Author Topic: Talking Thermodynamics  (Read 120476 times)

Offline Captain Jerry

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Re: Talking Thermodynamics
« Reply #1215 on: August 17, 2019, 01:24:45 PM »
Great explanation. I got it| So "cold" is like negative energy, sucking the energy out of the metallic atoms.  Is there a "cold front" as the boundary between agitated atoms and drowsy atoms progresses through the copper? Is that why filling a copper mug with ice, adding vodka and ginger beer will :cheers:  slow the thought processes if ingested?  I have seen this happen!
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Offline steam guy willy

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Re: Talking Thermodynamics
« Reply #1216 on: August 18, 2019, 01:11:45 AM »
Hi MJM , thanks for the further info and i suppose the next question is ...what frequencies do the molecules vibrate at ?? also  i possibly won't be doing that experiment  with the tea !!

Willy

Offline paul gough

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Re: Talking Thermodynamics
« Reply #1217 on: August 18, 2019, 02:27:30 AM »
Hi MJM, Thanks for the detailed explanations. For me these things are best understood if I can convert to a visual representation in my mind and your descriptions have enabled me to 'see' what is going on. Regards, Paul.

Offline MJM460

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Re: Talking Thermodynamics
« Reply #1218 on: August 18, 2019, 12:58:25 PM »
Hi Captain Jerry, I am glad that the explanation made sense, sometimes I wonder about my more wordy posts.  It’s good to have you on board.

I find it more logical to think of energy as a positive quantity, as negative motion in random directions as exhibited by the individual atoms does not make sense to me.  Energy is a scalar quantity, not a vector, so has no specific direction.  The flow of energy of course goes in the direction of the temperature gradient, and cannot go in the reverse direction unless you provide an energy input in the form of work.

It progresses across a material in a more gradual manner, not as a step or wave.  To get a feel for this on a very coarse scale, imagine the thickness that block we talked about earlier divided into ten equal parts, and look at the temperature gradient along a line through the block, perpendicular to the surface.  At the start, the temperature is the same all the way along the line, so no temperature gradient, no heat is flowing.  Now we raise the temperature of the surface.  Instantaneously for the first increment of thickness, there is a temperature gradient, so heat flows from the hot surface to the cold surface, but there is still no temperature gradient across the second or subsequent parts of the block.  The temperature gradients tell us that heat is flowing in one side, but not out the other.  The energy equation tells us that the difference between energy in and energy out is stored by the material heating up.

As time progresses, the temperature at the boundary between the first two increments rises due to the heat inflow.  This reduces the temperature gradient across the first increment, but introduces a temperature gradient across the second one.  And so on.  So some heat is stored in the material, some is lost from the other side, and the temperature gradient builds up.  Probably easier to sketch out than describe.

It is possible to apply the appropriate equations, to calculate the time all this takes, but eventually the temperature gradient becomes linear and constant, no further heat is stored.  It can also be done graphically with some calculated parameters for time.

You may realise that what I have described is a simple example of finite element analysis.  We had to do that exact example by hand when I was studying thermodynamics, as no one had a computer capable of doing it at all.  At that time, the UNIVAC computer had a whole 16k of memory for program and calculations, and was fed with punched cards.  And only the geeks got to understand, let alone use that.  Now days, no one, not even students would do it by hand.

And yes, I can see that the ginger beer, vodka and ice in a copper cup would slow the thinking, but which are the critical variables?

Hi Willy, great that my explanation made sense.  But the vibration issue is a bit harder.  For vibration, there must be a restoring force to keep reversing the direction.  I believe that in gases anyway, the molecules travel in straight lines per Newton’s law, until they collide with something, another molecule or a wall of the container, to exert a force which results in them taking off in another direction, like a 3-D billiards game.  I have a physics book which actually calculates the average velocity of the molecules, and the average distance between collisions.  But apart from a little review early in this thread, I have hardly looked at it in around fifty years.

I suspect even in solids, a similar thing happens, though whether the collisions are hard elastic collisions like the billiard balls or more like soft objects where the repulsive forces interact.  I am on shaky ground trying to describe that too closely.  There are vibrations in the system, as we can identify materials by the light they give off, but I will leave it to your theoretical physicist to explain that one.  And more particularly, to tie the two ideas together.

Hi Paul, I think the reason I like engineering and physics is that I can picture how things happen, rather than just learn the result.  My memory never worked well in that area.  Good that the explanation was helpful.

