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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55 views

What ds>dQ/T mean?

I read the derivation on page 216 over here: https://www3.nd.edu/~powers/ame.20231/notes.pdf First it considers an irreversible process between state 1 and 2 followed by a reversible process between ...
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4answers
7k views

Is thermal/heat energy the same thing as internal energy?

I learned that internal energy of an object is the sum of the kinetic energy and potential energy of the molecules of the object. Is this energy the same thing as the heat energy the object contains?
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2answers
120 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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3answers
24 views

When does the Boltzmann distribution apply?

What are the requirements for a system to be described by the Boltzmann distribution in equilibrium? For example, should all the particles be identical? No attractors in the phase space? ...
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1answer
284 views

How to calculate the heat transfer rate between the liquid and the gas?

Let water (liquid) be in a chamber (with no heat transfer to the walls). The gas is the residual space above the liquid, meaning that the contact surface is the surface of the water. Given the gas ...
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2answers
22 views

U and PV changes in thermofluid processes

If we have a steady flow open system, how can we know the change in U and PV? I mean if we input work in the system, how can we know that the work goes to U increase or PV increase? Also, can you give ...
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1answer
406 views

Does a negative line tension in a 3-fluid immiscible interface make sense?

This question is inspired by this question/answer pair: Is this formula for the energy of a configuration of 3 fluids physically reasonable? Consider three immiscible fluids forming contact surfaces, ...
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3answers
198 views

Is the Boltzmann brain problem really clearly established as a problem?

Starting from the assumption of a cosmology in which Boltzmann brains dominate over evolved ones, it is not immediately obvious to me that there is a real problem, since only Boltzmann brains ...
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1answer
29 views

Room with open door and heater : use inside boundary conditions or heat source for PDE?

I want to model the stationary temperature in a room (=square) that has one open door (=side) is otherwise perfectly insulated is heated through a (circular) heater that is placed somewhere in the ...
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1answer
34 views

Thermofluid mechanics inclined plane

As shown in the attacked image, a tank has an inclined wall at an angle of 450 to the horizontal. On this wall, there is a 1m square door that is hinged at A and has a simple latch at B. The distance ...
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0answers
44 views

Maximum temperature possible on earth

What is maximum temperature that can we have on earth on a single day? Lets say an air mass is static over an area and there is no way for air mass to move, sun warming it up would increase ...
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2answers
34 views

Thermal Capacity

An electrical heating coil of power $P$ is used to transfer thermal energy to a body of mass $m$. In a time $t$ the body changes temperature by $\Delta T$ . What is the thermal capacity of the body?
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1answer
144 views

Heat balance: how to obtain T(t) with energy in the form u(T(t))

I have the heat balance equation for a cooling case in the form: $$\frac{dU(T(t))}{dt}=-J$$ with $U$: energy (J) $T$: temperature (K) $t$: time (s) $J$: heat flux leaving the system (W) I have ...
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1answer
32 views

Entropy for $N$ number of particles [on hold]

If there are $N$ number of non-interacting and distinguishable particles which have either Energy $E_1$ or $E_2$ , then a. What will be the entropy $S(n)$ for such system? ($n$ is the number ...
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1answer
16 views

Force applied between neighboring atoms resulting in increase in separation

I am attempting a past paper question in practice for an upcoming exam and I am a little lost with the following question: Using lennard Jones model for potential. An external force $F$ is applied to ...
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1answer
28 views

2nd law of thermodynamics - simple steam power plant problem

A steam power plant consists of a boiler, a turbine, a condenser and a pump. The temperature of the inner walls of the boiler is 350 °C and the temperature of the condenser cooling water is 20 °C. ...
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1answer
113 views

relation between speed and temperature in relativistic gas

What is the correct formulation of the relation between the speed and temperature in the relativistic kinetic gas? I have found one mentioned in problem 3.24 of the third edition of Pathria's ...
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1answer
33 views

Does density of air decrease or increase with altitude and relation of this with temperature?

