Tagged Questions
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0answers
37 views
How does the volume change affect an adiabatic process?
A gas consisting of rigid diatomic molecules was initially under standard conditions. Then the gas was compressed adiabatically, 5 times the original volume. Find the mean KE of rotating molecules ...
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1answer
25 views
A Calorimetry Problem
I have a question in calorimetry from an old competitive exam. The question is:
The temperature of $100$ grams of water is to be raised from $24 ^\circ$C to $90 ^\circ$C by adding steam to it. ...
5
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2answers
160 views
First law of thermodynamics?
The first law says that the change in internal energy is equal to the work done on the system (W) minus the work done by the system (Q). However, can $Q$ be any kind of work, such as mechanical work? ...
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4answers
85 views
The Preference for Low Energy States
The idea that systems will achieve the lowest energy state they can because they are more "stable" is clear enough. My question is, what causes this tendency? I've researched the question and been ...
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3answers
74 views
How do you determine the heat transfer from a P-V diagram?
I doubt this question has been addressed properly before, but if there are similar answers, do direct them to me.
I am currently studying the First Law of Thermodynamics, which includes the p-V ...
3
votes
1answer
60 views
Why does compressing a piston increase the internal energy?
When we compress a piston, its total internal energy increases, however I don't understand why.
As the piston compresses, the temperature should change, as the total energy density increases.
As a ...
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3answers
132 views
Integrating factor $1/T$ in 2nd Law of Thermodynamics
How would you prove that $1/T$ is the most suitable integrating factor to transform $\delta Q$ to an exact differential in the second law of thermodynamics:
$$dS = \frac{\delta Q}{T}$$
Where $dS$ is ...
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votes
2answers
66 views
Is it possible to “add cold” or to “add heat” to systems?
Amanda just poured herself a cup of hot coffee to get her day started.
She took her first sip and nearly burned her tongue. Since she didn't
have much time to sit and wait for it to cool down, ...
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1answer
47 views
Liquid oxygen how do they use it as fuel?
Rockets are said to be using liquid oxygen as fuel. How do they use liquid oxygen since it's just oxygen, it only helps in the combustion process. How can it be a fuel on its own?
2
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2answers
185 views
Increase thermal efficiency of combustion engine by using heat of coolant/exhaust?
I can't be the only one who's ever thought of this, but obviously it hasn't caught on:
In terms of energy density, fossil fuels are the best thing around short of enriched uranium (and, flammability ...
0
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1answer
50 views
How hot would a disc have to be to create strong drafts that lift it upward?
I read this article on xkcd.xom, from its spin-off blog "What If?" and that answer made me wonder.
It was mentioned in that article that a very hot but indestructible box would eventually be able to ...
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1answer
128 views
If human energy use were to increase by 2.3% per year, would we use all solar energy by 1400 years?
Assuming 2.3% per year exponential growth of human energy consumption (so, roughly speaking, that corresponds to multiplying consumption tenfold every century), it's argued that human annual energy ...
2
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1answer
112 views
free energy and entropy of 2D soap froth
This is a (exploratory) computational project. The soap froth was created by injecting bubbles into a chamber formed by two rectangular plates which are 0.16cm. From the moment the soap froth was ...
3
votes
3answers
184 views
Understanding mathematically the free expansion process of an ideal gas
I'm trying to understand mathematically that for the free expansion of an ideal gas the internal energy $E$ just depends on temperature $T$ and not volume $V$.
In the free expansion process the ...
1
vote
2answers
100 views
Can the vibrational energy of a engine be used to increase efficiency?
In whole mechanics, we never talk about wasted vibrational energy!
As we see in our daily life, a lot of energy is wasted as vibrational energy in every engine, for example as we fire our car's ...
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5answers
675 views
Why does the nature always prefer low energy and maximum entropy?
Why does the nature always prefer low energy and maximum entropy?
I've just learned electrostatics and I still have no idea why like charges repel each other.
...
2
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4answers
503 views
Is energy the ability to do work?
Here was my argument against this, the second law of thermodynamics, in effect says that, there is no heat engine that can take all of some energy that was transferred to it by heat and do work on ...
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1answer
69 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 ...
4
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3answers
324 views
Can I take heat from the air and convert it to electricity?
Its a summer day and the air in my house has been heated up. I could switch on my air conditioning, but then I'd be using energy from the grid in order to reduce the amount of energy in my house.
...
2
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1answer
76 views
What are the properties and impediments of a liquid air fueled engine?
I recently came across a very interesting article that suggested the possibility of using liquified gases like air, nitrogen, or oxygen as a power source for cars. It appears that this company is ...
2
votes
2answers
802 views
Calculating work done on an ideal gas
I am trying to calculate the work done on an ideal gas in a piston set up where temperature is kept constant. I am given the volume, pressure and temperature.
I know from Boyle's law that volume is ...
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2answers
153 views
Carnot cycle: What happens to the rest of the energy?
In a Carnot cycle, the work potential is $W =Q_{in}( 1-\frac{T_0}{T_R})$ where $T_0$ is the temperature of the sink, $T_{R}$ is the temperature of the source heat reservoir, and $Q_{in}$ is the heat ...
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1answer
120 views
Why does an ice rink use a primary and secondary coolant?
A normal refrigeration cycle uses one coolant, so why does vapor compression cycle of an ice rink you a primary coolant and a secondary coolant?
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2answers
346 views
Why can't I evaporate water without wind, just heat? (not boiling,evaporating!) Or can I?
So here is the thing, I searched all over the internet for this but all the sources say that I need wind because the process of evaporation goes as follow:
Water particles at the top layer with ...
0
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1answer
680 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 ...
3
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3answers
747 views
How can I determine density of a gas only given temperature?
