entropy - useless?

[macmet]

This is my first year working and although I took matls in school my job is more mechanical based. And now it's the day before christmas and I've decided to take it easy and just do some work-related reading on combustion.

So, now I'm wondering...When I was in school I did a lot of thermo classes dealing with gibbs, entropy, enthalpy. But working as a mech eng will I ever really use entropy or gibbs again? Since I started work the only time I have heard any reference to entropy was related to turbines.

Could it be true that most of what I spent four years learning I will never use again? Where is entropy and free energy important in mechanical engineering?

Have a happy holiday season everyone.

 

[mbeychok]

macmet:

Mechanical engineering covers quite a wide field. Some mechanical engineers may never use thermodynamics and others may do work that involves quite a lot of thermodynamics.

For example, if you work in the field of automotive engine design, or steam turbine design, thermodynamics is involved in analyzing the cycles involved (i.e, such as the Carnot cycle, the Rankine cycle, the Otto cycle, etc.). If you get involved in aircraft engine design, again thermodynamics will be involved.

If you work in the field of fluid mechanics or heat transfer, you will also encounter thermodynamics.

However, if you have a career in pressure vessel design for example, or in machine design (like precision lathes, or in robotics), you may never encounter thermodynamics.

So, as they say, it depends .... !!

Happy holidays,


Milton Beychok
(Visit me at www.air-dispersion.com)
.

 

[TXiceman]

I have spent 36 years in the industrial refrigeration field and constantly use my thermo and heat transfer background. I sledom have any need for mechanical design,but do have to use it ocassionally. So it is really dependent on the area of mechanical engineering you decide to work.

Ken

TXiceman

http://www.rae-corp.com

 

[chicopee]

Eventho, there are disciplines learned in college that you may never use, it is still knowledge that may help you interact with engineers, scientists and other professionals. That knowledge will help you understand and feel confortable with what others may talk about and not make you feel totally out of place. Knowledge is power and valuable in this highly technical world.

 

[abeltio]

chicopee... you are right! Knowledge is power, so K = P
and Time is Money so: T = M

we know that P = W/T (work per unit time)

therefore:

K = W/M and M = W/K

this means that no matter how much Work W you do, the money you earn is inversely proportional to your knowledge... also, as your knowledge tends to zero, the money you make tends to infinite... this is Dilbert's Salary Theorem.

Also, if anybody does not understand this simple demonstration... he/she should not worry... the salary is surely pretty good.

now, going to macmet's question:

entropy is important at work for the following...
1. if you do a thermodynamical analysis of a process and the overall entropy change is < 0... something is wrong. re-check your calculations.

2. allows you to understand geeky jokes at the workplace.

3. allows you to tell the geeky jokes you heard at work in a party, and you will be the only one laughing... except perhaps another engineer or physics geek.

cheers

saludos.
a.

 

[macmet]

Thank you to everyone that called. I didn't do a very good job of making my question clear. I was hoping for specific uses of entropy and g.f.e. in all sorts of applications, like Abeltio mentioned.

Cheers

 

[sailoday28]

Consider conditions of steady state and a perfect gas with constant specific heats going thru a shock.
Solving conservation equations of enegry, mass, momentum will yield two solutions for change in pressure.
The correct change can be determined by considereing change in entropy, one which is + and the other negative.
Since entropy must increase, one can conclude that pressure increase corresponding to the entropy increase is correct.

For other equations of state, a shock can lead to more than two solutions for changes in pressure, temp, etc . Some solutions may not be real (ie, imaginary solutions) others may correspond to a decrease in entropy and therefore not valid. Use of change in entropy, in this type of analysis thereby gives insight to correct answers.

 

[EngJW]

I took two thermo courses and in the last one the prof said that we would absolutely come away from it with a clear understanding of entropy. Well, I didn't understand it then and still don't (I spent 30 years in machine design, that's the excuse).

Maybe somebody can explain it in plain English. All the textbooks use the same old equations and graphs with muddy explanations.

Some examples like Mr. Sail gave would be great.

 

[25362]

One common concept in statistical mechanics is that entropy is a measure of the disorder of a system. Anyone who has tried to reunite a pair of socks after tumbles in a drier is acquainted with entropy. Cream mixes irreversibly into coffee. This is coterminous with the fact that over time the universe slides on a one-way course toward disorder (larger entropy).

