# Tagged Questions

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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### Heat Pumps and COP?

Heat Pump COP (Coefficient of Performance) $$COP=1-\frac{T_c}{T_H}$$ With Heat Pumps is the efficiency/COP more dependent on the hot or the cold reservoir and why?
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### Change in entropy when mixing water at different temperature

Suppose two amounts of water of the same mass, but with different temperature, are mixed. Then the entropy of the hot water decreases, but the entropy of the cold water increases due to heat transfer. ...
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### Time dependence of mixing temperature of a solid with a liquid

let's assume we have the following situation: A person throws a stone with known temperature, mass and known heat capacity in water (known temperature, etc.). How can I calculate the temperature of ...
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### Will temperature, pressure and volume effect the value of universal gas constant in any way?

Effect of variables on constants: Will temperature, pressure and volume effect the value of universal gas constant in any way?
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### Is there a temperature at which ice is denser than water?

Normally ice would float on water because its density is less compared to that of water as a liquid. But is it possible that its density will increase due to a very low temperature or is ice in any ...
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### What is the rate of heat transfer between 25C water and 20C air?

This might be an elementary question, but I was only able to find the formula for heat transfer only in one substance. *Surface area: 1 meter square
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### Refractive index of plasma dependance on temperature

How does the refractive index of plasma changes with temperature? Temperature is not high enough for new ionization. Would it be like in gas p/T dependence ? http://farside.ph.utexas.edu/teaching/em/...
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### 'Equations of State' - is it wrong to derive dynamic relationships from these equations?

When we speak of 'Equations of State' I believe we are most often referring to a steady state equilibrium relationship between particular state variables. Am I wrong? For example, the famous ideal ...
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### Efficiency of engine in the case non-boundary work done on the system

The engine efficiency is defined as $$\eta = \frac{W_{\mathrm{produced}}}{Q_{\mathrm{absorbed}}}\tag{1}$$ But suppose that, in one of the processes that takes place in the cycle of an engine that ...
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### Can the work in a isochoric process be non-zero?

I came up with a doubt regarding isochoric irreversible processes. Question: Is it always true that, for any isochoric process, reversible or not, the work exchanged by the system is zero and the ...
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### Isothermal compression

What happens when you have gas on a channel with volume V* and p* at atmospheric pressure in an isothermal process and then at time t=0 water acting as a piston is used with a rapid change in pressure ...
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### The use of intensive variables in thermodynamics

Using intensive variables simplify the analysis of thermodynamics . Also equations are divided by the total mass in order to use the "specific" version of a variable . But what's the difference if it ...
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### How do you calculate the heat build-up inside a building or container as a result of the sun?

I need to calculate how much heat builds up in a closed environment eg a building or a Chromadek container. One of the factors I need to take into account is the sun. I know there are various ...
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### Behaviour of Altimeter at different temperatures

I have realized that the Altimeter of my bike computer (it uses Air Pressure to determine altitude and needs to be calibrated every start) is not consistent at different temperatures. At the Start ...
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### Why is the efficiency of human cells less than the efficiency of an Otto engine?

I always used to think (I don’t know why!) that the efficiency of human (and animal and plant) cells should be equal to or near the efficiency of a Carnot engine or at least should be the highest ...
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### Correct terminology or way to refer to the 2 “types” of ice sublimation

I'm trying to get the right terminology for various forms of phase change. I am familiar with the phase change / triple point diagram for water, and we have various terms for the transition of a ...
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### Physical meaning of enthalpy

I've been reading about thermodynamics and reached the topic about enthalpy . I've understood its derivation but I don't understand its physical meaning ... Also I don't understand why they have ...
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### Derivation of continuum expression of the first law of thermodynamics

Continuum expression of first law of thermodynamics: $$\frac{D E_t}{D t}=\nabla\cdot({\bf \sigma\cdot v}) - \nabla\cdot{\bf q}$$ (I've seen it in my physics book) How this equation is derived? ...
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### General expression of internal energy change in a closed system

let's say we have a classical closed system. What I've understood is that during a reversibile process the internal energy $E$ undergoes a change which could be expressed (as generally as possible) as ...
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### How is a irreversible process (conventionally) represented on a $T-S$ plane and why cannot it be (really) represented?

A reversible process can be represented on a $T-S$ plane, and the area under the curve is the heat exchanged by the system. On $P-V$ plane a irreversible process is conventionally represented with a ...
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### Thermal energy of a system

If I had a hot cup of tea and I added to it cold milk, what would happen is that tea will lose some of its thermal energy to the milk. But why do we notice that our liquid is no longer as hot as ...
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### Relation between entropy and internal energy

I am confused as to what is the relation between entropy and internal energy. Entropy is always presented as a measure of the randomness in a system. So when we supply heat to a well insulated system ...
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### “Big Crunch” Ending of Universe?

