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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Boltzmann constant--why units aren't joules per degree PER MOLECULE

I didn't think the information I looked at did a very good job of explaining the Boltzmann Constant, kB, but once I did this next simple calculation, it seemed clear that what I didn't understand were ...
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2answers
103 views

Are there materials that get softer with temperature decrease?

Could be there material that begins melting/softening when it's temperature is lowered? I would say no, but I've seen enough physics to know that not always life is so easy. Moreover I think I've ...
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2answers
101 views

What's the efficiency of real heat engines?

Real heat engines always have lower efficiency than the Carnot efficiency. I wonder how efficient real engines can be? Can their efficiency get anywhere near the Carnot-limit?
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56 views

Thermal expansion of two bolts

I have this question asking me about the thermal expansions of two bolts, a steel one, and a brass one. I have the equation: $\ (\delta L/L)/T = \alpha $ I don't understand how I can use this ...
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8answers
180 views

Why do turbine engines work?

I know roughly how a turbine engine (let's say a gas turbine producing no jet thrust) is supposed to work: The compressor forces fresh air into a combustion chamber, where it reacts with fuel to ...
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3answers
110 views

Does one square centimenter of the sun core really radiate this amount of energy?

I have been thinking that since the core of the sun maintains its temperature at 15 million degrees Kelvin, then every cubic centimeter of this core is receiving a certain amount of energy to keep it ...
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50 views

Latent heat of condensation

Regarding atmospheric processes, I understand that energy is required to evaporate water by moving molecules further apart i.e. a phase change from liquid to gas. The air then ascends into the ...
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1answer
72 views

Which air will give more effective cooling ? Dry or Moist?

Yesterday, I had an innovative idea. I live in India, and in summer season, the temperature can reach up to 45 degree Celsius. We use Split 1.5 Ton AC in our small office. The idea is to put an ...
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1answer
35 views

Can the thermal energy in an object be aligned, creating a thermal jerk?

Thermal energy is related to the speed at which atoms vibrate, therefor thermal energy is a form of motion. In a stationary object, all of these vibrating atoms 'impact' each other in such a way that ...
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121 views

Principle of Caratheodory and The Second Law of Thermodynamics

Background Constantin Carathéodory formulated thermodynamics on a purely mathematical axiomatic foundation. His statement of the second law is known as the Principle of Carathéodory, which may be ...
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1answer
52 views

Enthalpic Excess - Regular Solution Upon Mixing

My professor, in his handout, says the following about this diagram : For $\beta>+2$ there are two minima and phase separation occurs driven by unfavourable enthalpic interactions. I ...
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2answers
71 views

The entropy of the universe in an irreversible cycle [closed]

Suppose we have an ideal gas performing an irreversible cycle composed by: an isothermal transformation at $T_{1}$; an isobaric transformation at $P_{A}$; an isothermal transformation at $T_{2}$; an ...
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1answer
63 views

Steady state average of physical quantities

Consider the following Hamiltonian: $$ H = \sum_n \left[\dfrac{p_n^2}{2m_n} + U(x_l) + V(x_{l+1} - x_l) \right], $$ that corresponds to a 1-D system of particles with nearest-neighbor interactions ...
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21 views

Hamiltonian (temperature?) and frame of reference

So we can define a particle by defining its kinetic and potential energy, knowing that we can get a wavefunction describing a particle. But the kinetic energy involves motion, and motion can be ...
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2answers
73 views

First Order Phase Transition - Heat Capacity

At constant pressure : $\frac{dH}{dT}=C_p $ Hence, why doesn't the graph of $C_p$ against $T$ show two perfectly horizontal lines (with the obvious discontinuity) ? I mean the gradient of $H$ ...
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0answers
20 views

If maximal work is extracted, in what manner does the heat flow between bodies?

