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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How would I calculate the convection coefficient in transient convection?

So I have faced a problem dealing with transient conduction and I need a little help with the problem solving concepts. I need to determine how long it would take to reach the final temperature but I ...
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2answers
833 views

Transient radiation--heating a slab

Hey guys I really need help on this problem. A ceramic slab of dimentions 5cm x 10 cm x .25 cm has to be heated to $177\,^{\circ}{\rm C}$. The ceramic slab travels on a conveyor belt traveling at ...
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1answer
505 views

Relation between external magnetic field intensity H, magnetisation M and the entropy?

How are the external magnetic field intensity H, magnetisation M and the entropy related to each other? i.e. if I change the magnetic field intensity by dH what will be the change in entropy dS in ...
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2answers
2k 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 ...
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1answer
170 views

Thermodynamics and cross entropy

I am facing with the concept of cross entropy. I would like to know the thermodynamic and statistical meaning of cross entropy (if exists)?
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1answer
89 views

System moves away from equilibrium $\rightarrow$ it has energy added?

Suppose there is an isolated system $A$ at time $(-\infty, t_1)$, whose entropy is $S=S_{max}$, i.e. it is at thermodynamical equilibrium. Between moments $[t_1, t_2)$ the isolation is violated and ...
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1answer
2k views

Time it takes for Temperature Change

I have just been thinking about it for a while and would like to see if there is a way to do this problem. The Setup: We have an insulated cup with mass $m_c$ and specific heat $s_c$. The cup is at ...
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1answer
194 views

Reference paper to support information — energy relation ($kT \ln2 \rm\frac{J}{bit}$)

In answer to Maxwell's Demon Constant (Information-Energy equivalence) there is stated that one bit of information allows to perform $kT \cdot \ln2$ Joules of work. Which paper supports the ...
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176 views

Is a closed system receiving heat guaranteed to have entropy below maximum?

Isolated system will preserve its entropy unchanged (for fully reversible system) or it will increase its entropy (for system with irreversible processes). For a closed system, it can receive energy ...
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6answers
28k views

Why are volume and pressure inversely proportional to each other?

It makes sense, that if you have a balloon and press it down with your hands, the volume will decrease and the pressure will increase. This confirms Boyle's Law, $ pV=k=nRT $. But what if the ...
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4k views

Why is glass a good conductor of heat?

AFAIK Glass is insulator, it doesn't have free electron. It's said metal is a good conductor of heat because it has free electron, glass doesn't have free electron, why it is a good conductor of heat? ...
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2answers
246 views

How would I go about solving this transient convection problem if the mean fluid temperature is constantly changing?

Let's say I have a ceramic slab on a conveyor belt that is initially at $450\,^{\circ}\mathrm{C}$ and there is air being blown over it at a speed of $35 \frac{m}{s}$ with an ambient temperature of ...
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1answer
2k views

Equations instead of psychrometric charts

I want to create a program that will accurately simulate a condensor. I want to use the data in psychrometric charts. But I cannot and hence want to use equations that show similar data. Any idea ...
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1answer
239 views

Interacting system and relaxation times

I got a question I'm not sure how to state precisely or is it even valid. Any help is most welcomed. I stripped the question of all details because I wanted to emphasize my problem, but should ...
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1answer
174 views

nature of glass transition

I am reading in some book: "The glass transition is similar in appearance to a second-order phase transition, but it is not a true thermodynamic phase transition. This is because the transition ...
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2answers
6k views

Is there a simple way to derive a T-S diagram from a p-V diagram for arbitrary processes?

Often, for thermodynamic processes only a p-V diagram is shown. Even without hard figures, the shape of the curve can be helpful to evaluate the process. However, it is hard to figure out for real ...
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5answers
10k views

If I take a bottle of air into space, and open it, where does it go?

It seems to me that space doesn't have any/much air, and if my bottle is full of air, when I open it, where does the air go?
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2answers
509 views

Intuition behind the concept of heat

Even if elementary, I'm afraid I am still not comfortable with the concept of heat. I can picture heat as some kind of incompressible fluid that is transferred from a body to another in such a way ...
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2answers
2k views

Why did the microwave oven only heat my coffe half as much as expected?

