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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31 views

Momentum which is transfered to the wall of container by a molecule

now I study thremodynamics course, and I cant understand why for ideal gas in equilibrium state when a molecule collides with the wall of container it transfers to it the momentum which is equals to ...
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32 views

Where do combustibles(gas etc.) fit into the first law?

The first law can be written as $dU=dW + dQ$. As I understand it $W$, work, is mechanical energy from winding up springs, things spinning, flying etc. It's kinetic and potential. As I understand it ...
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29 views

CMB and Gibbs free energy

I was doing my homework and I have a doubt. I proved that if I have the radiation energy density $$u = \frac{U}{V} $$ and $$P = \frac{1}{3}u $$ Then the Gibbs free energy is 0. But, how can I to ...
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1answer
82 views

How fast does heat travel via conduction?

I have read this question which seems to ask an identical question, but I'm not sure - it had far too many words I don't understand, let alone the equations. Perhaps someone can answer with a ...
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1answer
120 views

Why is it difficult to mix helium and nitrogen gases?

I recently learned an interesting fact: That it's difficult to mix helium and nitrogen gases in a compressed gas cylinder. Gas suppliers that need to mix the two gases have to rotate the cylinders for ...
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33 views

What does temperature coefficient of resistance depend on?

I tried looking up on Google if there was an expression for the temperature coefficient of resistance(TCR) of a material in terms of other fundamental factors, but couldn't find any. Specifically I am ...
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1answer
32 views

Monatomic gas work

Recently I was trying to solve the following problem: monatomic gas expanded from 0.2 to 0.5 m^3 and pressure increased from 404 to 808 kPa. Find work done by gas, heat gained and change in internal ...
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1answer
34 views

Negative absolute pressure with positive absolute temperature

Could I ask if the derivative defining pressure $dU \over dV$ or ${∂S \over ∂V}|_{E,N} $can be negative in processes occuring in system not cosmological but statistical(gases or solids or liquids-I ...
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1answer
54 views

Is there any experimental setup to test if we are Boltzmann brains? [closed]

I am not sure if this subject belongs to mainstream physics. my question is motivated by the fact that I am not sure we could ever test if we are Boltzmann brains. The same happens with string theory, ...
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20 views

Expansion of solids on heating

I read that when we heat a solid, it expands! Based on circumstances it can expand in area, length and volume! My question is , that do they expand to a certain extent or keep on expanding, if we ...
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1answer
84 views

The whole black vs. white T-shirt issue [duplicate]

I was outside with a friend the other day and we ended up talking about clothing choices in the heat, and he repeated the usual statement that one should wear white instead of black in order to stay ...
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2answers
213 views

Can temperature be a complex number?

Is it possible for a temperature to be a complex number? I want to say "no" but I can't be so sure. If it is possible I would like to know of an example. I found an interesting article which treats ...
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38 views

Newton's Law of Cooling on a calorimeter with two systems

Having the previous calorimeter with adiabatic walls, we've proven, based on Newton's law of cooling, that the equations are... $m_{b} \times C _{pb} \frac{\mathrm{d T_{b}} }{\mathrm{d} t} = ...
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21 views

Superconductors in a magnetic field — mixed phase

In my Stat Mech class we're learning about the phase transition between superconducting and non-superconducting metals in the presence of a magnetic field (Chapter 7, Kittel). I understand that the ...
2
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1answer
67 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 ...
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1answer
92 views

Will my coffee end up colder if I wait to put in the milk? [duplicate]

When I receive a cup of coffee, and I'm not yet ready to drink it, will it lose less heat if I immediately add the milk, rather than wait to add the milk just before I drink it? (Bonus points for ...
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2answers
66 views
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59 views

How does this capillary-action setup not become a perpetual motion machine?

If you have a very thin glass tube and you place it into water, let's say the water in the tube rises to the height of $x$ mm from the surface of the water. What would happen if you poked a hole in ...
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30 views

Internal heating effects of a circuit element?

The heat-up of a solid circuit component in an electric circuit (for which no chemical reactions, phase changes reactions or other stuctural changes take place) may include: Joule heating $\dot ...
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35 views

Does a fixed amount of gas have slighty more mass when at a higher temperature?

According to relativity, energy and mass are equivalent. Does this mean that the energy added to a fixed amount of gas when it is heated adds slightly to the mass of the gas? The difference would be ...
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4answers
116 views

How can there be entropy change in this system?

How can there be an entropy change in this system? Suppose if I have a system consisting of liquid water, $1\, \mathrm{kg}$ at $290\,\mathrm{K}$, I stir it, and do say, $10\, \mathrm{J}$ of work on ...
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37 views

Determine the amount of energy used for baking a cake

Let's suppose we have a room of temperature $T_0$ with an oven of volume $V$, both filled with air of density $\rho$ and specific heat $c$. We need to bake a cake in the temperature $T_1$. Thus we ...
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1answer
2k views

Why do two flames kept side by side merge into a single flame?

I have read about why a candle flame is in the shape of a tear drop in the presence of gravity. The top portion which is sharp is directly above the wick, where the soot particles from the wick rise ...
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2answers
28 views

Deriving the partition function in MaxEnt

I'm trying to understand this paper on Maximum Entropy by Jaynes, and am stuck on something which should be rather simple. We're attempting to maximize the entropy $-\sum_i p_i \ln(p_i)$ subject to ...
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1answer
61 views

If entropy is a state function, then why is all the talk about reversible vs. irreversible processes?

So I'm preparing for my Thermodynamics undergrad exam, and I just can't wrap my head around the significance of reversibility vs. irreversibility of a process in relation to entropy. I mean if entropy ...
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1answer
437 views

If a microwave oven disk rotates to warm up food, why doesn't it go up/down/sideways?

