286 views

Violations of Dulong-Petit rule as an upper limit to heat capacity

Does any known substance have a heat capacity at constant volume ($C_V$) per mole of atoms greater than $3k_B$ ~ 24.94 J/(mol K)? In order to count, the substance must actually be made of atoms, that ...
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2answers
826 views

Solids with very large specific heats

I've noticed that for many solids, specific heat is around 1 Kj/Kg*K is there a theoretical explanation for this? What common solids have the highest heat capacity per mass and per volume?
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3answers
551 views

cm^3/g as a unit of adsorption

I recently saw cm^3/g as a unit for amount adsorbed. Usually, you see either kg adsorbate/kg adsorbent or mole adsorbate/kg adsorbent. Does anyone know the meaning of this unit?
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3answers
4k views

Why do power lines sag when they are heated up?

I was reading some information about the 2003 power blackout in the Northeastern US. Beginning early in the afternoon of August 14, 2003 big transmission lines began to fail in First Energy's ...
2
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1answer
369 views

Coffee cooling in a cup

Will coffee in two differant size styrofoam cups in the same room conditions cool at the same rate? One cup is 8 oz. and the other would be 12 oz.
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1answer
497 views

free adiabatic expansion

My question is mainly an engineering question. Assume I have a turbine in which I adiabatically expand compressed air. The air cools down and does work to its surroundings, which is captured by the ...
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2answers
664 views

Energy of unmixing

Mixing of two different fluids is associated with an increase of entropy. Conversely, separation of two gases must be associated with a decrease of the entropy of the two fluids. Is there a minimum ...
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3answers
5k views

Derivation of polytropic process equation

I know that a polytropic process equation is often the result of an empirical fit of a P-V curve that allows one to derive some nice analytical equations for the process. Is there actually a ...
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3answers
822 views

Does opening or closing the window in a non-AC car in the summer affect how much heat is felt inside?

This will perhaps look like a very basic and trivial question. But I find it confusing. As an experience, when you are travelling in non-AC car in summer, have people felt if putting on or putting off ...
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1answer
467 views

Reversible Stirling Engine

So it has been asserted that a Stirling engine with proper regeneration can be made reversible. It will consist of two isothermal quasi-static processes connected by two constant-volume processes. The ...
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2answers
2k views

Heat equation on ball - one-dimensional description

I want to solve the transient heat equation on a ball. The boundary condition is the same over the hole outer surface. So this should reduce to a one-dimensional problem in radial direction. However I ...
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1answer
1k views

Work for constant volume compression by adding moles of gas

I often see the expression $W = V \Delta P$ for the work of a constant volume compression where there are a fixed number of moles and the compression is caused by heating. Is this the work equation ...
11
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4answers
354 views

Discontinuities and nondifferentiability in thermodynamics

In physics and engineering sources, calculus-based formalisms - whether differential forms on a manifold, or "differentials" of functions of several variables - are presented as a way of modeling and ...
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3answers
347 views

How to think physically about basic “fields”

"Field" is a name for associating a value with each point in space. This value can be a scalar, vector or tensor etc. I read the wikipedia article and got that much, but then it goes it into more ...
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1answer
275 views

Heat reflection on distant planets

I was watching a documentary last night on the first planet discovered outside our solar system. The first one apparently is a giant gas planet that orbits very close to its sun over a very fast ...
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1answer
315 views

Is it possible that Atomic Electron Probability Density is a result of Heat?

The Schrödinger Equation provides a Probability Density map of the atom. In light of that, are either of the following possible: The orbital/electron cloud converges to a 2d surface without heat ...
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1answer
356 views

What is a quasicontinuum?

I'm currently going through statistical physics, especially on Fermi energy when I came across a term called "quasi-continuum", what exactly is it?
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0answers
585 views

How do I derive the critical temperature for bose condensation in two dimensions?

In class we derived the 3D case, but there's a step I don't understand: $$ N = g \cdot {V \over (2 \pi \hbar)^3} \cdot \int\limits_{0}^{\infty}{1 \over{e^{\left( E_p \over{K_B T}\right)}-1}} d^3 p = ...
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1answer
273 views

Does the Grand Canonical Ensemble allow for exchange of particles or not?

I was doing some reading on wikipedia and found it interesting that one page says the Grand Canonical Ensemble does not allow for exchange of particles, however another page says it does. So I went on ...
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0answers
88 views

How to compute the heat flow for a specific material for some given boundary temperature?