A sticker on my microwave oven states its output effect to be 750W, which is 180 calories per second. This means that heating 250g of water by one degree celsius would take 250/180 = 1.4s. Now, my ...
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5answers
10k views

Is fire plasma?

Is Fire a Plasma? If not, what is it then? If yes why, don't we teach kids this basic example? UPDATE: I probably meant a regular commonplace fire of the usual temperature. That should simplify ...
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4answers
1k views

Maximum efficiency for a counter-current heat exchanger (double flux controlled motorized ventilation)

I am not sure if I can explain the question correctly because I don't know the name of this mechanism in English. This is my explanation attempt: In a house, a tube is expelling the air from the ...
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2answers
2k views

How fast is heat transferred by conduction?

How fast is heat transferred by conduction? Is there some simple, but quantitative way that starts from some properties of the material (e.g. its thermal conductivity) and makes rough predictions, for ...
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3answers
4k views

Given constant T, why does P affect internal energy?

It has always bugged me that tables for water (and other) properties have the capability to look up internal energy as a function of both temperature and pressure. If we limit the discussion to ...
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1answer
366 views

What is lambda R in Richardson's Law?

I've got to calculate the thermionic emission through a diode, so I need to use Richardson's Law. However, one thing's got me confused - according to the Wikipedia page: $$J = A_GT^2e^\frac{-W}{kt}$$ ...
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1answer
1k views

When a color LCD/LED display is off it is black, when on it is colored. What color to light is an LCD/LED display

When a modern display using pixels is turned on its colors are different than the black that the screen is actually made up of. When put in light, such as sunlight, does the screen react to the color ...
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422 views

For how long can we make an air balloon stay flying via remotely controlled heat system?

I wonder how a function $$f=f(h, m_{\mathrm empty}, V_{max}, T_{h}), $$ with the top height $h$, the empty mass $m$, the maximum volume $V_{max}$, and the temperature of the heater $T_{h}$ would look ...
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1answer
122 views

Intuitive picture for spin-fluctuations contribution to specific heat of He3

Usually when discussing Fermi liquid theory, it is stated that due to the quasiparticles effectively behaving like a free electron gas with effective mass, the specific heat is linear in $T$ at small ...
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2answers
2k views

Why is oil a poor conductor of heat?

Why is oil a poor conductor of heat? Does it help in insulation?
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4answers
2k views

Is the efficiency of a stirling engine affected by dead volumes?

Consider the p-V diagrams of an ideal and real stirling cycle, as found here The article suggests that dead volumes reduce the efficiency of a stirling engine. On the other hand, a simple thought ...
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1answer
2k views

Why is the lid of the cookware kept on induction cooker not hot?

Induction cookware cooks food by inducing an electro magnetic field in the ferro-magnetic cookware. Since iron offers a lot of resistance to the current, the current is converted into heat in the ...
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1answer
437 views

Per unit area, is there more heat transfer through the open top of a mug or the side walls?

I had a mug of hot coffee cool too rapidly for my liking the other day, which made me wonder what was the greater contributor to heat loss for a typical open-top ceramic mug: the open top, the walls, ...
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2answers
1k views

What is the physical or mathematical meaning of the Gibbs-Duhem equation?

The Gibbs-Duhem equation states $$0~=~SdT-VdP+\sum(N_i d\mu_i),$$ where $\mu$ is the chemical potential. Does it have any mathematical (about intensive parameters) or physical meaning?
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739 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 ...
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3answers
5k views

How to calculate the evaporative cooling rate needed to protect a house from forest fire

Recently in our area there has been a large forest fire and I've been looking into home defense from such things. I am not a physicist - but can do some basic math. I was wondering how I could ...
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1answer
272 views

How to express the heat capacity in terms of heat?

