Covers the study of (mostly homogeneous) macroscopic systems from a heat/energy/entropy point of view. Maybe combine with [tag:statistical-mechanics].

learn more… | top users | synonyms (1)

0
votes
1answer
124 views

What is the difference of work $W$ and thermal energy $Q$ in thermodynamic Stirling-process for ideal gas?

What is the difference of work $W$ and thermal energy $Q$ in thermodynamic Stirling-process (in simple form) for ideal gas? I think that you need work to preserve this process and you bring thermal ...
0
votes
1answer
2k views

Heat Exchanger Calculation

I have a tank of oil at 55 degrees c. I plan to run a copper pipe 8mm in diameter (1mm thickness) into a coil 15m long inside the tank. For all purposes of assumption, the copper pipe is perfectly ...
0
votes
1answer
58 views

Action on Lard Oil

If water is mixed with lard oil and heated (creating some super-critical liquid with water), how does this affect the volatility of the mixture in comparison with its purity..? So, My question is: ...
0
votes
1answer
72 views

Performing work on a box of gas by lifting it, and first law of thermodynamics

What happens if we lift a box of ideal gas? Work is done to the box but no heat is getting into it. So does it's internal energy increase by the amount of work done? Or is it that lifting is not ...
0
votes
1answer
336 views

How to evaluate heating value on the basis on these data?

Heating value of liquid-gas mixture was tested with primus which has gas burner. In the initial state aluminum container with lid protected from the wind has temperature of 0 celsius and in it 0,54 ...
0
votes
2answers
233 views

Supermarket refrigerator - why is it noticebable colder in this shop area?

As we know fridge can't cool room in which it is (according to Second Law of Thermodynamics, heat emitted by fridge is greater than heat absorped). However, when we go next to the fridge in ...
0
votes
1answer
3k views

Work on ideal gas by piston

Imagine a thermally insulated cylinder containing a ideal gas closed at one end by a piston. If the piston is moved rapidly, so the gas expands from $V_i$ to $V_f$. The expanding gas will do work ...
0
votes
1answer
292 views

Dependance of temperature on color of metals

I asked this question because I supposedly did last year, Stanfor Klein which belongs to the Solar Energy Laboratory of the University of Wisconsin says that "the color of a car does not affect its ...
0
votes
1answer
132 views

Evolution of Gibbs energy change of reaction with temperature

The Gibbs energy change of reaction is given by $$\Delta G^0_{rx}=\Delta H^0_{rx}-T\Delta S^0_{rx}$$ From the look of it seems the direction of variation of equlibrium with $T$ is given by the sign ...
0
votes
1answer
163 views

heat of a laptop when placed on a table, when placed on a pile of books and when placed on a bed [closed]

I heard somewhere that it is dangerous to place a laptop on a bed. The reason given was that placing the laptop on the bed blocks (assume that the laptop does not have sideway fans - it only has air ...
0
votes
1answer
9k views

Water Electrolysis Calculations

From Wikipedia (http://en.wikipedia.org/wiki/Water_electrolysis#Efficiency): The electrolysis of water requires a minimum of 237.13 kJ of electrical energy input to dissociate each mole. Each ...
0
votes
1answer
124 views

What is a qualitative explanation for order of values of standard entropies of formation for isomers of hexane?

I have literature values for the standard enthalpy and Gibbs energy changes of formation at 298 K, $\Delta H_\text{f}^o$ and $\Delta G_\text{f}^o$, for product in the ideal gas state. Since $G=H-TS$ ...
0
votes
1answer
620 views

At what point can we assume the tip of a fin is adiabatic?

Let's say there is a fin that is 1mm thick, extends 8mm from the surface, and is 10 mm wide. The fin is exposed to a moving fluid. Can we assume the adiabatic tip condition and use the characteristic, ...
0
votes
1answer
120 views

Equipartition theorem for flowing gas

If an ideal gas is flowing with a velocity $v$, how is the equipartition theorem applied. Normally, we can say that $\frac{1}{2}mv_{x,rms}^2=\frac{1}{2}k_BT$. We can do the same thing for $v_y$ ...
0
votes
1answer
2k views

What's the temperature distribution of a plane wall insulated on one side with no internal heat gen.?

Let's say we have a plane wall that is insulated on one side, exposed to a fluid on the other side, and has no internal heat generation. Would the shape of the temperature distribution be a constant? ...
0
votes
1answer
309 views

When to use Heat Diffusivity eqn and when to use Fourier's law to find temperature distribution?