In the biochemical microscopic realm it has been shown that under certain conditions entropy can be a force of organization: the strange phenomenon in which an increase of entropy (ie, disorder) in one part of a system forces another part into greater order.

Biochemists are aware of this trick of physics being used to advantage by living cells. A colloidal cocktail containing small and bigger spheres rearranges in a way that the larger spheres crowd toward the edges of the container (for example, a cell membrane) to make room for the smaller spheres to be disorderly. Similarly to people waiting out a dance in a discotheque crowding against the walls to enlarge the space in the dancing floor.

Under the microscope it has been found that the small spheres bombard randomly the larger ones causing them to pile up on the walls or corners, being pelted by the smaller spheres on their exposed sides. This "force" of entropy appears to act when no other stronger fields are present (electrostatic, etc.) and depend on the concentration of small and large particles.

If the flexible membrane were to absorb the large spheres entropy would be maximized. The theory goes on saying that a living cell contains proteins and other bodies floating around that could act as small spheres pushing out larger sacks of hemoglobin into the bloodstream. Cell fluids being salty would serve to screen out the much stronger electrostatic forces that could overcome entropy's gentle influence.

Engineers hope to harness this force to develop crystals that reflect visible light perfectly at all angles to improve lasers and fiber-optic cables, among other projects.

The above has been taken from an article in the magazine SCIENCE (March 28, 1998) titled: Gentle Force of Entropy Bridges Disciplines, by David Kestenbaum.

 

[cecilo]

i am a service engineer in a Caterpillar dealership. there is indeed very little of my mechanical engineering in the job. all that is required so far is basics which i left after my second year in college

 

[abeltio]

cecilo... do you perform engine tests? in that case... you are still using entropy... if you do a test on an engine and the test results indicate that the efficiency is > than the efficiency of a Carnot engine you know something is not right... you would be violating the 2nd principle of thermodynamics and to do that the entropy change of the universe would be < 0.
perhaps you would just say: - wow, this test results are bollox...
and you would redo the test... but don't underestimate yourself... your knowledge of entropy is still there, somehow.

cheers.
happy new year.

saludos.
a.

 

[chicopee]

Here is a thought provoking idea about entropy and this idea is not new as I remember discussing it in the early 60's in college. We live in a universe where entropy is increasing all the time-so is it possible of another universe in which entropy is decreasing? any opinion??

 

[rmw]

Here is a take on entropy in the universe;

http://www.math.tulane.edu/~tipler/

Click on the link "Why the laws of physics imply the Omega Point Theory" under 'Main areas of research' and follow it up by reading the link to "Famous Physicist defends..." the last link under 'Main areas...."

Interesting reading.

Dr. Tiplers book "The Physics of Immortality" got into it in depth. (As in buried me.)

rmw

 

[JStephen]

"One common concept in statistical mechanics is that entropy is a measure of the disorder of a system." That idea gets mis-applied and mis-understood a lot, I have found. Specifically, assuming that visible disorder somehow equates to greater entropy.

Anyway, I've found few uses for entropy in tank design. I think what you're overlooking is that entropy ideas are in the background of a lot of common ideas. Anytime that you have the efficiency of an engine or pump or similar apparatus, entropy is lurking in the background.

 

[rmw]

Any time you pump a tank full of a fluid, you have an entropy change.

rmw

 

[bspry]

Using entropy can be useful in calculating properties of devices such as turbines and pumps. Using an isentropic efficiency you can relate the state of the entry and exit of these devices.

For example a turbine may have an isentropic efficiency of 85%; therefore the actual power developed by the turbine is its isentropic efficiency * the potential power output if the turbine was operating isentropically (constant entropy). For a pump the inverse is true, because isentropic efficiency are <1.

Also to clear a few things up, some people have posted that if a thermodynamic process results in a decrease in entropy that something is wrong. This is not totally true. Entropy can be transfered and the definition of a change in entropy is:
the integral of dQ/T plus sigma (entropy production for an irreversable process). if the heat transfer term is negative (out of your control volume) and greater that sigma the net affect is a loss in entropy.

However, the entropy of the universe is always increasing.