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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621 views

Where does air pressure come from?

Where does air pressure come from? I thought it was from gravity or the speed of the gas resulting from its heat. However, analyzing my own hypotheses, I think that my 'heat conjecture' is probably ...
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4answers
646 views

Why does a bubble take a spherical shape?

I suspect this has something to do with thermodynamics and the isoperimetric inequality and I'm interested in a mathematical derivation of this result.
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1answer
952 views

How cold would a bowling ball near absolute zero make a room?

If a bowling ball at absolute zero suddenly appeared in my room, how cold would the room get? Would I die? My initial estimation is that it one bowling ball at 0K (i.e., 300K below room temperature) ...
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2answers
1k views

Can you extinguish a fire by cooling down the fuel?

I know that temperature plays a crucial role in the process of ignition, as most combustible materials will spontaneously start burning in presence of enough oxygen when heated above the kindling ...
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4answers
17k views

Entropy Change During Reversible Processes

I'm confused about the Second Law of Thermodynamics. The Second Law of Thermodynamics prohibits a decrease in the entropy of a closed system and states that the entropy is unchanged during a ...
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3answers
723 views

Less than absolute zero possible? [duplicate]

Possible Duplicate: Temperature below absolute zero? According to this article http://www.sciencemag.org/content/339/6115/52 (preprint: http://arxiv.org/abs/1211.0545) it is. What do you ...
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3answers
4k views

Fast cool down of “things” in the kitchen

Is there any fundamental physical reason (thermodynamics/entropy?) behind the fact that there doesn't exist home appliance for fast cool down of food/drinks? I know that there are some methods (...
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3answers
205 views

How can a strong water current be cold

This is a layman question. If heat is the motion of atoms, how can a fast moving water current be cold?
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3answers
932 views

What is the most efficient way to use hand dryer?

What's the most efficient way to place your hands under the hand dryer? Let's assume that dryer creates simple downward flow of hot air. Here are some examples:
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3answers
262 views

How to “read” the temperature of an abstract system?

How can I interpret the parameter temperature $T$, if I'm not given the description of the system in terms of the equation of state, $E(S,V\ )$ or $S(E,V\ )$ and so on. In many systems it makes sense ...
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1answer
879 views

heater in a perfectly insulated box

Imagine a perfectly insulated box, placed inside the box is an electric heater. The heater is switched on and the box is left to reach equilibrium with its surroundings. What is the final temperature ...
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3answers
415 views

How to think physically about basic “fields”

"Field" is a name for associating a value with each point in space. This value can be a scalar, vector or tensor etc. I read the wikipedia article and got that much, but then it goes it into more ...
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4answers
197 views

Really how does the entropy of the universe increase?

Universe means the system along with its surroundings. I have always got this statement while studying the second law; be it a thermodynamics book (Sears, Salinger) , physical chemistry book (Atkins, ...
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2answers
1k views

Why is the Carnot engine the most efficient?

It seems that the only condition used in proving that the Carnot engine is the most efficient is that it is reversible. More specifically, the Carnot engine can be run in reverse as a refrigerator. ...
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4answers
3k views

How does the entropy of an isolated system increase?

The change of entropy is defined $$\Delta S = \int \frac{dQ_\mathrm{rev}}{T}.$$ If a system is isolated the heat transfer between the system and the surroundings is zero ($dQ = 0$), thus $\Delta S = ...
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3answers
2k views

Why the free energy is called 'free'?

The free energy, $F$ of a thermodynamic system at a given temperature $T$, is defined as, \begin{equation} e^{-\beta F} = \mathcal{Z} = \sum_{\{configuration\}} e^{-\beta E(configuration) } \end{...
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2answers
2k views

Performance of a thermos bottle relative to contents

I'm not a physicist but I majored it at high school (a long time ago) and I study university math. Me and my roommate discussed whether the performance of a Thermos bottle is influenced by how full ...
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3answers
3k views

Irreversible process

I have this problem. I have an ideal gas that goes through an irreversible adiabatic decompression. I have the initial state (P,T,V), and the final pressure, and I have to calculate the entropy ...
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2answers
2k views

Does negative temperature in Carnot cycle yield a counterexample of the second law of thermodynamics?

By Carnot Theorem, the efficiency of Carnot cycle is$$\eta=1-\frac{T_C}{T_H}$$ where $T_C$,$T_H$ are the absolute temperature of the cold reservoir and hot reservoir respectively. Since $T_C > 0$, ...
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3answers
338 views

Do I have the meaning of the property temperature correct?

OKay my book just starts out talking about the vague definition we have for temperature and we ended up with the Zeroth law of Thermodynamics which states: Two systems are in thermal equilibrium ...
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2answers
18k views

What happens to the absorbed light energy?

When light comes across with a solid material, some of it is reflected, some of it passes through and some of it is absorbed. I understand the reflection and passing through, but I don't understand ...
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2answers
76 views

Gravothermal catastrophe: looking for simple explanation

I am beginning to try to understand the gravothermal catastrophe. I was hoping someone could provide an explanation to help me understand what the gravothermal catastrophe is and why it is important, ...
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2answers
134 views

1st Law of thermodynamics (conservation of energy) and quantum tunneling

Doesn't quantum tunneling contradict the 1st Law of thermodynamics? As far as I remember my school physics teacher told us that quantum tunneling is the reason why we can observe the alpha decay, as ...
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2answers
67 views

gravitational force and irreversibility

If we place a ball at a certain height it falls and the process is irreversible. Is there any entropy change associated with the falling of ball? If so why?
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3answers
156 views

Why is the change in entropy for a given energy input dependent on the temperature?