There are two bodies starting with different temperatures and specific heat capacities (for simplicity assume that the bodies are solid to the volumes are constant). Let the rate of heat flow from ...
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3answers
192 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, ...
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1answer
34 views

Chemical Potential with Pressure at Constant Temperature

The above picture shows how the Gibbs free energy of a solid, liquid and gas vary with pressure. I have found no similar graphs on the internet that match the above. Instead I often find these ...
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3answers
68 views

Is there a conveniently small unit of volume for talking about molecules?

When discussing energy transformations on the molecular scale, we usually use electronvolts as the energy unit. This is handy because chemical bond energies are a few electronvolts in magnitude. I ...
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4answers
801 views

Wrapping plastic in aluminium foil to protect it from heat

Does it make any sense to wrap the plastic handle of a pan in aluminium foil to protect it from overheating when placing it to the hot oven?
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1answer
47 views

Exception to weight of hot vs cold water?

I have seen the various posts regarding the comparison of weight of heated and cold water.But is there any contradiction?I live in really hot conditions and as such tap water literally 'boils' ...
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2answers
92 views

Specific Heat equations - paradox!

We know that enthalpy is $h = u + pv$. Where $u, p, v, h$ are internal energy, pressure, volume, and enthalpy respectively. Now specific heat at constant volume is calculated as $c_v = \frac{\partial ...
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4answers
103 views

role of entropy in gibbs free energy intuition

I have looked at various derivations of the Gibbs free energy equation and the underlying definition of the gibbs free energy. However I have been unable to attain direct insight/intuition over the ...
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1answer
43 views

Why formation of rain is adiabatic?

I recently went through the wikipedia page for Rain. It said that There are four main mechanisms for cooling the air to its dew point: adiabatic cooling, conductive cooling,.... It proceeds as ...
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0answers
35 views

Which metal will be on the surface after solidification?

I know that there are some metals that just "like" to be on a surface, so if I make an alloy of such metal and some other metal, the first one will be on the surface after solidification of initially ...
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28 views

Energy stored in a battery?

I want to calculate the energy lost from a battery to heat, and the energy stored in a battery. Are these formula's correct? $ E_b$ = $I$$V$ x $t$ $ E_H$ = $I$$R$$^2$ x $t$ $E_b$ = energy stored in ...
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1answer
43 views

How can I calculate the speed at which water evaporates when held at 100 degrees Celsius?

If I have a mixture of some higher bp solvents and water, and I heat it to 100 degrees celcius so that the water in the mixture begins to boil, how can I calculate the rate at which water leaves the ...
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1answer
49 views

California “firenados”

There are spectacular rotating towers of fire that can be seen in recent television images of fire in California. In this video, we see that these fire towers can rotate either clockwise or counter ...
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1answer
133 views

Flory-Huggins ternary phase diagram with a neutral component

I am searching the literature for the Flory-Huggins phase diagram with the following components : polymer, solvent, and a third component that does not interact with the other components (just entropy ...
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1answer
35 views

Chemical potential in terms of temperature and pressure

For part (a), I know how to take the partial derivatives of S to get chemical potential, pressure. But there seems that I still need one equation to correctly express chemical potential in terms of ...
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1answer
81 views

Can water evaporate when frozen?

If an ice cube is stored in a freezer could it evaporate given enough time? Is there a temperature below which water cannot evaporate (even just a little)?
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0answers
15 views

Water Displacement in the Oceans As a result of Global Warming [duplicate]

Not sure if this is the right place to ask this but here goes. If the north pole ice sheets melt, everyone agrees this would be a bad thing, however one of the reasons they state for this being a bad ...
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2answers
45 views

Does the ice melt faster when breaks free from larger sheet and drops into Antarctic Ocean waters?

My question is, when Ice is intact with the large sheet and exposed only to air, it melts at some pace. But does it melt faster or slower when the big piece breaks free from the sheet and drops into ...
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2answers
117 views

Thermo-Emf variation with temperature

In the following experiment for seebeck effect After a certain temperature, the thermo-emf begins to fall. Why does this happen? What is happening microscopically at this level to cause such an ...
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2answers
357 views

If temperature is average KE per particle, and heat is total KE of all the particles, how can molar heat capacity vary?

