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 (3)

12
votes
3answers
820 views

Intuitive understanding of the definition of entropy

In Wikipedia, the definition of entropy goes like this: $ d S = \dfrac{\delta q_{\rm }}{T}$. The literal interpretation of this equation is that some amount of heat transferred into a system, if the ...
7
votes
7answers
2k views

Introduction to differential forms in thermodynamics

I've studied differential geometry just enough to be confident with differential forms. Now I want to see application of this formalism in thermodynamics. I'm looking for a small reference, to learn ...
26
votes
6answers
12k views

How do whisky stones keep your drink cold?

From a discussion in the DMZ (security stack exchange's chat room - a place where food and drink are important topics) we began to question the difference between how ice and whisky stones work to ...
15
votes
5answers
2k views

Why isn't absolute $0 K$ temperature possible?

So $T$ is defined as $$T = \left(\frac{\partial E}{\partial S}\right)$$ and $S$ is defined as $$S = k_B \ln \Omega$$ where $\Omega$ is the number of accessible states of the system for a given ...
14
votes
5answers
3k views

Entropy of radiation emitted into space

In several papers I see something equivalent to the following expression for the entropy of radiation given by an astronomical object such as the Sun (assuming the object can be approximated as a ...
11
votes
2answers
758 views

Quantum entaglement and the arrow of time

I have seen several claims to that quantum mechanics is required to explain the arrow of time which I take to mean the macroscopic irreversibility of physical systems. This is presumably to resolve ...
7
votes
3answers
439 views

Why does ice melts, waits for 100 degrees and THEN vaporises? Why is not the process of expansion of things continuous?

What I am asking is this: Why can't a body be solid, then solid-ish, then solid-like, then liquid-like, then liquid-ish, then liquid, then vapor-like and then vapor? Why is there a rigid temperature ...
7
votes
2answers
707 views

The statistical nature of the 2nd Law of Thermodynamics

Ok, so entropy increases... This is supposed to be an absolute statement about entropy. But then someone imagines a box with a 10 particle gas, and finds that every now and then all particles are in ...
3
votes
2answers
218 views

Symplectic geometry in thermodynamics

There seems to be analogues between Hamiltonian dynamics and thermodynamics given the Legendre transforms between Lagrangian and Hamiltonian functions and all of Maxwell's relations. Poincarè tried to ...
37
votes
4answers
8k views

How close can you get to lava before burning?

As the title asks: How close can you get to lava before burning? I know that it depends on an number of factors; speed of lava flow, wind direction/strength, type(?) of lava flow (related to speed, ...
16
votes
5answers
1k views

What was the entropy of the universe at the time of the Big Bang?

(I asked this question in Philosophy.SE; but I was advised to direct it here, despite it is, in my opinion, somewhat too speculative for physics.SE). High entropy generally means high disorder; and ...
15
votes
5answers
32k views

Does wrapping a wet paper towel around a glass bottle really speed up the cooling process?

There are claims like this one that you can improve the cooling speed of beverages when you put them wrapped in a wet paper towel inside the refrigerator/freezer. I've just tried it by myself and ...
11
votes
1answer
598 views

Thermodynamically possible to hide a Dyson sphere?

You build a Dyson sphere around a star to capture all its energy. The outer surface of the Dyson sphere still radiates heat at much higher temperature than the cold space background, so you're easy to ...
10
votes
6answers
3k views

Perpetual motion machine of the second kind possible in nano technology?

First of all sorry for my English - it is not my native language. During my engineering studies at the university the thermodynamics professor told us that the "second law of thermodynamics is not ...
8
votes
5answers
2k views

How is thermodynamic entropy defined? What is its relationship to information entropy?

I read that thermodynamic entropy is a measure of the number of microenergy states. What is the derivation for $S=k\log N$, where $k$ is Boltzmann constant, $N$ number of microenergy states. How is ...
7
votes
3answers
491 views

Axioms behind entropy!