In my thermodinamics class, we saw that $$dS=\frac{dQ}{T}$$ and $$\Delta S=\int_{a}^{b}\frac{dQ}{T}$$ My question is: why for the same energy input $dQ$ the entropy increases more in lower ...
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2answers
552 views

Heat Temperature and my dinner

So today while revising for my QM midterm I decided to raid the cupboards for a well earned lunch, only to find nothing there but noodles and rice. So I put two metal pans of water on two pre-heated ...
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1answer
2k views

Why do vapour cones form around jet fighters?

Apparently this phenomenon has nothing to do with jets breaking the sound barrier and has something to do with the Prandtl-Glauert singularity as described on Wikipedia. But, the Wikipedia article isn'...
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2answers
1k views

Why can we say that $\bar{d}Q=TdS$?

When we introduce entropy we do this by saying that: $$\bar{d}Q=TdS.$$ Now I was wondering why this should be true? I know that by looking at a Carnot cycle, we do get this relation for reversible ...
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2answers
373 views

Why won't the efficiency of a heat pump increase when $T_\mathrm{cold}$ decreases?

Why won't the efficiency of a heat pump increase when $T_\mathrm{cold}$ decreases? In the carnot cycle, efficiency increases when $T_\mathrm{cold}$ gets colder and $T_\mathrm{hot}$ gets hotter. Are ...
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4answers
4k views

Alternative liquid for Galileo thermometer

So a friend of mine broke my Galileo thermometer recently. The glass tube and the liquid inside were lost, but the bulbs survived. I've cleaned out an old tall glass candle, and tried filling it with ...
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2answers
309 views

Paradox while Cooling & Heating in Air? [duplicate]

I was thinking about how asteroids get "burned" up in the upper atmosphere as they approach the earth surface due to the atmosphere of earth heating the asteroids immensely as a result of air ...
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2answers
224 views

Why doesn't water in water barometer boil?

I have read that the pressure in a water barometer at the top of the water column is around 0.5 psi and at such low pressures water should boil at around ~26°C (Room temperature). [1] [2] How ...
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2answers
219 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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1answer
709 views

Error in Sear's and Zemansky's University Physics with Modern Physics 13th Edition (Young and Freeman)?

I was reading up on the Ideal Gas Equation in University Physics with Modern Physics by Young and Freeman when I chanced upon a seemingly illogical mathematical equation. Can anyone rectify this ...
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5answers
859 views

Integrating factor $1/T$ in 2nd Law of Thermodynamics

How would you prove that $1/T$ is the most suitable integrating factor to transform $\delta Q$ to an exact differential in the second law of thermodynamics: $$dS = \frac{\delta Q}{T}$$ Where $dS$ is ...
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5answers
5k views

Why does a larger thermal conductivity provide a smaller temperature gradient?

I was thinking about Fourier's Law in heat transfer today and for some reason I am just not understanding the relationships it gives us. Fourier's tells us that if the heat transfer rate is kept ...
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3answers
279 views

What are some creative illustrations of the nature of dissipative forces?

I'm teaching a conceptual introduction to physics for American 13-15 year old students this summer. One of the main ideas I want to hit on is the relationship between energy conservation, equilibrium,...
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1answer
329 views

Wasn't the Hawking Paradox solved by Einstein?

I just watched a BBC Horizon episode where they talked about the Hawking Paradox. They mentioned a controversy about information being lost but I couldn't get my head around this. Black hole ...
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2answers
8k views

Will the hole on a metal disc expand or contract upon heating? [duplicate]

Possible Duplicate: Will a hole cut into a metal disk expand or shrink when the disc is heated? The metal disc of diameter D1 has a hole in it and the diameter of the hole is D2 (D2 Now, if I ...
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1answer
450 views

The full entropy quote

What is the full text (and possibly the source) of the summary of the 3 laws of thermodynamics that goes something along the lines of "Can't break even, can't win and can't even stop playing the game"?...
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2answers
67 views

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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4answers
310 views

Can we have air flow without air noise?

I want to have air flow in my room, but I don't want a fan to move the air because of the air noise. I think the noise comes from all air molecules going to the same direction. If I can move the air ...
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2answers
56 views

Why are metals worse conductors when heated?

When metals, (such as in circuits), are heated, their ability to conduct electric current is hampered. Why is this? Does the transition towards liquid disrupt a metal's ability to conduct, or is ...
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2answers
74 views

Mirages under water?

In Polynesian shallow water, the temperature difference from short place to place is so high that one can see the same trouble refraction that exist in air above fire or metal sheet in summer. So I'm ...
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1answer
847 views

Why do we ignore rotational energy in monatomic gases? [duplicate]

I understand that the average energy of each degree of freedom in a thermodynamic system is $\frac12kT$. And so, for an ideal monatomic gas, there are three degrees of freedom associated with the ...
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2answers
235 views

Average surface temperature of Earth [duplicate]

I had a question in my school exam. Will the average surface temperature of the Earth be lower or higher, if there was no atmosphere? Now, the answer expected is "The avg temp will be lower, because ...
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3answers
4k views

How can I measure the volume of a balloon?

I'm working on a project and I need to measure the volume of a balloon. In fact, I need to measure its radius. I want you to give me advice and ways of measuring it.
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1answer
159 views

What would a normal thermometer read at the Sun's photosphere?

I was reading about the Earth's Thermosphere, and I quote this: "The highly diluted gas in this layer can reach 2,500 °C (4,530 °F) during the day. Even though the temperature is so high, one would ...
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6answers
192 views

Thermodynamics only deals with homogenous systems?

In Thermodynamics quantities like pressure, temperature and entropy are associated with overall states of a macroscopic system. In that case, we do not talk about "the quantity $Q$ at the point $p$ of ...