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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228 views

Problem about thermodynamic relations

We know that for constant pressure thermodynamic processes, $dH=dQ_p$. My question is, does it implies that only reversible work is possible in this processes so that $dw=0$ because $dv$ is zero? In ...
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1answer
20 views

Why do $C_p$, $\alpha$ and $\kappa_T$ all approach to infinity when two phases coexist at the mean time?

My textbook (writed by Chinese) says that when two phases of a system coexist at the same time, this system's isobaric thermal capacity $C_p=T(\frac{\partial S}{\partial T})_p$, coefficient of ...
5
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1answer
1k views

Inflating a balloon (expansion resistance)

I am doing a quick calculation on how to calculate the pressure needed to inflate a perfectly spherical balloon to a certain volume, however I have difficulties with the fact that the balloon (rubber) ...
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1answer
22 views

Two rising bubbles of differing temperatures

Consider two bubbles rising through some medium (water, for example). Given that the two bubbles have the same initial size, pressure, and temperature but differing velocities, which bubble will be ...
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19 views

Calculate Temperature change in a solid due to resistive heating

Trying to calculate temperature increase in a solid due to resistive heating by a current flow, is this along the right lines? First I have calculated resistance due to resistivity, then calculated ...
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1answer
32 views

Why are glass beads used in a fractionating column?

Most sources do not go beyond saying that glass beads help increase the surface area for condensation of the liquid of the higher boiling point while using the method of Fractional Distillation to ...
3
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1answer
45 views

If Black holes are maximal entropy how can they evaporate?

According to Hawking/Bekenstein a black hole represents the highest amount of entropy for a given volume, (actually the entropy is related to the surface area of the black hole but the fact that they ...
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2answers
55 views

Entropy always increases in a closed system - what if the universe is open?

An interesting question I was asked: Entropy always increases in a closed system - what if the universe is open? Does that mean that entropy can decrease in such a system? Of course, I think there is ...
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16 views

Inductance heating, Temp Change [duplicate]

I am trying to find the temperature increase of a solid(copper) in a gas (air) due to an electric current passing through the solid (inductance). I need to encompass any temperature losses from the ...
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1answer
23 views

Confusion about Isobaric processes

Suppose in a ballon of gas is given an extra bit of heat, so as to getting the ballon's internal energy slightly up and this will work out to achieve a little bulk for the ballon. Though the whole way ...
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1answer
20 views

Effect of wall friction on stagnation pressure and temperature

We have incompressible air flowing through a cylindrical pipe, from section A to section B. The process is considered to be stationary. My theory book says that, because of the frictional losses on ...
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1answer
36 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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1answer
11 views

Does frequency of mechanical vibration of material affect its blackbody radiation spectrum

Can we see the difference in emissivity for instance between the same object vibrating in two different frequencies ?
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12 views

Rugged Energy Landcapes (Free Energy vs Potential Energy Questions)

A spin glass has what is called a "rugged energy landscape." That is, when you cool down below a certain temperature, the system divides into many wells, all corresponding to slightly different ...
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0answers
9 views

Heating during expansion in Joule Thomson - Mechanism? [on hold]

When a real gas(like H2,He) expands through a porous plug into free space, they heat up. How? How does it get the extra energy?
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2answers
36 views

Maxwell's area construction, why?

In the standard derivation of Maxwell's area construction (which can be found on page 4 of this pdf) the following equation is often written: $$G(p_1,T)=G(p_0,T)+\int^{p_1}_{p_0}Vdp $$ When the two ...
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1answer
159 views

Why does the equation for effectiveness of heat exchangers have $C_{\text{min}}$ in the denominator?

For an ideal heat exchanger of infinite length, $$ Q_{max}=m_{h}C_{h}\left[T_{h,i}-T_{o}\right]=m_{c}C_{c}\left[T_{o}-T_{c,i}\right] $$ \begin{eqnarray*} m_{h}C_{h}T_{h,i}+m_{c}C_{c}T_{c,i} & ...
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1answer
2k views

Why does friction produce heat?

What causes two objects sliding against each other to produce heat? Why don't they generate visible light or something else?
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16 views

Project Orion pusher plate computer modeling

Linked from the space.stackexchange forum: http://space.stackexchange.com/questions/15051/orion-project-npp-pusher-plate-computer-models-and-survivability Project Orion is discussed as if it is a ...
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1answer
37 views

2nd law of thermodynamics - simple steam power plant problem [on hold]

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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0answers
52 views

Would it be correct to state that all the forms of everyday matter (mountains, tables, buildings, people, etc.) are in a metastable state?

We had a fire going in the backyard of a friends house this weekend, and after thinking about the chemical reactions occurring in the wood (and anything else we threw into the pit), I started to ...
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0answers
13 views

Theoritical Electrical Energy of battery [on hold]

i m a student and looking for some answer related to my research, i m recently working on Ni-fe battery ie. nickel-iron battery. I would like to know what formula or law is applicable to calculate ...
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0answers
23 views

How can I get the exit temperature of water entered 100 degree in a pipe surrounded by 30 degree water?

Assume any value for: Length of the pipe The material of the pipe Velocity Heat transfer coefficient. I tried considering it as parallel flow heat exchanger but the water in this example isn't ...
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1answer
68 views

Thermal average, thermal fluctuations

I've a doubt concerning the physical meaning of "thermal average" and the "thermal fluctuation" in the canonical ensemble. Let's consider a very simple thermodynamic system: N particles, at fixed ...
4
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3answers
56 views

Why do Temperatures Equalize

I have some Oxygen at Temp A in one container and some Nitrogen at Temp B in another container. If I mix these two containers eventually both the Oxygen and Nitrogen will be at the same temperature. ...
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1answer
37 views

Confusion about inverse square law and temperature increase of an object

I know the inverse square law means that radiation intensity decreases in proportion to the radial distance from the source (I is proportional to 1/r^2). Does this apply to the temperature increase ...
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3answers
45 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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3answers
84 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
123 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 ...
3
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1answer
292 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
25 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 ...
4
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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
201 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
34 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 ...
0
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1answer
45 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
48 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
40 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
36 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 ...
0
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1answer
17 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
116 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
44 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
28 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
39 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
25 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
21 views

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

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
33 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
23 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?