The concept of entropy is very ubiquitous, we learn about its uses starting from Information Theory (Shannon entropy) up to its basic definition in statistical mechanics in terms of number of ...
5
votes
1answer
111 views

In a Monte Carlo NVT simulation How do I determine equilibration

I'm running an NVT (constant number of particles, volume and temperature) Monte Carlo simulation (Metropolis algorithm) of particles in two dimensions interacting via Lennard-Jonse potential ($U = ...
3
votes
4answers
22k views

Why does a gas get hot when suddenly compressed? What is happening at the molecular level?

My guess is that the molecules of gas all have the same speed as before, but now there are much more collisions per unit area onto the thermometer, thus making the thermometer read a higher ...
3
votes
3answers
411 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 ...
21
votes
2answers
689 views

Why are expressions such as $\operatorname{ln}T$ used in thermodynamics where $T$ is not dimensionless?

In all thermodynamics texts that I have seen, expressions such as $\operatorname{ln}T$ and $\operatorname{ln}S$ are used, where $T$ is temperature and $S$ is entropy, and also with other thermodynamic ...
18
votes
2answers
2k views

Do chemical bonds have mass?

When an exothermic reaction occurs, the energy in the chemical bonds of the reactants is partially transferred to the chemical bonds of the products. The remaining energy is released as heat. For ...
18
votes
7answers
21k views

Does an empty refrigerator require more power to stay cold than a full one?

Given that everything else is equal (model of fridge, temperature settings, external temperature, altitude), over a given duration of having the door closed, does it require more electricity to cool ...
16
votes
2answers
40k views

What exactly is the difference between advection and convection?

After reading Wikipedia articles on advection and convection, I still cannot determine whether there is a consensus on a difference between these two terms. Sometimes, the term convection seems to ...
14
votes
2answers
2k views

Which ball falls faster, the cool one or the hot one?

Suppose we're on the top of the Tower of Pisa (or a larger version of it) with two identical cannonballs. We heat one up (say, to 200 degrees Celsius, or some other high temperature before it starts ...
11
votes
3answers
958 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 ...
10
votes
3answers
886 views

How hot is the water in the pot?

Question: How hot is the water in the pot? More precisely speaking, how can I get a temperature of the water as a function of time a priori? Background & My attempt: Recently I started spend ...
10
votes
2answers
13k views

Can entropy be equal to zero?

I've searched for it but I only found contradicting answers from "scientists": Dr. David Balson, Ph.D. states: "entropy in a system can never be equal to zero". Sam Bowen does not refutes the ...
9
votes
2answers
1k views

Entropy of the Sun

Is it possible to measure or calculate the total entropy of the Sun? Assuming it changes over time, what are its current first and second derivatives w.r.t. time? What is our prediction on its ...
8
votes
3answers
951 views

Where does the kinetic energy go?

A uniform cylinder was placed on a frictionless bearing and set to rotate about its vertical axis. After a cylinder has reached a specific state of rotation it is heated without any mechanical support ...
8
votes
4answers
6k 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 ...
6
votes
3answers
4k views

How do blankets keep you warm?

I heard a famous physicist (was it Feynman?) argue that blankets do not keep you warm by trapping heat but by trapping air next to the body. Is this true?
6
votes
2answers
766 views

Why does $U = T S - P V + \sum_i \mu_i N_i$?

From the first law of thermodynamics: $$ dU = TdS - PdV + \sum_i\mu_i dN_i.$$ Quoting Wikipedia: As conjugate variables to the composition $N_{i}$ the chemical potentials are intensive ...
6
votes
2answers
805 views

Microwave oven + water: dielectric heating or ion drag?

