Covers the study of (mostly homogeneous) macroscopic systems from a heat/energy/entropy point of view. Maybe combine with [tag:statistical-mechanics].

learn more… | top users | synonyms (1)

2
votes
1answer
118 views

Can anyone explain the idea behind dS ∝ dV/V?

In a lecture on entropy, one of the equations $dS ∝ \frac {dV}{V}$ was explained as "a fractional change in volume as a measure of the increase in randomness" (related to $\frac{dQ}{T}$) How does ...
2
votes
2answers
791 views

Does coffee cool faster than tap water?

A friend of mine, based on casual observation, believes that coffee will cool faster than ordinary hot tap water. Being curious about this I have tried to investigate it myself, but I'm not well ...
2
votes
1answer
62 views

How can you tell between gas, liquid and crystal in a microscopic way?

I know that molecules in ideal gas can move freely, and molecules in crystal are bonded to some specific location. But can I describe this in a more quantitative way? Do gas molecules have more ...
2
votes
2answers
990 views

Can any solid of low heat capacity release or gain energy slower than another solid of higher heat capacity?

If our sense of contact temperature is influenced by the thermal conductivity of the material: Can any solid material with a low heat capacity exist that feels closer to human body temperature than ...
2
votes
1answer
312 views

Is there an easy way to get water at roughly 70°C in our kitchen?

Some green tea requires to pour water at 70°C. I have no thermal sensor or kettle with adjustable temperature with me. Do you know a way to get water at roughly 70°C like “boil water and wait for x ...
2
votes
2answers
6k views

In an ideal gas, if temperature increases, will pressure or volume increase?

This question came up when I was talking about the atmosphere. Someone had mistakenly claimed that as temperature increases, the density of the atmosphere should increase as well. I reasoned from the ...
2
votes
1answer
113 views

Would a super-hot object launch itself into the air?

In this XKCD What If, an indestructible hairdryer outputs 2 GW of power, turning the earth under it into gas: Periodic explosions of gas beneath the box launch it into the air, and it starts fires ...
2
votes
1answer
289 views

Is a tankless water heater more or less efficient than an electric stove at heating water?

Consider the following scenario that I actually encounter frequently: I just finished washing a pot in the sink, and now I want to boil some water for cooking noodles. If I fill the pot with hot ...
2
votes
2answers
602 views

Doubt over Kelvin-Planck statement of thermodynamics' second law

I have a doubt over the Kelvin and Planck's statement of thermodynamics' second law, in particular applied to a cycle. Let's take a Carnot cycle as an example, and let's call the first two ...
2
votes
1answer
528 views

How much does sunlight affect inside temperature?

Suppose we have a $3\cdot 3\cdot 3\,m^3$ room of which one side is glass. And suppose that the other 5 sides have no effect on temperature (super isolation). We know from physics how to calculate ...
2
votes
1answer
2k views

Sackur-Tetrode equation - clarification required - problem with units

I'm a 2nd year physics undergraduate and recently I've volunteered to give a short presentation on the Sackur-Tetrode equation derivation and its use at removing the Gibbs paradox. I've looked on the ...
2
votes
3answers
785 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 ...
2
votes
1answer
62 views

Heat deposition at optical focus in a clear medium

This is an applied physics question for an engineering problem. The well known experiment we have all done--burn paper and wood with a magnifying glass and the sun's rays. My question is, if one were ...
2
votes
2answers
926 views

Calculating the change in entropy in a melting process

I have a homework question that I'm completely stumped on and need help solving it. I have a $50\, \mathrm{g}$ ice cube at $-15\, \mathrm{C}$ that is in a container of $200\, \mathrm{g}$ of water at ...
2
votes
1answer
1k views

Does an object's color change its rate of cooling?

