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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Specific heat and latent heat of fusion [closed]

A thermally isolated vessel contains 100g of water at 0 degree c. When air above the water is pumped out, some of the water freezes and some evaporates at 0 degree c itself. Calculate the mass of the ...
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2answers
73 views

If you heat a closed tube, does its mass change?

This is a result of a debate with friends. We were more concerned with the nano- and smaller-scale effects than the larger effects. Let's assume that we have a closed tube filled with air. We weigh ...
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1answer
71 views

A possible way to initiate a nuclear fusion reaction without a nuclear fission trigger [on hold]

A few years back I read in either Popular Science or Popular Mechanics about a new kind of high explosive that had an energy density many times higher than the most potent ones previously developed, ...
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1answer
160 views

Increasing the efficiency of solar cell systems

As far as I know, there are currently two main approaches to utilising solar radiation for maximum energy conversion to electricity. These are either direct conversion to electricity, using ...
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0answers
23 views

Derivation of adiabatic T-V realtion [closed]

This is giving me hard time to the point of extreme annoyance with the algebraic manipulation. I have $$P^{-R} T^{C_v+R}$$. How do I get to $$\text{TP}^{\frac{(1-\gamma )}{\gamma }}$$ Appreciate any ...
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11 views

Solving a Non-Isentropic rankine Cycle [closed]

So I have been using a computer program to determine the various state parameters of a Rankine cycle for homework and I've hit a bit of snag. I can't seem to fully solve for the system with the knowns ...
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1answer
48 views

How much work can we extract from a black hole?

Recently, I've read an article on scientific regarding the possibility of a stairwell into a blackhole (unsurprisingly, it isn't possible). I've found the following question more interesting: ...
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3answers
193 views
+50

Causality principle and Entropy (Second Law)

I was reading about the light cone in relativity and I got to the point where in order to avoid paradoxes one can introduce the causality principle: Causality Principle: For every inertial ...
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24 views

Determine the pressure on the low-pressure side of a gas flowing through an orifice [closed]

A gas flows through a small orifice in a pipe as shown above. On the higher pressure side, the gas is at 1Mpa and temperature of 300K. The pressure reduces to 1kPa after it flows through the ...
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42 views

Heat equation with with time varying boundary condition [closed]

I am trying to solve the heat equation: $$\frac{\partial U}{\partial t}=k \frac{\partial^2 U}{\partial x^2}$$ with the following boundary conditions: \begin{align} U(x,0)&=0 \qquad t<0\\ ...
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2answers
54 views

Practical understanding of energies in LHC collisions

Is visible light emitted at the point of collision in the LHC? If so, how bright? How much water could be heated up by 10 degrees celsius (this is the energy from collision only, not the energy to ...
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1answer
25 views

What polytropic index should I use for spherical accretion onto a degenerate object?

Take a degenerate object, such as a white dwarf, embedded in a gas cloud. The cloud is small enough such that the size of the object is non-negligible in comparison - that is, the white dwarf takes up ...
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1answer
28 views

Rate of evaporation of water with mass loss at set temperature and pressure

I have a cylindrical tank with an 'm' amount of water in it. The tank is sealed except through an exhaust with a fan that keeps a lower than atmospheric pressure (but not vacuum) inside the tank. The ...
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0answers
8 views

Ratio between thermal conductivity heat transfer coefficient

Consider the Robin boundary condition for the diffusion/heat equation $u_t=a(t)u_{xx}+f(x,t)$: $$\mathrm{-k(t)u_x(0,t)=h(t)u(0,t)}$$ or $$\mathrm{u_x(0,t)+\frac{h(t)}{k(t)}u(0,t)=0}$$ where ...
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2answers
97 views

Susceptibilities and response functions

It is often confusing whether a susceptibility is the same as a response function, specially that often they are used interchangeably, in the context of statistical mechanics and thermodynamics. Very ...
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1answer
17 views

Thermodynamics Work Done At constant pressure and charles law

If we fix a piston in the mouth of a cylindrical container in such a way that the piston settles when the pressure from outside the piston is equal to that of the gas within the container, and ...
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2answers
56 views

How do we know that the Law of Cooling is true?