When you place a water or food in a microwave oven, it heats. Which process commits more energy to that: dielectric heating, or ion drag i.e. resistive heating? AFAIK, in distilled water (which is a ...
5
votes
4answers
6k views

Water in vacuum (or space) and temperature in space

So, water in vacuum will boil first and then freeze. I don't know how the freeze happens. As pressure lowers to zero, what happened to freezing point? (I know heat taken by vapor, and the water cool ...
5
votes
3answers
1k 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 ...
4
votes
1answer
221 views

Independence of thermodynamic variables

A given thermodynamical system has a number of state variables, not all of which are independent. Suppose that we have a system which can be specified by $k+1$ extensive variables: $U,X_1,\cdots,X_k$. ...
4
votes
1answer
216 views

Explicit form of the entropy production in hydrodynamics

I'm trying to understand how hydrodynamics arise from a precise, mathematical formulation of thermodynamics, learning mostly from Landau's "Hydrodynamics". So Landau starts from formulating the ...
4
votes
2answers
3k views

Connection between entropy and energy

An isolated system $A$ has entropy $S_a>0$. Next, the isolation of $A$ is temporarily violated, and it has entropy reduced $$S_b ~=~ S_a - S,\space\space\space S\leq S_a.$$ Is it true to say: the ...
3
votes
2answers
447 views

Why is the Carnot engine the most efficient?

It seems that the only condition used in proving that the Carnot engine is the most efficient is that it is reversible. More specifically, the Carnot engine can be run in reverse as a refrigerator. ...
1
vote
1answer
3k views

Carnot efficiency, coefficient of performance for a refrigerator

I was reading that a new type of refrigerator might reach a coefficient of performance (COP) of 10. This seems quite the achievement and the authors state that their approach might achieve a Carnot ...
0
votes
4answers
26k views

Why change in internal energy is zero in isothermal process

In isothermal process $\Delta U =0$. But I am having trouble understanding it. Say we have an ideal gas, and say my temperature is constant but I move the pressure, volume from $(P, V) \to (P-dP, ...
25
votes
4answers
5k views

How is it possible that it can get hotter in the car than it is outside?

The Law of Thermodynamics says that two bodies will eventually have equal temperatures. How is it possible that when you leave your car in the sun, it gets hotter in the car than it is outside? Why ...
8
votes
2answers
588 views

Chemical potential in Thermodynamics

In many scenarios, on computing the partial derivative of the internal energy (U) with respect to mole number (N) is negative. This implies that adding more moles of the substance decreases the U of ...
7
votes
1answer
2k views

Adiabatic expansion in van der Waals gas [closed]

Given a Van der Waals gas with state equation: $$\left( P+\frac{N^2 a}{V^2}\right)\left( V-Nb \right)=NkT,$$ show that the equation of an adiabatic process is: $$\left( ...
6
votes
3answers
597 views

Can temperature be defined as propensity to transmit thermal energy?

I was recently surprised to learn that defining temperature isn't easy. For a long time, it was defined operationally: how much does a thermometer expand. Also surprising, temperature isn't a ...
5
votes
3answers
7k views

Would being underwater help survive a nuclear bomb?

If I jump in my pool, on the river near my house knowing that a nuclear bomb, or atomic or H-Bomb exploded around 10 km from my house, would I survive? The way I see it is that water will protect me ...
5
votes
1answer
239 views

Fluids in thermodynamic equlibrium

I am reading about the Euler equations of fluid dynamics from Leveque's Numerical Methods for Conservation Laws (Amazon link). After introducing the mass, momentum and energy equations, some ...
5
votes
1answer
1k views

Greatest volumetric heat capacity

Is there any substance with bigger volumetric heat capacity than water? According to this table water has the biggest known VHC. But I can't believe that in the 21. century we have no special material ...
5
votes
3answers
6k views

Differentiating the ideal gas law

In reading Fermi's Thermodynamics, to show that $C_p = C_v + R$, the author differentiates the ideal gas law for a mole of gas ($PV = RT$) to obtain: $PdV + VdP = RdT$. Now, the only way I am able to ...
4
votes
4answers
142 views

Why do dark objects emit more than lighter ones?

For the purposes of this question, "lighter" and "darker" refer to the absorptive qualities of the objects. Darker objects absorb more light, and therefore appear darker. I'm trying to understand the ...