Assume I have a bounded material with heat sources inside. The material is known (i.e. I know heat capacity and all relevant data) and the temperature of the boundary is fixed. I solved the (steady ...
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1answer
518 views

Temperature vs AC energy consumption

I'm need to understand the following: to keep the room at confortable temperature (70 degree, for example), how does the amount of energy consumed by the AC grow as the outside air temperature rises ...
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0answers
684 views

Why is it when you microwave cold coffee and then add milk it creates a foam head? [closed]

As compared to when the coffee is just hot from brewing. I suspect it has something to do with the way the microwaves are affecting the molecules of the coffee.
17
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6answers
1k views

What exactly is heat?

Is it energy? Is it energy per unit volume? Is it energy per unit time i.e power? What is it?
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3answers
7k views

Why does microwaved food get cold faster

My evidence is entirely anectodal and non-scientific, but I've noticed food gets cold faster when it's been heated in a microwave instead of a stove. Is this true? And if it is, why does it happen?
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4answers
10k views

How does a maple syrup evaporator work?

Some background info on what an evaporator is: It is a system of metal pans set over a heat source. Sap constantly enters the first pan controlled by a float valve to keep a constant depth. The pans ...
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5answers
1k views

Does the foam on top of boiling maple sap affect the rate of evaporation?

This is a serious question from someone engaged in evaporating large quantities of water to turn sap into syrup at this time of year. Probably some background will help. When sap boils vigorously it ...
3
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2answers
300 views

How might a resonant antenna and black body radiation interact?

How does an antenna behave when it is cooled so that its black-body radiation is emitting energy at its resonant frequency? Edit: To clarify, its not how they're related in general, but how might ...
18
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4answers
12k views

Why does the air we blow/exhale out from our mouths change from hot to cold depending on the size of the opening we make with our mouth?

Why does the air we blow/exhale out from our mouths change from hot to cold depending on the size of the opening we make with our mouth? It's not just a subtle difference, but significant in my ...
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2answers
569 views

Can a water bubble be frozen into ice bubble?

To maintain the surface tension which formed our original bubble (in order to keep the bubble from breaking), we may change the temperature/pressure of air on both sides of the bubble varyingly, with ...
2
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1answer
903 views

time dependence of temperature equalization

Suppose you have two thermodynamical systems $X_1$ and $X_2$ (for example water and air) with different temperatures ($T_1 > T_2$). Now put them into thermal contact. Is there a formula which ...
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1answer
268 views

Cooling a ball of Iron

If I have an ocean with constant temp 10 degrees celcius and I throw in an 8 gram iron ball that's 50 degrees celcius, I'd like to find how to calculate the amount of energy that flows into the water ...
2
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1answer
462 views

Internal Energy and entropy in a open system

Here http://en.wikipedia.org/wiki/Internal_energy we can read $$U= TS-PV+\sum_i \mu_i N_i$$ Let's suppose i=1 and a ideal gas. We know: $$U=N/N_A c_v T$$; $$PV=NKT$$; $$\mu=\frac{\partial ...
4
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1answer
635 views

Thermodynamics of evaporation

If water is introduced in a container maintained at 20 °C in vacuum conditions, a gaseous phase will appear and the pressure will stabilize at the vapour pressure for the given temperature inside the ...
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3answers
614 views

Light “diode” and 2nd law of thermodynamics

If I had a light "diode" - an object that only allowed light (at least for a range of frequencies) to travel through it in one direction, would this necessarily allow violations of the 2nd Law of ...
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5answers
1k views

What is the most energy efficient way to boil an egg?

Starting with a pot of cold tap water, I want to cook a hard-boiled egg using the minimum amount of energy. Is it more energy efficient to bring a pot to boil first and then put the egg in it, or to ...
5
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1answer
179 views

Evaporating water in -30ºC

I don't really know anything about physics even though I pretended studying it for years. How is this explained?: ...
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3answers
702 views

Why is a hot air balloon “stiff”?

1) Why is a hot air balloon stiff? 2) Is the pressure inside the balloon higher than the pressure outside (atmospheric pressure)? 3) If the pressure inside is higher than the outside, how is it ...
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2answers
1k views

Chemical potential of particles with zero mass

Why massless particles have zero chemical potential?
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0answers
288 views

Maxwell's Demon - laser cooling

There’s an interesting article on Scientific American that tells how to cool individual atoms to within a very tiny fraction of Absolute Zero. It uses two laser beams acting on a very cold rarified ...