If temperature is defined as the average kinetic energy per particle, and heat energy is defined as the total kinetic energy of all the particles (or more strictly, heat transferred is the total ...
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0answers
35 views

How to calculate the amount of energy in joules/s that gas oven uses?

I know there may be missing information, but my question is really simple and yet I haven't found an answer to it. To make things easy, let's assume we have a $40×40×40$ cm oven that we want to keep ...
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2answers
128 views

Description of the heat equation with an additional term

I have the following equation: $$\frac{\partial U}{\partial t}=k\frac{\partial^2 U}{\partial x^2}-v_{0}\frac{\partial U}{\partial x}, x>0$$ with initial conditions: $$U(0,t)=0$$ $$U(x,0)=f(x)$$ ...
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2answers
33 views

Would adding water after heating decrease the overall heat, when compared to adding the water before heating for the same period of time?

I was rather cold last night, and warmed up a cup of water to drink in the microwave. I put it in for 60 seconds, but it came out boiling hot, so I put a bit of cold water in with the hot water, and ...
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1answer
332 views

Why does a water drop on a hot plate at $150^o C$ evaporate faster than on a plate at $200^o C$?

I recently read that: "A drop of water landing on a hot plate at $150^o C \:(300 F)$ evaporates in a few seconds. A drop of water landing on a hot plate at $200^o C \:(400 F)$ survives a whole ...
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2answers
36 views

First law of thermodynamics applied to atmospheric fluid

I was reading an article that talks on a climate prediction model. This article proposes the following equation (energy balance): $$\rho_0 c_v\left( \frac{\partial T}{\partial t} + \sum^{3}_{j=1} ...
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0answers
45 views

How come evaporation is possible? [duplicate]

If I have wet clothes and put them on the ground and let the Sun shine on them, they dry as water evaporates. But water should evaporate at 100 degrees Celsius so how come they go dry if it's just ...
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7answers
3k views

Homemade salad dressing separates into layers after it sits for a while. Why doesn't this violate the 2nd law of thermodynamics?

The oil, vinegar and other liquids in homemade salad dressing separate into layers after sitting for a while, making the mixture become more organized as time evolves. Why doesn't this violate the ...
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1answer
59 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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3answers
227 views

What makes now special?

I recently came across the paper Relation between the psychological and thermodynamic arrows of time (arXiv). Their argument makes sense to me, however their concept doesn't seem to address what ...
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0answers
15 views

Understanding how clouds form: warm air near the earth's surface?

I'm trying to understand how clouds are formed. I've read that it begins with air near the earth's surface heating up. But I'm hearing two different reasons for how this happens: Air pressure near ...
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1answer
45 views

Heat generated on thin disc plate in magnetic field [closed]

There are the eddy currents in thin circular copper plate thickness h and radius a, in a harmonic magnetic field which is perpendicular to plate and the effect of auto-induction is negligible. Now we ...
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2answers
136 views

Is there an equation to calculate the average speed of liquid molecules?

I seem to remember from first year physics that we can calculate the RMS speed of a stationary, ideal gas with $v=\sqrt{\frac{3RT}{M}}$. Does a similar equation exist for liquids?
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3answers
74 views

Is it possible to have $\Delta T\neq 0$ with no heat exchange $Q=0$, no work $W=0$ done, and no change in internal energy $\Delta E=0$?

Is it possible to have $Q=0$, $\Delta E=0$ and $W=0$, but $\Delta T\neq 0$? In particular, if there is no change in internal energy, doesn't that imply it is an isothermal process, and therefore that ...
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0answers
64 views

How to design a house to be cooled passively? [closed]

I live in Louisiana these days, in an area that is known for its numerous antebellum plantation homes (circa early 1800s). While touring one of these homes it was clear that almost everything about ...
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3answers
885 views

How does cold air move through a room

If you turn on a fan in a warm room, it feels as if cold air is being pushed from the fan out in the direction that it's facing, but what's actually happening on a molecular level? When an object is ...