I think the air density decrease as due to gravity more air woul be near the earth but as the hot air raises up doesn't it have momentarily increased the density at height (like in up troposphere) ...
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2answers
43 views

Increase in entropy principle

If we consider a system to undergo an irreversible process from state 1 to state 2 and a reversible process from state 2 to state 1, then through Clausius inequality $\int_{1}^{2} \frac{\delta ...
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0answers
24 views

Dimensional Analysis Problem [on hold]

In the formula: $$ P = nRT/(V-b)* e^{-a/(RTV)}\ $$ Find the dimensions of 'a' and 'b', where P = Pressure , n = No. of moles , T= Temperature , V = Volume and R = Universal gas constant.
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2answers
37 views

A 50 g Ice Cube with an initial temperature of -10°C is put in 400 g of water at 40°C. Find final temperature [on hold]

My calculations: $$Q_g + Q_l = 0$$ $$Q_g = -Q_l$$ $$mc\Delta T = -mc\Delta T$$ $$(0.05)(2100)(T_f+10) = -(0.4)(4200)(T_f-40)$$ $$105(T_f+10) = -1680(T_f-40)$$ $$105T_f + 1050 = -1680T_f + 67,200$$ ...
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0answers
22 views

Eddy current damping heat generation

Background According to this source (page 7): https://deepblue.lib.umich.edu/bitstream/handle/2027.42/109373/me450w10project16_report.pdf?sequence=1 the "braking" torque a magnetic field on a ...
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0answers
16 views

How does one compute the maximum height reached by the water droplets in this experiment and what is the theory behind such phenomenon?

Hot Water Fountain - Partially fill a Mohr pipette with hot water. Cover the top of the pipette with your thumb. Turn the tip upwards and observe the fountain exiting the tip. Investigate the ...
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1answer
32 views

Are all heaters exactly as energy efficient as each other?

I many applications, efficiency is measured in terms of how much of the input energy is wasted generating heat instead of the actual type of energy you want. For example, lightbulbs generating heat ...
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1answer
21 views

relativistic temperatures energy assumptions

why is the assumption $k_bT>>mc^2$ made for relativistic temperatures, where did the kT come from and how is it related to mc^2?
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2answers
42 views

how can energy conservation equation be different in different situations?

Neglecting potential and kinetic energy: energy conservation equation in a closed system is dU=Q-W and in an open system with steady flow is dU+d(PressureEnergyPV)=Q-W or in other words dH=Q-W. How ...
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0answers
20 views

Relation between Canonical and Grand Canonical Ensemble for a two levels system

For a two-level system of classical non interacting distinguishable paricles, labelling energy levels as $+\epsilon, -\epsilon$, in the canonical ensemble I found for the partition function ...
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2answers
34 views

Thermodynamics : Heat transfer : Conduction or convection?

I have an hot object, that is place in a room of 20°C. My goal is to be able to estimate how warm the object will be in T seconds. To be able to do that, I must first understand what is really ...
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1answer
49 views

I can get a heating strip - why can't I get a cooling strip?

A typical heating strip is a flexible silicone material containing a resistive heater. However, I would like the opposite - something I can wrap around my experiment and use in a similar way to the ...
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1answer
28 views

First Law of Thermodynamics and Signs

I have a question in my text book about the first Law of Thermodynamics. It gives 3 situations with the systems for each in brackets: 1) Rapidly pumping up a bicycle tyre (air in the pump) 2) ...
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0answers
21 views

What are the calculations specifically needed for High Altitude Ballooning? [on hold]

I want to know the lifting gas needed to launch the vehicle including the positive lift I want to predict the possible retrieval site I want to know the burst altitude AND MAYBE ALL WITHOUT ...
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0answers
18 views

Ratio of the change of internal energy under the specified condition given below [on hold]

What will be the ratio of the change of internal energy of a diatomic gas U1 & U2 for a unit rise of temperature in constant pressure and volume respectively.
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3answers
62 views

Degrees of freedom and temperature

I quote the following lines directly from the Wikipedia page titled "Heat capacity": "...rotational kinetic energy of gas molecules stores heat energy in a way that increases heat capacity, since ...
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3answers
137 views

Why engines don't melt?

