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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How does the entropy change during the cooling of a hot coffee in a cold cup?

The second Law of Thermodynamics states that entropy always increases in the universe: things become more disorganised. This means, that if I have a hot coffee in a cold cup, then the heat will ...
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0answers
20 views

Why do internal combustion engines only burn to carbon monoxide, not dioxide? [migrated]

Why don't diesel and regular car engines burn all the way to CO2?
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1answer
23 views

The relation between Internal Pressure and flow rate in a balloon [duplicate]

I'm working on a project. We have two balloons, connected by a hose with a valve.We know when we open the valve, by according to internal pressure of each balloon, air goes form the higher pressure ...
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0answers
49 views

Loosening a brass screw from an aluminium hole [on hold]

I'm trying (desperately) to remove a screw from my espresso machine but it won't budge. The screw is a dark yellowish colour making me think it's brass (or maybe bronze). I've been told that the ...
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5answers
4k views

What is the speed of sound in space?

Given that space is not a perfect vacuum, what is the speed of sound therein? Google was not very helpful in this regard, as the only answer I found was 300 km/s, from Astronomy Cafe, which is not a ...
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4answers
82 views

Why don't the collisions of the particles in a gas affect its temperature?

Temperature is defined as the mean kinetic energy of the system. Now the particles are constantly colliding with each other; as a result of which kinetic energy of each particle changes. ...
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0answers
72 views

Calculation of water bulk modulus using equation of state

I simply want to calculate the bulk modulus of water at 50C and increasing pressures. I think I am correctly calculating the new specific volume from the original conditions at (25C and 1atm) to 50C ...
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6answers
4k views

Are engines more efficient on cold days?

According to Carnot efficiency formula $\eta=1-T_C/T_H$, can we say that the engines of cars are more efficient on cold days where $T_C$ (the temperature of the surroundings) is less than on hot days? ...
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1answer
33 views

Heat capacity $C$ at low temperature

The internal energy, $U = Nk_bT$ where $N$ is particle number, $k_b$ is Boltzmann constant and $T$ is temperature. Therefore, the heat capacity $C$ is given by $C=\frac{dU}{dT}=k_b$. However, in ...
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3answers
70 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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4answers
72 views

Relation between temperature and pressure?

I was just working on a special question but I ignored the effect of temperature on it and now it becomes very important to me. What is the relation between Pressure and Temperature? Suppose we ...
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1answer
17 views

Boiling water at low pressure with heat supply

I understand that water boils at lower temperatures at lower pressures, but I want to know what happens when external heat is supplied to a body of water at low pressure. Suppose I have a hot object ...
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15 views

Constant volume gas thermometer

Well, in the notes I took in thermodynamics' class I have a graph that plots Pressure of boiling point $P_b$ over Pressure of freezing point $P_f$ at constant volume in terms of Pressure of boiling ...
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0answers
23 views

Validity of topological thermodynamics?

I've been reading some material by R. Kiehn, developing a topological approach to non-equilibrium thermodynamics through Cartan forms, where the fundamental claim is that irreversible processes are ...
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3answers
95 views

I open the fridge - do I let the heat in or the cold out?

Conventionally, heat flows from hot to cold. How legitimate is it to redefine things in terms of "cold" rather than "heat" eg Cold flows from cold to hot?
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1answer
31 views

How did steady-state universe deal with entropy?

Until the 1960's the general consensus was that the universe is infinite years old (steady-state). The second law of thermodynamics says that entropy always increases, so we'd expect that within a ...
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0answers
20 views

Mechanical equivalent and reversible process

I am confused and missing something. So mechanical equivalent means a 1 Calorie = 4.18 J assuming all heat is converted into work. Wont that be same as reversible process. As reversible process is ...
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1answer
75 views

Does the second law of thermodynamics put limit on efficiency of engines?

