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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Heating soil by 1C and maintain the temperature

I would like to calculate how much energy I would need to heat soil by 1C for, let's say, a month. So far, my findings are as follows: First of all let me say I could not attach all the reference ...
2
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3answers
181 views

Interpretation of systems in different state of matters BUT almost identical configuration

For a start, let me clarify that by "almost identical configuration" I mean same volume, temperature and number of molecules (but different pressure). One could for instance take two identical systems ...
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19 views

Existence of a gas with multiple adiabat-isotherm intersections

From here. The problem asks if there exists a real gas such that one of its isotherms intersects with one of its adiabats at multiple points on the $pV$ diagram. I ask if the following approach is ...
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1answer
122 views

Find Solution to the Heat Equation with Bessel Functions and Initial Values

I'm trying to find the time dependent solution to the heat equation in a very long cylinder (the problem only depends on radius, r, and time, t) with the initial value $T(r,0)=T_1$ and the Dirichlet ...
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1answer
31 views

Boiling water function

i want to make a program that realistically boils water with all the variables but know near nothing about the math i got the energy transfer equation down but this seems to be giving me some ...
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25 views

13 watt coffee cup warmer and fish tank? [on hold]

I have a small fish that I wanted to keep warm and seperated from his larger fish tank. I have a large glass measuring cup in which I'll place water. (Up to 500 ml) My room temperature is 75 degrees ...
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1answer
23 views

Is average kinetic energy equal to the total thermal energy of a gas?

"Average KE" as in this equation: $$K_{average} = \frac{3}{2} kT$$ Since potential energy in ideal gas model is eliminated, I guess this equation is also for the total thermal energy of a gas/a ...
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96 views
+100

Is the stability matrix of a linearised RG flow always diagonalisable?

This is a follow up on "Why are the eigenvalues of a linearized RG transformation real?". My question is simple: Is there some physical (or mathematical) reason for the stability matrix of ...
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0answers
38 views

Efficiency of a compressed air engine

I am doing some personal research on the efficiency of compressed air cars. I am currently trying to come up with an 'ideal' cycle to use as a reference for efficiency (c.f. Otto Cycle for petrol ...
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2answers
125 views

What is the heat sink in an internal combustion engine?

I believe that the heat source is the spark plug because it heats up the fuel-air mixture and that the working substance is the fuel-gas mixture. Then, I know that the explosion created by the fuel ...
2
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1answer
222 views

Newton's law of cooling for the heat equation boundary condition

Newton's law of cooling says the temperature of an object satisfies $$ \frac{dT}{dt} = -k(T(t) - T_0),\tag{1} $$ where $T_0$ is the surrounding temperature. See these HTML notes for example. Now if ...
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3answers
81 views

Does wax go through a sharp phase transition when it melts?

When ice melts, the system goes sharply from being solid to being liquid. There is no intermediate state where it is soft. This is a true phase transition. The thermodynamic potential is not an ...
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1answer
96 views

Is equilibrum condition consequence of second law of thermodynamics?

Equilibrum state is generally stated to be most stable state. Is equilibrium state consequence of second law of thermodynamics at microscopic level. i.e a system tends to reach the equilibrium state ...
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3answers
42 views

How is Work by a System Interpretated in the First Law?

Good evening. I've happened to be sitting down today and just couldn't wrap my head around this question which seems rather simple at first. From reading about the first law and sign conventions as it ...
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5answers
2k views

Does the Moon's core still contain significant heat?

On earth, using earth-sheltering techniques can significantly reduce the temperature fluctuations on a structure. Would the same statement be true as well on the Moon? Does the Moon's core still ...
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2answers
182 views

Liquid column “recoils” in a sealed cylinder when hit by a piston — is it possible?

Consider a cylinder filled partially with a liquid (e.g. water). The cylinder is sealed, and is at held at room temperature (e.g 298K). At equilibrium (or when no external disturbance is imparted to ...
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1answer
38 views

Single story, hotter outside, some shade from porch, enough to cool house with/without air flowing?