The motivation for this question comes directly from this thread. The proposition is that the color of something changes how fast it cools (note: specifically the rate of cooling, not taking into ...
2
votes
2answers
429 views

Entropy exchange of a free fall

I have a problem in which the tell me that you drop a bag of 50 kg of sand from 10 meters high, and you have to caltulate the entropy difference of the sand, asuming that the speific heat of the sand ...
2
votes
1answer
1k views

Isentropic Processes

I'm having trouble understanding why reversible adiabatic processes are isentropic. I understand that in a reversible adiabatic process there is no heat exchange and so $dQ = TdS = 0$. However, if ...
2
votes
2answers
130 views

Eutectic systems behavior near 100%-0% composition and low temperature

I can't understand why in this phase diagram , in the area where the system is approaching the 100% $\alpha$ or 100% $\beta$ composition, the state is just a pure solid. I mean, where does the ...
2
votes
2answers
254 views

Can the vibrational energy of a engine be used to increase efficiency?

In whole mechanics, we never talk about wasted vibrational energy! As we see in our daily life, a lot of energy is wasted as vibrational energy in every engine, for example as we fire our car's ...
2
votes
2answers
399 views

How cold should it be outside for a hot coffee mug to break?

So I like to go outside for a morning coffee with a cigarette. In winter here it's usually between -5C to -25C, and sometimes it gets down to -30C and colder. Assuming that my coffee is about 75-80C, ...
2
votes
1answer
1k views

How relative humidity and temperature affect rainfalls

The question is simple as stated in the title. How relative humidity and temperature affect rainfalls? I can't find a satisfatory answer in the web. Who can help me to understand me this relation?
2
votes
1answer
176 views

Minimal Maxwell's Demon

I would like to understand where the waste heat is generated in the Maxwell's demon problem. To this end I've come up with the simplest scenario I can think of. If my scenario is workable I am hoping ...
2
votes
1answer
259 views

Gas pressure equalisation: Where does the excess energy go?

Assume for purposes of discussion a closed container with a concentration of gas on one side and near-vacuum on the other. We could let the gas pressure equalise naturally, or we could construct a ...
2
votes
1answer
157 views

can hydrogen stay frozen in vacuum?

I've look into the hydrogen state diagram, and it seems that it can be frozen under pressure. Question: Does this mean that hydrogen cannot be kept frozen in a vacuum chamber?
2
votes
1answer
765 views

Physics of the electric hot plate

For an electronics experiment, I began wondering about the electric hot-plates (specifically the temperature dependence over time). If I were to measure the given temperature over time, I assume that ...
2
votes
2answers
616 views

Is it Possible to have Adiabatic Processes other than $PV^\gamma$ for the ideal Gas?

Is it possible to represent an adiabatic process for an ideal gas by a formula other than $PV^\gamma=Const$?: Relevant Considerations: We always need to connect a pair of arbitrary points/states ...
2
votes
2answers
536 views

Demonstrate magnets adhere to conservation of energy pursuant to the laws of thermodynamics

I am looking for a way to demonstrate that magnets adhere to the laws of thermodynamics, in particular the requirement that energy in a closed system be conserved. To adhere to the requirement that ...
2
votes
1answer
282 views

Why is the crust of my meat pie only slightly warm?

I just cooked a meat pie in the oven. Almost immediately after pulling it out of the oven, I felt that the crust was only slightly warm, but when I cut it open the filling felt very warm. I can't ...
2
votes
2answers
4k views

Why I will feel colder when I get out from a swimming pool? [duplicate]

A friend of mine told me that because water heat capacity is higher. He also mentioned that it was similar to alcohol on skin effect. Assume that there are 3 situations. 1. I did not get wet 2. I got ...
2
votes
1answer
295 views

Man on treadmill and energy/mass conservation

Suppose we put a very obese man inside a large air tight isolated room with a treadmill and he runs on the treadmill for 24 hours and at the end of the day he sees his weight has come down (mostly ...
2
votes
2answers
579 views

how does heat energy start to speed up a gas molecule?

If it was possible to place a single gas molecule in a cell and freeze it to near absolute zero. What would the molecule do as it thawed out? Would it translate the heat energy into it's electrons ...
2
votes
1answer
173 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 ...
2
votes
1answer
4k views

How does smoke move in the air and how can I direct it so it will go to a place I want it to go?