In thermodynamics it is claimed that the efficiency of no engine can be one. But look at the efficiency of let's say Carnot engine $\eta=1-T_c/T_h$. Now one can take $T_c$ arbitrarily close to ...
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1answer
19 views

Gas compressed and cooled

If a certain amount of a gas is compressed into a cylinder by the action of a piston that reduces its volume (lets say at a low speed), the kynetic energy of the gas molecules increases, what is ...
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0answers
21 views

Brownian Ratchet Plausibility

Alright I'm going to throw whatever reputation I have on the line here. And yes this is a serious question. Apologies for the shoddy imagery. I had a couple ideas to get the Brownian Ratchet to ...
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0answers
20 views

Power and energy physics [closed]

A power plant based on a gas turbine produce 500mw of electrical energy. It has 48% effiency, and dumps its waste heat into the atmosphere at 20 degrees C. if you assume the plant functions near the ...
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2answers
45 views

Are sound waves adiabatic or isothermal?

I am doing a presentation on sound waves and I need to know if they are adiabatic or isothermal. I know that they can generate heat, but is the amount of heat created so small that it can still be ...
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1answer
19 views

Emissivity of vacuum for radiation heat

What is the heat radiation emissivity of vacuum? For air as well? What is the difference? I understand that the vacuum has reflectivity of 0, So what is the other two values are in $$ reflectivity + ...
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0answers
25 views

Gas cloud gravitation problem [closed]

Consider the following problem from the 2014 F=MA contest: A spherical cloud of dust in space has a uniform density $ρ_0$ and a radius $R_0$. The gravitational acceleration of free fall at the ...
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1answer
31 views

When I'm in a warm room how much of the warmth I feel is due to radiation from surfaces?

Say I'm in a room heated to 23C, once the temperature is in balance all the objects will also be 23C and radiating heat towards me. How much of the warmth I feel is due to this radiation, and how much ...
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1answer
46 views

Calculation of the efficiency of a Carnot engine for photon gas

I am working on a problem considering the Carnot cycle for a photon gas. The cycle has two isothermic processes at $T_{H}$ and $T_{C}$ and two adiabatic processes. The entropy of the photon gas is ...
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2answers
50 views

How to make the mixed water warmer?

Freezing cold. Nastily I still have to do the laundry by hand. And to keep my hands from being frozen I have to add some hot water to the freezing tap water. Let's just suppose that I have the same ...
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1answer
64 views

Could the uncertainty principle theoretically be violated at 0 K? [duplicate]

Ok so please excuse me if the following mental argument is completely ridiculous or obviously flawed :P I was reading about how, even at 0 K (assuming we could experimentally reach such a ...
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0answers
19 views

Calculating saturation mixing ratio for water in air

If I have air pressure (in, say, Pa) and air temperature (in, say, K) - how do I calculate the saturation mixing ratio for water? I can't seem to find it anywhere and it's a fairly useful quantity to ...
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0answers
11 views

Steady flow system energy problem thermodynamics? [closed]

Assume 5 lb/sec of fluid enter a steady-state, steady flow system with p₁ = 100psia, ρ₁ = 0.2 lb/ft³, v₁ = 100 ft/s, u₁ = 800 Btu/lb, and leave with p₂ = 0.05lb/ft³, v₂ = 500ft/s, and u₂ = 780 Btu/lb. ...
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2answers
42 views

Why does this entropy change formula for heating water blow up at $T_{1}=0$?