The temperatures of internal combustion engines are well above the melting points of the component materials. For example, temperatures can reach 2200° C whereas the melting point of aluminium is ...
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3answers
52 views

How does frost form above freezing temperature?

This is a follow-up to the best answer linked below: Why does ice form on bridges even if the temperature is above freezing? I've witnessed that phenomenon, recently. The temperature was above zero ...
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1answer
38 views

Is potential energy included in internal energy?

I thought internal energy is only the energy due to molecular motion and chemical bonds, but here it says it's also the potential energy of the system! Is that true?
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1answer
31 views

heat as a low quality energy

In a cyclic process, there is no change in internal energy. So the work done by the system must equal to the heat offered to the system. So if all heat is converted to work, how can heat by a low ...
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1answer
136 views

Specific Internal Energy Decreases with Increasing Pressure?

I was looking at "Properties of Compressed Liquid Water" (Table A-5) in Fundamentals of Engineering Thermodynamics by Moran. Table A-5 shows that the specific internal energy $u$ of compressed liquid ...
2
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1answer
394 views

Calculating new temperature of an object when air temperature changes

I'm trying to calculate the new temperature of an object when the air temperature around it changes, given a period of time. Basically I get periodic readings from an air temperature sensor in a ...
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0answers
39 views

Can the second law of thermodynamics be violated in a small enough system if tried repeatedly enough?

Second law of thermodynamics is observed in the universe because statistics favors it, right? And in large enough system this statistical tendency becomes certainty. Does it also mean that negative ...
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8answers
235 views

Ideal gas law: Will the piston move at all?

We have the following experimental setup: Before the experiment starts: $$p_1=p_2; \space V_1=V_2; T_1=T_2+\Delta T$$ The experiment starts and both the containers are heated so that the ...
2
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1answer
31 views

Is saying that “Heat always flow from hotter to colder object in a isolated system” equivalent to the second principle?

In other words, can we make exactly the same predictions as the second principle (that involves entropy) by this phrase without ever introducing entropy ?
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1answer
113 views

On Thermodynamics of Spontaneity of the Reduction-Oxidation Chemical Reactions

According to the first and second law for a closed system containing different chemicals we have \begin{align} &\delta Q - \delta W = dU = T dS - p dV +\sum_i \mu_i d N_i\\ ...
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1answer
21 views

Why use dimensionless heat capacity?

Perhaps this is blindly obvious, but in typical discussions of statistical mechanics (with, say, constant volume) one often finds that, rather than using the heat capacity $$ C_V = \frac{\partial ...
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0answers
18 views

How heated air moves [on hold]

Ok so how does air move when heated up inside a tube or cone? I am working on a wood stove and curious on how I can get air to flow faster or easier into the burn chambers to fuel the wood. Heat ...
4
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2answers
132 views

Reversible and Irreversible Process

I would like to ask a specific conceptual question which bothers me for quite some time! First of all i do know the difference in between reversible and irreversible processes. What is thought in ...
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2answers
47 views

Intuitive explanation of the shape of the Maxwell-Boltzmann distribution

At higher temperatures (for an ideal gas), the Maxwell-Boltzmann distribution is spread more widely and has a lower maximum. At lower temperatures, the spread is much more narrow and the peak is much ...
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1answer
23 views

Thermofluid mechanics Temperature

A steel alloy specimen (mass 4 kg) at 80 0C is submerged in water at 10 C0. At thermal equilibrium state the temperature of the steel-water system is 40 C0. Calculate the mass of the water used. The ...
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4answers
4k views

Surface Area: Volume and its relation with heat

In biology, it is stated that more surface area and less volume helps for keeping bodies cool. How can one explain this phenomena in terms of physics?
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2answers
181 views

Why doesn't the separation of particles by mass via gravity decrease entropy?

Let's say we've got a fluid of heavy and light particles inside a cubical flask, which is initially shaken up so that the density of heavy particles is uniform everywhere. Let's also say that these ...