Let's say that there is a circular conical section that has diameter $D=.25x$ without any heat generation and I need to find the temperature distribution. Originially I thought I could use the heat ...
0
votes
1answer
310 views

Expansion of Helmholtz energy

To get an expansion of Helmholtz energy of a) an ideal gas b) a Van der waals gas we must integrate $\left ( \frac{\delta A }{\delta V} \right )_{T}=-P$ I saw the solution is : Can you ...
0
votes
2answers
2k views

how much heat i need to reach constant temperature

Is there an equation I can use to calculate the heat required (as a function of time) to sustain water at some specif temperature? I already know how much heat I need to reach the desired temparature ...
0
votes
1answer
1k views

What happens to temperature when pressure is constant in a cylindrical piston of saturated liquid ammonia?

Let's say I have a cylindrical piston containing saturated liquid ammonia that is fitted with an electrical heater and a paddle wheel for stirring at an initial pressure and an initial temperature. ...
0
votes
1answer
618 views

Helmholtz Free Energy relation

Ok so in a lecture my professor gave us this definition: $dF=\left(\frac{\partial F}{\partial T}\right)_{V,N}dT+\left(\frac{\partial F}{\partial V}\right)_{T,N}dV+\left(\frac{\partial F}{\partial ...
0
votes
1answer
466 views

Uses and interpretation of the 'Bowen Ratio' ($B_o=SH/LE$)

The Bowen Ratio is the ratio of sensible heat flux to latent heat flux, so presumably it gives some information about the relative importance of these processes. But it is not clear how this ...
0
votes
1answer
605 views

Reversible Stirling Engine

So it has been asserted that a Stirling engine with proper regeneration can be made reversible. It will consist of two isothermal quasi-static processes connected by two constant-volume processes. The ...
0
votes
1answer
532 views

What role does gravity have on flame composition?

In 2000, Nasa conducted an experiment and found that gravity plays an indirect role in flame formation and composition (see the wikipedia article). What role does gravity play? Why does a flame ...
0
votes
1answer
15 views

Rapid/ Slow Expansion in Gases

How is rapid expansion of gases different from slow expansion?
0
votes
0answers
23 views

Can I measure the volume of a locked room by pumping gas through keyhole and measuring its entropy?

Suppose that I have a locked room and a keyhole in the door and I want to measure the room's volume. Suppose also that I have some "magical" "artificial" inert gas A that doesn't interact with ...
0
votes
1answer
20 views

Distinguishing Isentropic Process; Are all things in an insulated cylinder considered reversible adiabatic?

How much work is necessary to compress air in an insulated cylinder from 0.20m^3 to 0.01m^3. Use T1 = 20C and P1 = 100 kPa. I don't need to solution or the answer. I already have it. But I think ...
0
votes
1answer
53 views

How do we measure thermal energy?

My textbook says "The amount of thermal energy is found by measuring the temperature of the substance." However, how is this true? Temperature measures only the average kinetic energy of the particles ...
0
votes
0answers
48 views

Why does specific heat increase with temperature?

Except for monoatomic molecules, specific heat increases with increase in temperature. Why?
0
votes
0answers
11 views

Is Isochoric compression or expansion possible?

Stirling cycle is made of two isothermal and two isochoric processes. Heat is added isothermally, expansion and compression takes place isochorically, as given by the book. How can a gas be ...
0
votes
1answer
39 views

Can the entropy of a part of a closed system become less than the initial entropy of the entire system?

I am reading Sir Roger Penrose's book "What came before the Big Bang?". Here, I came across a chapter where he talks about how sunlight provides us energy in low entropy and how through various ...
0
votes
0answers
22 views

How do I determine the heat conductivity of a material? [on hold]

I have a cup. I want to determine how much heat is transferred between the inside contents (boiling water) and the outside surroundings. What kind of information and how do I determine how to ...
0
votes
3answers
48 views

Work in the First Law of Thermodynamics

The First Law can be stated as $\Delta U=Q-W$, where $W$ is the work done by the system. My question is what kind of work $W$ includes. $W$ certainly includes $PV$ work, i.e. expansion and compression ...
0
votes
0answers
40 views

Proving $-\frac{\partial P}{\partial V}|_{T,N}\geq0$ [on hold]

I have obtained the following condition from the entropy maximum principle: $$\left.\frac{\partial^2U}{\partial V^2}\right|_{S,N}\left.\frac{\partial^2U}{\partial ...
0
votes
0answers
38 views

What time of day to run the air conditioner? [closed]

I live in the Rocky Mountain West, where the summer temperature fluctuation from low to high is typically 30-40 degree F. We have a seldom used house on the market. We would like the temperature to ...
0
votes
1answer
26 views

Few doubts about saturated vapour pressure and unsaturated vapour pressure

I have some sort of confusions towards saturated vapour pressure and unsaturated vapour pressure.. So let me give you those. What is exactly saturated vapour pressure and unsaturated vapour ...
0
votes
0answers
47 views

Equipartition theorem and virial theorem differences?