So, I know this question, without shade or one-story, has been answered many times on the 'net, so I have some actual physics for the one-story. I am fairly certain my roommate mixed up thermal ...
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2answers
1k views

Physics-based derivation of the formula for entropy

I am looking for a derivation of the formula $$S~=~-\Sigma_ip_i \log (p_i).$$ for entropy, from first principles. I only wish to assume the laws of physics, and without involving concepts in ...
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3answers
55 views

Zeroth law of thermodynamics and related discrepancy?

Consider the case: Assume molecules to sphere and their |velocity| as an indication of their temperature. Now for head on elastic collision (i.e transfer of heat), assume the momentum is conserved ...
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0answers
19 views

What allows some objects to have a high specific heat capacity?

I know that temperature is a measure of thermal energy associated with the KE of molecules. But when heat enters an object, surely those molecules now start vibrating and translating, so this results ...
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0answers
22 views

Basic Thermodynamics: Adiabatic Process

we know that for adiabatic process $W=-\nabla E$. My question is here why do we consider $W=nC_v\nabla T$
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1answer
105 views

Spontaneity / Free Energy of Non-Isothermal Process

I'm trying to determine a lower bound for the work input necessary to make an entropy-reducing process "spontaneous" in the sense that the 2nd law is not violated. For a constant temperature and ...
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1answer
101 views

Why temperature of liquid drops after spraying thru a nozzle?

We have tested in our lab as mentioned in the picture. We connected hot water at 130degree F at 40Psig to a nozzle(bottle sprayer).we measured the temperature differences inside tank and after ...
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1answer
23 views

Accounting for the thermal excitation in the Richardson-Dushmann equation or the work function equation

When accounting for thermal excitation in a system that is not in thermal equilibrium, heat is constantly flowing through a material, should I account for the thermal excitation in the work function ...
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1answer
63 views

Is there a formula/relationship between $q$, $m$, $c$, $\Delta T$ and $t$ (time)?

I am currently doing an experiment where I am burning methanol to test for the heat of combustion. To test this, I am heating 100g of water and raising it by 20 degrees Celsius before extinguishing ...
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3answers
9k views

Entropy Change During Reversible Processes

I'm confused about the Second Law of Thermodynamics. The Second Law of Thermodynamics prohibits a decrease in the entropy of a closed system and states that the entropy is unchanged during a ...
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1answer
89 views

On Thermodynamics of Spontaneity of the Reduction-Oxidation Chemical Reactions

According to the first and second law for a closed system containing different chemicals we have \begin{align} &\delta Q - \delta W = dU = T dS - p dV +\sum_i \mu_i d N_i\\ ...
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5answers
85 views

What happens to the kinetic energy of a dropped ball when it comes to rest on the ground?

If we want to drop a ball from a height, we calculated that potential energy at bottom is zero and we say it is converted into kinetic energy. At that movement, if it is a kind of sand, we find it ...
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2answers
88 views

How does pressurized gas constantly push?

If a gas, such as hydrogen, is pressurized into an air tight container, a force in terms of pascals (or whatever unit you want to use) is exerted, correct? That is what pushes against every surface ...
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1answer
35 views

What causes a eutectic point?

What causes the melting point depression known as a eutectic point? The temperature depression of a eutectic point can be calculated from the enthaplies and entropies of fusion of the two ...
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1answer
187 views

Boltzmann Distribution of Electrons in Confining Potential

I have a particle simulation wherein many non-interacting electrons are trapped in a electric potential well. I would expect, and therefore I initialize according to this, that the electrons would ...
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2answers
49 views

What is the perfect Ice Cube? [closed]

I have a freezer with -25°C temperature and Soda with +30°C. What is the ice-cube-sized material I can put in the freezer that can bring my glas of soda to the lowest temperature? Are e.g. saltwater ...
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0answers
28 views

Expression of heat by the Brownian motion

folks. I was reading a paper from PRE and I'm not sure what's going on about the following equation. So for the system composed of two heat baths governed by Brownian motion, the entropy change of the ...
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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 ...
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1answer
370 views

Why does the Nusselt Number for a Constant Surface Temperature, Thermally Fully Developed Circular Pipe Converge To a Constant of 3.66?