My textbook says: Next we consider a more complicated problem: heating m grams of water, from $T_{1}$ to $T_{2}$. The entropy change is $$S_{2}-S_{1}=\int mc_{w} ...
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1answer
21 views

pressure as resistance

Does atmospheric pressure act as resistance against water that prevents it from boiling? Water boils at lower temperatures at lower pressures. Does this mean that water would naturally prefer to exist ...
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0answers
21 views

General boundary condition for 1D heat equation

I'm studying from Numerical Solution of Partial Differential Equations by K.W.Morton and D.F.Mayers (Amazon link). I'm confused with general boundary conditions. Could someone give me a clue? For ...
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1answer
25 views

time factor of heat increase (specific heat)

It takes 4 joules to bring 1 gram of water up 1 degree, but for how much time - 1 second? If not, how much and how to calculate it?
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27 views

heat transfer between objects

when we touch a very hot object, the edges of our skin interact with the object's high energy and velocity particles and molecules. why is it physically that we get "burned" by extremely cold objects ...
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0answers
15 views

speed of system to reach equilibrium [closed]

What is the time required by a system to reach an equilibrium from a large fluctuation, say twice or thrice the mean value of a variable?
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0answers
19 views

Chemical potential and origin of pressure in a solid

Pressure of a gas is related to the rate of change of momentum of the particles; the temperature is the mean kinetic energy of the particles. Can the chemical potential be given a similar physical ...
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0answers
33 views

Partition function for a two state system

We have a system of two energy states and we treat classical distinguishable and indistinguishable particles respectively. For the distinguishable case I thought that all in the left one one left ...
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1answer
39 views

thermodynamics trace calculation [closed]

I'm trying to calculate a trace to get the average energy. The Hamiltonoperator is $H = \sum\limits_k \varepsilon_k a_k^\dagger a_k$ and $N = \sum\limits_k a_k^\dagger a_k$. The trace to calculate is ...
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1answer
17 views

Reversible and Quasi static processes

Do we have any proof that reversible processes are always quasi static or is it just a fact that hasn't been violated till date? If there is a proof then please provide a link.
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1answer
31 views

Blowing on a hot liquid [duplicate]

Lets say you have a nice bowl of piping hot soup. You use your spoon and take a sip and realize its too hot. So you blow on it a few times and now its cooled enough that it doesnt burn your mouth. ...
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0answers
16 views

When does exergy destruction occur?

I am doing a project on developing a code for the first and second law analysis of the Brayton Simple and Modified cycles. The first law part is simple. But I have run into a roadblock when I started ...
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1answer
48 views

Where do limits of integration come from in the equation of heat transfer by conduction?

I was watching the third lecture of Diffrential equations on OCW. As an application, the model of heat transfer by conduction is provided. We derived this equation which models the system where $T$ is ...
5
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4answers
116 views

Thermodynamics thought experiment

There is some ideal gas in a container moving with some velocity on a smooth surface and you suddenly stop it( say by using your hands) , will the temperature of the gas increase? It seems to me that ...
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0answers
23 views

Dependence of Internal Resistance of Cell on Temperature and Nature of Electrode

I need to find out the dependence of Internal Resistance of a (Primary) cell on Temperature and Nature of Electrode. Temperature For that I am trying to derive a relation using the following ...
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1answer
58 views

Is Carnot engine the only reversible engine?

Is Carnot engine the only form of reversible engine? Is it possible to have a different form of reversible engine that goes through different processes? For a standard Otto cycle working with ideal ...
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0answers
13 views

Heat loss from hot cast iron oven

I am considering an unconventional pizza oven made from cast iron. What is a good formula for estimating non-radiative heat loss from a 450C/850F surface in a 20C outdoor environment? I am looking ...
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1answer
58 views

How to use the first law of thermodynamics for simple mechanical systems?

I'm confused about what exactly is $Q$ and $U$ and their signs. Consider a block initially having some kinetic energy which we stop and we want to find by how much amount its temperature increases. ...
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1answer
19 views

pV work in reversible processes. Why is $p_{ext}=p_{internal}$ in a contraction?

My textbook uses newtons third law to say to prove this. I think that's completely wrong. What I think happens is that we are doing an approximation that $p=p+dP$. In case you haven't figured it out, ...
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1answer
83 views

Can a liquid boil in a closed container?

The title says it all. When one fills a container (with water for example) and closes it so no air can escape from it, then heats the container continuously, can the liquid boil or completely ...