Thanks everyone for looking in,

MJM460
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Offline Captain Jerry

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Re: Talking Thermodynamics
« Reply #1219 on: August 18, 2019, 08:36:01 PM »
When I was In school, a physics classmate of mine could always be counted on to ask the obviously stupid question and after a chorus of groans from the rest of the class, we would often get into some of the most interesting conversations.  By having to rule out Howard's ridiculous hypothesis it was necessary to have a better understanding of the conditions being observed. For example, what happens at temperatures below absolute zero ( -459.67F or 0 Kelvin). A theoretically impossible condition because by definition, atomic motion would cease.


However, a group of German scientist at the University of Munich published their experiment in the Jan 4 issue of The Journal of Science in which they achieved a negative value on the Kelvin scale. Their findings suggest that the temperatures below O Kelvin are equivalent to temperatures above Infinity and that the temperature scale is circular and that the atoms in their experiment were in a state that was hotter than infinity. There are many more interesting possibilities from their finding, including heat engines operating at efficiency greater than 100%.


Why was such an experiment attempted? I am sure that someone like my old buddy Howard asked the question and then followed it up until it couldn't be ignored.  I hope I haven't muddied the water too much it is thinking like that that has led to expanded knowledge.


Jerry

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Offline paul gough

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Re: Talking Thermodynamics
« Reply #1220 on: August 19, 2019, 02:17:35 AM »
Hi MJM, Your refined description of energy flow in reply to Jerry, 'painted' a very clear picture of the process. Thanks for creative writing. Regards, Paul.

Offline MJM460

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Re: Talking Thermodynamics
« Reply #1221 on: August 20, 2019, 12:23:15 AM »
Hi Jerry, I must admit that I had always thought that molecular motion ceases at zero K and lower temperatures were not possible.  However I found out recently that Richard Feynman says that at 0 K there is still a tiny motion, but still that lower temperatures are not possible.  And who am I to argue with him?  A defeatist attitude I know, but it raises the question what is that residual motion, which I don’t think he answered in the books I have.  I wonder if it is a minor imbalance with the electrons whizzing around, which might give a small motion almost imperceptible unless you are very close to absolute zero.  But that is all conjecture on my part.

That article and experiment sounds interesting.  Thermodynamics as we know it says you need a lower temperature sink in order to cool something, which is surmised as not possible at zero K, so it is intriguing to think about how they did it.  But as we know, all laws of physics are only laws until proven otherwise, it’s just that the ones accepted as laws of physics have been well tested and no exceptions have yet been found.  It will be interesting to see if the the experiments can stand with full peer review and if others can replicate the results of that experiment.

But it all sounds very much like theoretical physics to me, very hard to see how we would use the principle in our model engine making.

Hi Paul, thanks for that.  It’s good to know the explanation makes sense.

Thanks everyone for looking in,

MJM460








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Offline Captain Jerry

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Re: Talking Thermodynamics
« Reply #1222 on: August 20, 2019, 01:20:25 AM »
I would not argue with Feynman either, but If I was able to talk to him, I might say "I know its not possible but if you thought it was, how would you go about doing it?"  It is only theoretical physics until it is put to the test.  Extreme temperature experiments may have no relationship to machining as a hobby, but the question certainly does. "How would you go about it?" is a big part of the challenge for me.  I know it can be done with CNC but how would I approach it with limited manual equipment.


By the way, when you talked about doing finite element analysis by hand, I guess you had a slip stick in your hand.  I haven't seen mine in years but I know its here someplace.


Jerry

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Offline MJM460

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Re: Talking Thermodynamics
« Reply #1223 on: August 20, 2019, 02:01:21 PM »
Hi Captain Jerry, well said on the theoretical physics.  I must admit to thoroughly enjoying Richard Feynman’s lectures, or at least the summaries I have in two books of “easy pieces”.  I have found them quite readable and truly informative.

I still have at least five “slip sticks” that I could lay my hands on.  I get two of them out occasionally to try and keep my hand in, though I would not want to have to do the complex calculations I was once able to do.  That was all I had until I bought my first electronic calculator as a duty free purchase on a flight after my third job, about ten years of my work.  At least I did not have an abacus, but I did have a mechanical calculator that could multiply and divide, or rather that could be used for those calculations.  But a little child dropped it so that was the end of it.  You can’t keep everything of interest forever unfortunately.