So, I am reading Introduction to Modern Cosmology by Andrew Liddle and have just learned that the universe is cooling as it is expanding. Now, I am mathematician knowing very little about physics, but ...
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### What is the relationship between Maxwell-Boltzmann statistics, Boltzmann distribution and Maxwell-Boltzmann distribution? [closed]

I have recently been studying some statistical thermodynamics and I am currently trying to understand all the different concepts of the course. I was wondering about differences between MB statistics, ...
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### An ideal black - body at room temperature is thrown into a furnace. It is observed that? [closed]

An ideal black - body at room temperature is thrown into a furnace. It is observed that? (A) initially it is the darkest body and at later times the brightest. (B) it is the darkest body at all ...
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### How is one element capable of radiating (almost) every wavelength? [duplicate]

Black bodies are considered as emitting all wavelengths. A incandescent light bulb has (nearly/almost) also the continuous blackbody spectrum near-perfect color rendition. In this bulb is only one ...
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### Compressibility factor and deviation from ideal gas behaviour

I have recently started to study the deviation of ideal gas behavior. The source from which I am reading doesn't give me an intuitive understanding of the compressibility factor. I dont understand how ...
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### At what rate does a wet clothing decrease the temperature of a human body?

The question is required for my project, but it is frankly too hard for me... The variables that I consider are: Human Body: 36.5(C) Air: 20(C) Sunlight Intensity: 1,368(W/m^2) Wind Speed: 20(km/hr) (...
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### Is the top of an iceberg floating above the waterlevel equal to the expanding of the whole iceberg?

When water freezes it expands and is getting less dense. But is this expansion of the total iceberg equal to the top of an iceberg floating just above sealevel?
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### Why don't air cool over time?

Say you have a balloon full of a type of gas, it is completely isolate from everything else. Let the temperature of the gas be 20 degrees (or any temperature, it doesn't matter) Now, the ballon is ...
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### Using the ideal gas law to figure final pressure [closed]

I'm studying for the GRE and I'm happy with Charles', Boyle's and Gay-Lussac's laws. I'm doing some questions on the ideal gas law. Please hear me out. This may come across as a homework style ...
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### What is the time required for water at 10 deg C to reach room temperature?

I have a small container (100mm X 80mm X 60mm) filled with water at 10° C. The container is made out of Aluminum and is not insulated, and is resting on a wooden table. Room temperature is about 27° C....
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### Why did our universe become transparent to light approx. 300,000 years after the Big Bang started?

Most books say that at age 300,000 years, our universe had cooled to approx. 3000 degrees Kelvin, which allowed free-electrons to bind with atomic nuclei, which allowed light to travel, unimpeded, ...
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### Specific Heat of tissues vs kcal (Weird Thermodynamics question)

I'm a YouTuber who over-examines video game physics for the purposes of both education and entertainment, and I've run into a bit of a snag while researching my next video. I'm looking into heating ...
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### How are extremely high/low temperatures achieved on Earth?

Low Temperature Superconductors (LTS) have their critical temperature below 30K. How are they cooled to such low temperatures? The operating temperature of a Tokamak is greater than 10 keV (over ...
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### Is reversible work a point function?

The potential of a conservative force is equal to the reversible work done on or by a system. But since the potential of a conservative force is represented by a point function, this would seem to ...
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### Books on entropy [closed]

What books introduce entropy in a intuitive, elementary way (at most, for a person with undergraduate physics studies)? The book should not necessarily introduce entropy in relation only to ...
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### Will a glass of water left in the fridge evaporate?

A commenter suggested that a glass of water left in a fridge would evaporate. I couldn't tell if this is true. I don't want to do the experiment since it would take too long; so I thought instead I'd ...
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### Accessibility by reversible processes and the Second Law of Thermodynamics

One common way of motivating the existence of Entropy as a state function is the following. Let us take the Clausius/Kelvin-Planck statement of the second law, from which we can deduce Clausius' ...
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### Water Boiling Time Calculator

Is there a way to calculate out how quickly a body of water will boil? My primary concern is to measure in relationship to different temperatures. For example, heat at 100 degrees Fahrenheit versus ...
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### Why does wood go straight into ignition, while other materials melt first?

Background: I was cooking eggs (very difficult) with a plastic spatula which was not very good, so when I set it on the border of the pan it began melting. In order to keep cooking, I reached for a ...
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### Is work done in isothermal process process greater than work done in isobaric process?

One of my friend was discussing today about the work done by ideal gases in three different processes. They are isothermal, isobaric and adiabatic. We googled to know about the differences between ...
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### newtons law of cooling [closed]

I need help with this calculus problem i just cant get After 10 minutes in Jean-Luc's room, his tea has cooled to $43^\circ ~\rm C$ from $100^\circ ~\rm C\;.$ The room temperature is \$23^\circ ~\...
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### How to justify a thermodynamic cycle practically? [closed]

I have been working on some new thermodynamic cycle and was managed to create the formulation and the P-V diagram for it but when i showed my work to my professor he said that i am gonna have to ...
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### Moving heat problem: instant toast?

Toast-making usually takes a couple minutes using something like a conventional toaster oven to brown the surface of a piece of bread, making toast. However, this duration may be inconvenient. What ...
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### Would water, given enough heat, instantly vaporize?

(Please excuse the wrong usage of words, I'm not a trained physicist. Please point out any misuse.) So far, my understanding of water evaporating is the following: Water always evaporates, whatever ...
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### Reversible processes in which mechanical or thermal equilibrium is not reached

The definition of a reversible thermodynamic process requires in any instant the mechanical equilibrium (equal pressures) and thermal equilibrium (equal temperatures) of the system in a quasi-static ...
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### Change in entropy of thermodynamic environment during isobaric or isochoric processes

When an ideal gas follows a isobaric or isochoric transformation (no matter if it is reversible or not) I'm not sure what is the change in entropy of the thermodynamic environment. First of all, ...