The first law of thermodynamics divides the internal energy change into contributions of heat and work. $$\text dU=\omega_Q-\omega_W,$$ Here I chose the notation to emphasise that the two parts are ...
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1answer
411 views

Boltzmann Brain Immortality

This is a somewhat philosophical question. Given that even after person A's death, there's a non-zero chance that a brain fluctuates into existence with exactly the same structure and memories as A's ...
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1answer
1k views

How much oxygen would be consumed on a 1 cm squared surface which is on fire?

I'm trying to figure out how much oxygen the Human Torch produces when he is on fire. I figure if I knew how much oxygen on average (per second?) is consumed by a 1 cm squared surface which is ...
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1answer
1k views

Why is compressible flow near the choke point so efficient?

Imagine a steady state, one-dimensional, compressible flow in a horizontal pipe of constant cross sectional area. This flow can be isothermal, adiabatic (Fanno), or diabatic (Rayleigh). As an ...
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1answer
2k views

Finding electric power generated using heat transfer

I'm working through an example I have been given to study. Suppose I have a 2m X 4m photovoltaic panel on my roof that is irradiated with a solar flux of $G_s = 700W/m^2$. Given: $\alpha_s = 0.83$ ...
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2answers
421 views

What's the classical limit of the pressure of the ultrarelativistic Bose gas

The pressure for the ultrarelativistic Bose gas is $$p~=~U/(3V) ~\propto~ (kT)^4/(hc)^3.$$ It looks to me like it diverges for $h \to 0$. Looking at the derivation, it diverges because $h$ is the ...
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1answer
362 views

Does a negative line tension in a 3-fluid immiscible interface make sense?

This question is inspired by this question/answer pair: Is this formula for the energy of a configuration of 3 fluids physically reasonable? Consider three immiscible fluids forming contact surfaces, ...
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3answers
316 views

Does the second law of thermodynamics tell me how the entropy changes?

In thermodynamics I can e.g. compute the properties of ideal gases with certain energies $U_1,U_2$ in boxes with certain volumes $V_1$ and $V_2$. Say I have two such boxes and they have some specific ...
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3answers
673 views

Why does large curvature of spacetime imply high temperature?

I`ve just stumbled about a sentence which says that high curvature of spacetime implies that any matter present is at high temperature. This somehow confuses me, so my probably dumb question(s) are: ...
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1answer
3k views

How much disolved oxygen is removed by boiling water?

Apologies if this is a chemistry question I've read that drinking water contains dissolved oxygen to the tune of $10\:\rm{ppm}$. I've also read that raising the temperature of water will remove some ...
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1answer
657 views

Can we explain physical similarities between Black Scholes PDE and the Mass Balance PDE (e.g. Advection-Diffusion equation)?

Both the Black-Scholes PDE{*} and the Mass/Material Balance PDE have a similar mathematical form of the PDE which is evident from the fact that on change of variables from Black-Scholes PDE we derive ...
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2answers
429 views

What's the validity of the thermodynamic definition of entropy?

Thermodynamic Entropy Variation is defined as $$\Delta S = \int_i^f \frac{dQ}{T},$$ where $i$ and $f$ are the initial and final states of the process. My question is: does this equation apply to ...
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3answers
1k views

Using 'Euler's Homogeneous Function Theorem' to Justify Thermodynamic Derivations

I've been working through the derivation of quantities like Gibb's free energy and internal energy, and I realised that I couldn't easily justify one of the final steps in the derivation. For ...
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1answer
124 views

What is a qualitative explanation for order of values of standard entropies of formation for isomers of hexane?

I have literature values for the standard enthalpy and Gibbs energy changes of formation at 298 K, $\Delta H_\text{f}^o$ and $\Delta G_\text{f}^o$, for product in the ideal gas state. Since $G=H-TS$ ...
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1answer
246 views

Hygiene thermodynamics 2

Two kinda related questions here: Is evaporation rate and temperature difference related? There is an experiment of pouring cup of hot water out the window during winter. The water evaporates almost ...
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4answers
2k views

Sauna thermodynamics

Why does it get hotter (feel hotter) in a sauna when one pours water over the hot stones? Wikipedia says that the water condenses onto the skin, but the actual air humidity is so low that I doubt ...