Let's assume a close room with 1-2 people who only one of them smoking cigarette. What is the equation describe the smoking spreading? is it diffusion? what are the parameters is so? Is there a ...
2
votes
2answers
212 views

How much energy does lowering an object into a black hole generate?

An object of mass m is slowly lowered into a black hole of mass 1000 m. Is the amount of braking energy larger than $0.6 mc^2$? Now what if, after lowering the mass close to the event horizon, we ...
2
votes
1answer
1k views

Understanding the Seebeck effect

Thermoelectricity is, as I understand it, the difference in voltage between the hot and cold ends of two dissimilar materials. If two materials are connected at two different junctions, the hot ...
2
votes
1answer
1k views

Heat exchange depending on coolant flow direction

Consider a simplest case of a heat exchanger - two parallel pipes of flowing liquids (say, hot and cold) that have physical contact along some part of their length. Hot water of a certain temperature ...
2
votes
2answers
82 views

In a gas of particles, how is the displacement vector related to the number density?

Suppose I have a gas of particles that is initially uniformly distributed so that the number density is $n_0$ (number of particles per unit volume), and then I displace the particles by the vector ...
2
votes
2answers
1k views

Evaporation from a capillary tube

Consider a capillary tube (say from a liquid / capillary thermometer), that means a tube of small internal diameter which holds liquid by capillary action . The tube is fulfilled with water and closed ...
2
votes
1answer
1k views

time dependence of temperature equalization

Suppose you have two thermodynamical systems $X_1$ and $X_2$ (for example water and air) with different temperatures ($T_1 > T_2$). Now put them into thermal contact. Is there a formula which ...
2
votes
2answers
39 views

Difference between a reversible change and a reversible process?

Question In thermodynamics what is the difference between a reversible change and a reversible process? Additional information I am new to the topic of thermodynamics and getting confused about ...
2
votes
1answer
58 views

The outside of a steel teapot is hot to touch. Does this mean that the water inside is necessarily at least as hot?

Some teapots, like this one Are made from thin steel which conducts the heat of the teapot quite well, meaning that you can't comfortable hold the sides while they're full of hot water. Tonight I ...
2
votes
1answer
46 views

Finding the expression for the time it takes for a water tank to be half empty

The problem is to find an expression for the time it takes for a cylindrical water tank is half empty. I am given that the water tank has a diameter of $D_0$ and a hight $H$. The water outlet is ...
2
votes
2answers
58 views

what is the temperature (and pressure) of the oxygen inside an oxygen bottle?

In the hospital or in a lab, one can see the huge oxygen bottles. A question is, what is the temperature of the oxygen inside the bottle? And what is the pressure inside? We know the critical ...
2
votes
1answer
51 views

Why should slope of pressure versus volume always be negative?

I read that the slope of the pressure, ($P$), versus volume ($V$) graph should always be negative for thermodynamic stability, but there was no further explanation in the text. Could someone give an ...
2
votes
3answers
66 views

Feasible way to cool water (ice) to temperatures around -20 degree celsius

Is there an easy way to cool water to temperatures around -20 or -30 degree celsius that is economically and technologically feasible for a high school student. Can it be done using typical household ...
2
votes
1answer
84 views

2nd law of thermodynamics - thought experiment

I have designed this simple thought experiment that seems to contradict 2nd law of thermodynamics. Could you please find a mistake in my reasoning? ...
2
votes
1answer
53 views

Planck's postulate for oscillators or for light?

I know that Planck originally postulated that the energy of an oscillator in a black body was quantised to $E=nh\nu$ but did he know at the time that this meant the energy of light was also quantised ...
2
votes
2answers
57 views

Any general method of determining changes in speed of sound with pressure?

I need to correct speed of sound measurements in various gases for changes in ambient pressure, typically 700-1100 mBar. Is there any general theoretical model available for this? Especially something ...
2
votes
1answer
43 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
votes
1answer
90 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, ...