The classical virial theorem and the classical equipartition theorem are clearly related. A version of the virial theorem is, \begin{equation} \bigg\langle \sum ^{3N}_{i=1} x_i\frac{\partial ...
0
votes
0answers
28 views

Sign convention thermodynamics

Say a system is inside a volume $V$ with surface normal vector $n$ pointing out from the volume. Then there are force vector fields (nonconservative and conservative) depending on parameters external ...
0
votes
1answer
26 views

How can I find the temperature of a welded part of two metals? [closed]

Let's say you have two bars, different metals(Assume it's Iron and Copper) connected end-to-end(welded). The other end is Dipped(either Fe or Cu) to Icy Cold water and the other end is Steamed. How ...
0
votes
0answers
25 views

Efficiency of an Internal Combustion Engine

This question is about How correct are those comments on efficiecy of internal combustion(IC) engines which calcuate it using 2nd law of thermodynamics.The maximum efficiency of a heat engine can be ...
0
votes
0answers
19 views

Maxwell relations | U goes down pressure?

Differentiating Helmholtz function $ A = U - TS $ with respect to $V$ we get: $$ (\frac{\partial A}{\partial V})_{\scriptscriptstyle T} = (\frac{\partial U}{\partial V})_{\scriptscriptstyle T} ...
0
votes
0answers
15 views

When is strain rate constant?

I have started a tricky problem involving strain rates. To simplify it I was wondering if I can assume the strain rate is constant (and therefore that the shear stress is constant with distance above ...
0
votes
0answers
8 views

Reaction enthalpy-work relationship in a closed piston-cylinder assembly undergoing a chemical reaction

In an closed and insulated piston-cylinder assembly under constant pressure. From first law I should expect $-p(V_f-V_i) = U_f – U_i$ and thus $H_f = H_i$ and $h_f = h_i$. If an exothermic reaction ...
0
votes
0answers
30 views

Change in internal energy

One definition in my textbook states that total derivative of internal energy $dU$ has the form: $$dU = C_vdT +(\frac{\partial U}{\partial V})_{\scriptscriptstyle T}dV $$ Which I interpret this way: ...
0
votes
1answer
23 views

Heat loss during evaporation of a liquid boiling due to Bernoulli effect

this is my first post here. I watched this video https://www.youtube.com/watch?v=h5wQoA15OnQ and at 51 seconds it describes Bernoulli's effect of a drop in pressure resulting from an increased ...
0
votes
0answers
18 views

How accurate is the Callendar-Van Dusen equation?

... over 0 degC to 100 degC? The equation itself can be found here I can do a very accurate one point calibration of a 1/10 DIN PT100 sensor at 0 degC. If I use the equation to correct for non-linear ...
0
votes
0answers
48 views

How much sunlight would 'fit' through a fibre optic cable?

Say a parabolic mirror were used to focus the full sunlight spectrum into a single strand of optical fibre. Assuming ideal conditions (perfect reflecting surface, focal point smaller than diameter of ...
0
votes
0answers
21 views

Must irreversible heat transfer be quasistatic?

Suppose there is an isolated system composed of two subsystems $A$ and $B$ in thermal contact with each other, but mechanically and diffusively insulated from each other. The system starts off with a ...
0
votes
0answers
40 views

Does mixing violate conservation of energy?

If two fluids in an isolated system mix, the entropy of the system increases. The volume and the number of particles in the system, however, are constant. Thus $$dU=TdS+PdV-\mu dN=TdS>0$$ which ...
0
votes
1answer
43 views

Illustration of heat processes

In isothermal process the work produced in a cyclic transformation is the sum of the small quantities of work $ \delta W$ produced at each stage of the cycle. This description of work I picture, for ...
0
votes
0answers
32 views

How does a gas behave if i let it out of a pressurised canister?

So I am working on a problem where I need to know how a gas behaves if it is let out of a canister. My first approach yielded some result, but i can't seem to get all the way, although it seems to me ...