I have flow in a pipe with a constant surface temperature boundary condition. The velocity profile is fully developed and the temperature profile is also fully developed. In my textbook, it says that ...
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2answers
128 views

Why is the outside run of high presure line on a ductless mini-split airconditioner insulated?

On every ductless mini-split air conditioner I've ever seen, both the high and low pressure lines are insulated between the compressor and the building. It seems like the liquid refrigerant coming ...
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2answers
9k views

Water Electrolysis Calculations

From Wikipedia (http://en.wikipedia.org/wiki/Water_electrolysis#Efficiency): The electrolysis of water requires a minimum of 237.13 kJ of electrical energy input to dissociate each mole. Each ...
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2answers
318 views

Will Neil Armstrong's moon boot marks really last for thousands of years?

This question concerns the residual heat (if any) contained within the Earth's moon. At the time of the Apollo moon landings, it was widely reported that the boot marks left by the astronauts would ...
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3answers
5k views

Why do reversible processes not increase the entropy of the universe infinitesimally?

The book Commonly Asked Questions in Thermodynamics states: When we refer to the passage of the system through a sequence of internal equilibrium states without the establishment of equilibrium ...
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3answers
809 views

Question about the proof that heat capacity goes to zero if temperature approaches $0K$

I don't completely understand the proof that is given for the claim that the heat capacity goes to zero, if the temperature approaches $0K$. They do it as follows, if $C_x$ is the heat capacity where ...
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1answer
153 views

Does an ice cube change its core temperature as it melts?

Let's say we have an ice water solution in an ambience with room temperature, i.e. there is a permanent exchange of energy. Let's suppose the ice cubes were from the freezer, which is at $-18°C$. The ...
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4answers
2k views

What happens when I place an ice cube into boiling water

My friend tried this experiment at home: She put an ice cube into boiling water. The water in the container stops boiling while the ice melts to water. My question: Why does the water stop boiling ...
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0answers
26 views

Finding root mean square speed of an ideal gas [closed]

Again a question ive never seen in my life... The molecules of an ideal gas have a root mean square speed of 520 meters per second at a temperature of 27 degrees celcius. Calculate the r.m.s speed ...
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1answer
2k 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 ...
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1answer
46 views

How long will pressure exist in this case?

I'm currently fabricating my project, which currently exists only on paper, and there's one question to which I have stumbled today. Suppose, that I have a pressure vessel capable of holding upto ...
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1answer
146 views

What causes a heat generating source to stabilize at a certain temperature? (Answered by myself, I think)

For example, we out a heat sink on a microprocessor to keep it cooler. I understand that if we run 100 watts of electricity through the microprocessor, it will generate 100 watts of heat, or 100 ...
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1answer
90 views

Why is not the entropy of earth increasing? [closed]

The entropy of earth is not increasing significantly in the past billions of years. So what is the reason? It must be because of the sunshine. But I still do not have a concrete picture.
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1answer
40 views

Alka Seltzer Model: formulas/models requested [closed]

This evening I became fascinated with how my Alka Seltzer tablet disintegrates over time within a small portion of Diet Lipton Citrus Ice Tea. I used a nearly frozen cup; tall, as one might request in ...
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0answers
119 views

Doping in physics [closed]

compare and contrast the principle involved in doping and bonding. Why do increase in thermal agitation in semiconductor result in increase in conductivity
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0answers
26 views

thermodynamics question [closed]

I've thought this question for a really long time and now i need help..... The question is "the volume occupied by a mole of liquid water at 100 degrees celcius is (1.87 exponent -5) cubic metres. ...