According to James Stewart's Calculus book (exercise 14, page 609), Newton’s Law of Cooling states that the rate of cooling of an object is proportional to the temperature difference between the ...
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1answer
16 views

Maximal work from cooling a portion of water

Let's consider a portion of water of mass $m$. We want to find out how much work (electric energy) we can acquire by cooling it from temperature $T_1$ to temperature $T_0$. The most efficient way is ...
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7answers
2k views

Drying clothes with the sun's heat, without any air

Will my wet clothes dry if I hang them under the sun, and if there is no air around the clothes? In other words, do I need both air and heat to dry wet clothes, or is heat alone (in the imagined ...
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1answer
45 views

Apparent Contradiction in Helmholtz Free Energy

I am looking at some mathematics of HelmHoltz free energy. Naturally, wikipedia is one of the (hopefully) more reliable sources of information. In its derivation section, the last equation states that ...
2
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2answers
72 views

Blackbody radiation and emissive power

According to blackbody radiation theory, and thanks to Planck, we now know that there is a energy density, $u(\lambda,T)$ [$J/m^3$], associated with a certain wavelength at a particular temperature. ...
2
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0answers
78 views

Is the Boltzmann brain problem really clearly established as a problem?

Starting from the assumption of a cosmology in which Boltzmann brains dominate over evolved ones, it is not immediately obvious to me that there is a real problem, since only Boltzmann brains ...
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2answers
1k views

Why is pale coloured skin said to absorb more UV?

Many resources state that light skin/pale skin absorbs more UV than dark-colour skin. Doesn't black absorb maximum radiation? For an example, see this article: Natural selection therefore ...
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2answers
45 views

Is liquid helium expensive because of its rarity or because of its low liquidification temperature?

I once argued with my roommate about this problem. Of course it is rare. But presumably the low liquidification temperature makes the cost high too. So which is the primary reason?
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0answers
11 views

Looking for $\frac{dH}{dT}$ in terms of total latent heat and the change in magnetic moment

As the title states, I'm looking for $\frac{dH}{dT}$ in terms of total latent heat $L$ and the change in magnetic moment $M$. I think I can use the equation $$dU = TdS - MdH$$ where $M$ is the total ...
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1answer
42 views

Change in entropy of the Universe for charging/discharging a capacitor

I just need to know what exactly happens when charging/discharging a capacitor. Is there any heat transfer between the capacitor and the surroundings? Is there a heat transfer between other parts of ...
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4answers
1k views

The unreasonable effectiveness of the partition function

In a first course on statistical mechanics the partition function is normally introduced as the normalisation for the probability of a particle being in a particular energy level. ...
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0answers
20 views

In the Leidenfrost effect why droplets flow upwards

Leidenfrost effect is a physical phenomenon when a hotter liquid when it comes to contact with another significantly hotter body then the liquid creates a vapor layer and the liquid flows.Now why when ...
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1answer
20 views

Is thinner glass more heat resistant?

I heard that vessels made of thin glass are more heat resistant (less likely to crack) than those made of thick glass. Is that true, if so, and why? On one hand, heat propagates more quickly through ...
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1answer
42 views

Do magnetic fields cause ionisation of gases?

I am doing my final year engineering project on Magnetic Field Assisted Combustion and was curious to see what people thought about it. Companies sell rare earth magnetic arrangements to be attached ...
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1answer
25 views

Specific heat ratio range [closed]

I know that the specific heat ratio, $\gamma$, exists in the range $1 <\gamma< 2$, and I am required to demonstrate a proof of this. I have come across the following proof (but I don't really ...
4
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1answer
36 views

Speed of sound of a gas mixture

Is there a general mixing rule for estimating the speed of sound of a gas mixture, given the speed of sound in the components? From the ideal gas relation: $$c = (kRT/M)^{1/2}$$ (where k is the ...
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0answers
11 views