This has been in my mind for a while... Well, actually everytime I heat any food with a greater amount of liquid in it: a microwave oven warms up food by inducing polar molecules in the food to ...
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1answer
53 views

Will an object with a higher temperature lose its heat faster than a cooler, yet warm object

I'm after the math, which I don't understand yet and can't find elsewhere. The variables are: Two identical cups, A and B The same volume of fluid is contained in each cup, test fluid is water. ...
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2answers
53 views

If pressure inside a container is lesser than atmospheric pressure, how long can the vessel resist deformation? [closed]

Is the strain caused by stress (due to the pressure difference) is negligible?
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1answer
18 views

What physical change has taken place in a solid, say ice, half way through a latent heat temperature verses time experiment?

During the classic high school latent heat experiment a solid such as ice is heated from below the melting point until it is a liquid above the melting point. During this experiment the temperature ...
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2answers
423 views

Is there a highest temperature? [duplicate]

Im new to physics and do not yet know of all of the terms and math. I was wondering about temperature. I read that there is the lowest temperature which is absolute zero where nothing happens, i.e. ...
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2answers
79 views

Why specific heat at constant pressure is greater than specific heat at constant volume?

I know the relation between specific heat at constant volume and pressure and I also know how to calculate it. Thing is, I don't understand its concept I want to know why at constant pressure, ...
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1answer
52 views

How can we be sure the Maxwell speed distribution equation is always a rational number?

The Maxwell speed distribution equation is given as $$f(v) = 4\pi \biggl(\frac{m}{2\pi kT}\biggr)^{3/2}\exp\biggl(-\frac{mv^2}{2kT}\biggr)v^2.$$ The left hand side gives the fraction of molecules ...
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1answer
43 views

Why dU may not equals dW in free expansion?

From a video lecture, it is mentioned that "dU≠dW in Joule's free expansion if the process is irreversible and adiabatic" Mentioned in around 36:00-38:00 in the video: ...
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2answers
80 views

Resonance in Benzene violating Second Law of Thermodynamics

In chemistry a few months ago we were taught the resonant structure of benzene, that states the double bonds upon the six carbon atoms flicker back and forth between the two possible states it can be ...
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36 views

Filling an empty box with ambient air

A rigid box has a volume of $V_1$ and is thermally isolated so that it exchanges no heat with its environment. Initially the box is empty, and the air around it has a pressure of $P_0$ and a ...
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1answer
89 views

What happens as you approach/cross the Planck temperature?

According to IFLScience, above the Planck Temperature (absolute hot) conventional physics break down. My question is what happens as you approach this temperature, and, if it is possible, what ...
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1answer
32 views

Amount of heat required to raise the temperature of a balloon which requires no energy for expansion?

Two moles of ideal helium gas are in a rubber balloon at $30^°C$. The balloon is fully expandable and can be assumed to require no energy in its expansion. The temperature of the gas in the balloon ...
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1answer
47 views

Is the pressure uniquely determined by the energy density?

I've been trying to figure out how many state functions one needs to fully specify a thermodynamic system. For example, suppose we are given the energy as a function of temperature and volume, i.e. U ...
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1answer
50 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 ...
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57 views

If time somehow went backwards would this violate one of the laws of thermodynamics?

If time somehow went backwards and all the physical events that had occurred in a sequence , would some of the events occurring 'backwards' or in the reverse order violate the principle of entropy or ...
2
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1answer
50 views

Effect of breeze on a very hot day [duplicate]

If air temperature is well above body temperature eg air temp of 50C then would a wind make the apparent temperature higher or lower. Clearly the wind would help take away heat due to increasing ...
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38 views

mathematical Physics + fluid mechanics, thermodynamics + mass transfer? beginner [closed]

I am completely new to the world of physics and am interested in learning the aforementioned topics. I have previously studied biochemistry, but am fascinated by physics and am interested in ...
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2answers
40 views

Heating suprecritical water in a closed water tank

How does the pressure of water in a closed tank evolve in the following setting: - closed tank of 2 liters (filled up with water) - water initially at 25°C and pressurized to 3 bars The water is now ...
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0answers
46 views

Is vortex tube reversible? [closed]

If you inject cold and hot air in outlets of a vortex tube, do you obtain a greater pressure at inlet?
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28 views

One question about critical exponents for first order transition free energy [closed]

I've got a problem to calculate critical exponents for theory given by Landau free action: $$ \tag 1 L = L_{0} - \frac{1}{2}(\nabla m)^{2} + atm^{2} + dm^{3} + bm^{4} - hm, $$ where $$ -\infty < ...
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1answer
59 views

Which laws of physics does this problem break?

One of the problems from the Google Code Jam programming competition, round 2 2015, goes like this: A kiddie pool is a big container in which you can put water, so that small children can play ...
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2answers
64 views

Definition of an irreversible process

I'm a little bit confused as to why quasi-static process cannot lose energy to friction in order to be reversible. This is how I'm thinking: Suppose you have a container of gas with a piston, and on ...
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3answers
698 views

How hot can metal get in sunlight? [closed]

I will make some 5mm blackened steel letters for a building. Now, in the sunlight in summer, how hot can they get? EDIT: i want to place some LEDs on the back of the letters. now the question is if ...
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1answer
42 views

Shape of metal to be heated in consumer microvave oven? [closed]

I tried to use a common 900 W microwave oven melt some alloy with low melting point (138 deg C, eutectic bismut/tin). The metal did not heat up, neither did the metal container, which makes me think ...
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24 views

Is it impossible to find the exact analytic form of the molar volume of liquid and gas in phase transition from the van der Waal equations?

I want to find the molar volume $v_g , v_l$ of gas and liquid respectively in the coexistence curve of phase diagram with given temperature when ideal van der Waal gas $$ P = ...