I have a homework problem where exhaust is traveling through an exhaust system (assumed to be air for simplicity) from and engine and then released into the atmosphere. The exhaust is at a temperature ...
2
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3answers
136 views
Heat Equation Equalities
While studying the heat equation, I ran into a few equalities that I cannot understand.
For example, Fourier's law of heat conduction claims that
$$\varphi(x,t)=-K_0\frac{\partial u}{\partial x},$$
...
1
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1answer
86 views
Where is the critical moment where the microcanonical ensemble enters the justification for the equilibium state?
As explained in many books, for the microscopic justification of the second law of thermodynamics (lets formulate it as the total entropy takes maximum among all possible exchanges of two systems), ...
6
votes
4answers
156 views
Is energy extensivity necessary in thermodynamics?
Given a partition of a system into two smaller systems, the energy $U$ is devided into $U_1$ and $U_2$, with
$$U=\mathcal{P}(U_1,U_2):=U_1+U_2,$$
so that $U_2$ is given by $U-U_1$. Here the ...
4
votes
1answer
716 views
What is the relationship between Energy, Entropy, and Information?
What is the relationship between Energy, Entropy, and Information?
I read this - What Is Energy? Where did it come from? - and the top answer says that 'energy' is an abstract number that is a ...
2
votes
1answer
147 views
Man on treadmill and energy/mass conservation
Suppose we put a very obese man inside a large air tight isolated room with a treadmill and he runs on the treadmill for 24 hours and at the end of the day he sees his weight has come down (mostly ...
3
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1answer
311 views
Wind Turbine Impact
OK, I'm not sure this is not a silly question, but here goes.
I've often wondered what the impact is of wind turbines on the weather. My question is, since wind turbines transform wind energy into ...
2
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2answers
220 views
how does heat energy start to speed up a gas molecule?
If it was possible to place a single gas molecule in a cell and freeze it to near absolute zero.
What would the molecule do as it thawed out?
Would it translate the heat energy into it's electrons ...
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1answer
2k 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 ...
3
votes
2answers
252 views
Connection between entropy and energy
An isolated system $A$ has entropy $S_a>0$.
Next, the isolation of $A$ is temporarily violated, and it has entropy reduced $$S_b ~=~ S_a - S,\space\space\space S\leq S_a.$$
Is it true to say: the ...
0
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1answer
69 views
System moves away from equilibrium $\rightarrow$ it has energy added?
Suppose there is an isolated system $A$ at time $(-\infty, t_1)$, whose entropy is $S=S_{max}$, i.e. it is at thermodynamical equilibrium.
Between moments $[t_1, t_2)$ the isolation is violated and ...
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1answer
135 views
2d or 1d conduction in this scenario?
There is a rectangular fin attached to a heat exchanger with a base temperature of 350K. The fin has uniform properties and experiencesa uniform heat generation. It also experiences heat transfer with ...
0
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1answer
277 views
Energy versus free-energy diagram
Energy versus free energy diagram. I haven't been able to find an adequate definition of these two terms in relation to each other. Could someone point me in the right direction, please?
From Borrell ...
0
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1answer
162 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, ...
1
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1answer
460 views
In what situations do I use the characteristic length of a fin to find the surface area?
So I'm learning about fins in heat transfer and it seems that there are two separate formulas for the surface area of a rectangular fin of length L, width w and thickness t. The fin is attached to a ...
3
votes
2answers
682 views
What meaning do changes in the absolute value of Gibbs free energy have in a simple expansion process?
Below is a simple representation of the thermodynamics of a steam turbine. Stream kinetic and potential energy changes are neglected and no other type of non-PV work is done besides shaft work. ...
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vote
0answers
166 views
Is the $mL_c$ value for triangular and rectangular fins the same value?
I am looking at the solutions that my professor put up and I feel that he did something wrong. Here is the question and I will give my stab at the solution so you can see why I think that it is wrong.
...
2
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1answer
271 views
How do I find average temperature given a temperature distribution?
I was told to find the temperature distribution of a wire with a current going through it. So I found $$T(x)=T_{\infty}-\frac{\dot{q}}{km^{2}}[\frac{cosh(mx)}{cosh(mL)}-1]$$
I need to find the ...
1
vote
0answers
238 views
How equivalent are heat energy and work energy in connection with a spinning flywheel?
Let's say we have two identical spinning flywheels, that have arbitrary geometry, and are made of copper.
Now we apply some heat energy at the center point of flywheel A, causing it to slow down a ...
1
vote
1answer
81 views
Terminology question about energy
I'm looking for the appropriate term to use for what gets "used up" as potential energy is converted to heat and work. For example, some of the the energy in solar radiation is converted by ...
0
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1answer
178 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 ...
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2answers
126 views
How much energy does lowering an object into a black hole generate?
An object of mass m is slowly lowered into a black hole of mass 1000 m. Is the amount of braking energy larger than $0.6 mc^2$?
Now what if, after lowering the mass close to the event horizon, we ...
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3answers
169 views
If humans were able to catch all sun energy reaching the earth for their use, will the climate change?
I guess that energy will be used up and, at the end, will contribute
to heat the earth, so I see no big differences...
please explain your point of view.
2
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3answers
82 views
Certain material heating water in a recipient
I don't know how to resolve a problem, but I don't want the answer since I'm almost going to have it resolved.
What the problem says is we have 85 liters of water at 7ºC in an iron pot of 29kg. We ...
0
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0answers
78 views
2 streams going into an engine and 2 come out. Find the enthalpy?
Problem: Steam supply to an engine is made of two streams that mix before entering the engine. Stream 1 flows at a $.01\frac{kg}{s}$ with an enthalpy of $2952\frac{kJ}{kg}$ and a velocity of ...