Pressure change in throttling & flow rate reduction in a water circulation

If the pressure in a water pipe line is reduced from 7kgf/cm2 to 4 kgf/cm2, what is the % reduction in flow rate?
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5answers
762 views

What exactly is fire? [duplicate]

What is fire? Is it a wave or is it matter? Where does fire come from? Does everything burn with fire? (for example: water and some metals don't burn).
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1answer
33 views

Steady State For An Air Conditioner

Here's the problem I'm trying to solve (from Blundell's Concepts In Thermal Physics): I just don't understand what "steady state" exactly means here. Does it mean that the temperature of the house ...
2
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1answer
44 views

Significance of Reversible and Irreversible Process

I would like to ask the following questions: What is the significance of reversible and irreversible nature of thermodynamic process? (I understand that reversible processes are quasi-static, ...
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1answer
14 views

Does the rate of heat dissipation slow as the temperature differential between the heated object and the surrounding environment decreases?

Does the rate of heat dissipation slow as the temperature differential between the heated object and the surrounding environment decreases, or is it constant? To put this into context, picture the ...
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1answer
21 views

General relation between power density of any engine and dissiapation rate and temperature

Many years ago ( before my university studies ) I read that renewable resources are fundamentally limited by laws of thermodynamics to produce energy very slowly (low specific power or power density) ...
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2answers
51 views

Violently shaking object

Would violently shaking something cause a temperature change? For example, if a container of water was shook violently enough; would it be possible to make it boil?
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2answers
1k views

What happens to a body, initially at 300K, kept in isolated space? Will it's temperature drop to 0k?

With regards to Thermal Radiation, given a stable body initially at 300 Kelvin placed in isolation, after continuous Thermal Radiation will it's temperature gradually reduce to 0 kelvin ...
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0answers
25 views

What will be the final temperature of water when heating at a a rate 10J/s?

If I supply heat of rate 10W to water of mass 250g and initial temperature of water is 29 degC then what will be the final temperature of water when steady state is reached. **Water is heating in open ...
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1answer
25 views

COP of Stirling engine

If we consider the theoretical maximum of COP for a heat pump, we will see that it is possible to make it infinite by approaching the temperatures of each source to each other. Yet, in a real engine ...
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2answers
60 views

Using Maxwell's demon alongside quantum mechanics to contradict 2nd law of thermodynamics

Suppose we have a steady state universe with a gas chamber resembling that of Maxwell's demon that is used to power this hypothetical heat engine as molecules transfer to their respectable sides based ...
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0answers
25 views

Water as heat source in a heat pump

Consider we have water flowing in a pipe, which is working as a hot source for a heat pump. We are able to know the temperature of the water in both ends of the pipe, yet we don't have information ...
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1answer
27 views

The potential profile of two batteries in series

I would like to know the potential profile in two batteries in series, like the battery in the attached image.
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1answer
44 views

Can sound frequency create more ideal conditions for fire?

You may have seen the recent story of a device that engineers claim can extinguish flames using sound frequency. An older article loosely explains the theory behind how this works: Sound travels ...
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46 views

Thermodynamics and Axioms and the like

Can thermodynamics and any important related information be expressed as a set of axioms with various 'rules of manipulation'?
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1answer
83 views

Why is $\int_{s} \mathbf{h}\cdot \mathbf{n} da = - \dfrac{dQ}{dt}$ & not $\int_{s} \mathbf{h} \cdot\mathbf{n} da = - \dfrac{dQ}{dt} .{dt}$?

I was reading the Lectures of Feynman about surface integral where a situation in which heat is conserved has been dealt. Let there be $Q$ heat energy present inside a body. Now, if there is net heat ...
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1answer
51 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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2answers
57 views

What is the correct form of heat diffusion equation taking into account temperature dependence of specific heat

What is the correct form of the heat diffusion equation in 1D if we take into account the temperature dependency of specific heat capacity? $$ \rho\frac{d(cT)}{dt} = ...