MJM460


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Offline steam guy willy

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Re: Talking Thermodynamics
« Reply #1224 on: August 21, 2019, 02:51:21 AM »
Hi MJM,   ..Last night i had a bath and i always run the water to a temp of 105 F... and i keep checking the temp over the 10 minutes it takes to fill.  however the temp of the water by the time it filled it only got to about 101  degrees ?!  when put my foot in it it was
really very hot and when i checked the water again after a few minutes with it switched off it was 112 F !! I then realised that the Thermometer was giving the wrong reading . I think this was because the battery might be a bit low as the device was on continually for the 10 mins ?? is this possible with these type of devices and there is no warning on it to show the battery may be down.!!

Willy

Offline Admiral_dk

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Re: Talking Thermodynamics
« Reply #1225 on: August 21, 2019, 12:43:15 PM »
Quote
is this possible with these type of devices and there is no warning on it to show the battery may be down.!!

Oh yes, very much so and for several reasons. Cheap sensors give a not very accurate voltage that is compared to a reference voltage and from that the temperature is calculated. So in your case I would say that the Reference Voltage can't be maintained @ low battery voltage => more or less useless readings.
Low voltage indications would drive the price up too .... A quality digital temperature measuring system will set you back hundreds of £.

Offline MJM460

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Re: Talking Thermodynamics
« Reply #1226 on: August 21, 2019, 12:50:13 PM »
Hi Willy, it is certainly frustrating when a digital instrument does not give reliable answers, there is nothing to see to alert you.

Until recently I have been using electronic calipers.  I have always noticed that when I start getting erratic readings it is a sign that the battery is flat.  Even though the battery warning light is not showing.  After spoiling a few parts I started to check with a micrometer when the measurements are getting close.

I presume that your thermometer could be similar. 

I have recently bought new electronic callipers which I hope will be better, but I have not got to use them much yet.  Keeping my fingers crossed that they will be better.

MJM460

Hmm! I see that Admiral has answered while I was typing, I think we are saying similar things.  Sometimes I am tempted to try those more expensive instruments for my boiler tests, but usually either reneg on the cost, or can’t find something suitable.  But I am an old fashioned shopper.  It’s sure to be available somewhere.
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Offline steam guy willy

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Re: Talking Thermodynamics
« Reply #1227 on: August 21, 2019, 08:22:53 PM »
Hi MJM a new question a bit more fluidy dynamic...Does pressure  air, steam, water, etc act on a surface at 90 degrees ??   If so if a piston was cone shaped would the pressure be trying to push it sort of sideways at the angle of it .....rather than strait downwards in line with the piston rod ???

this may be a bit silly ?? :-\

Willy
« Last Edit: August 22, 2019, 02:20:39 AM by steam guy willy »

Offline MJM460

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Re: Talking Thermodynamics
« Reply #1228 on: August 22, 2019, 12:41:36 PM »
Hi Willy, that is really getting back to the topic.  It gets to the basic  understanding of how fluid pressure works in a confined space.

I think it was covered in one of our very first fluid mechanics lectures, over fifty years ago now, the concept that pressure operates equally in all directions.  So for a little sample volume somewhere away from the walls within a confined space, the pressure acts the same in all directions.  Any pressure effect in any direction is exactly cancelled out by the equal pressure in the opposite direction.  Another way of putting it is that pressure is a scalar quantity that has no direction.

However, at the wall of any contained volume, the molecules causing the pressure can only come from one side of the surface.  The pressure acts at right angles to the surface.  Even molecules approaching the wall at an angle, are balanced by the effect of molecules approaching from the other direction.  Not on a one for one basis, but the sum total of the horizontal effect of all the angular collisions is zero, and only the perpendicular component of each collision contributes to the net force.

In your piston example, the pressure on the angled top of the piston acts at right angles to the surface causing a force proportional the surface area, acting at right angles to the surface.  The perpendicular to the surface direction is the direction of the force.  Force has magnitude and direction so force is a vector.

Now, if you remember your vector maths, a vector can be resolved into two components at right angles to each other, using the normal maths of a right angled triangle.  And each of those components is always smaller than the primary vector. 

If we resolve the force on the angled part of the piston as you have drawn it, the horizontal portion on one side of the piston is exactly balanced by the equal force on the other side.  The horizontal forces on the piston are balanced by internal stresses in the piston, meaning that any horizontal section through the crowned part of the piston is in compression.  There is no movement in the horizontal direction, so there is no work done or energy absorbed by the horizontal component of the force.

The vertical component acts in the normal manner to do work on the piston.  While the vertical component of the force at any point is smaller than the force acting on the sloped surface, the area of the sloped surface is larger than that of a flat topped piston.  The result is that the vertical component of the force over the whole piston top is exactly the same as the vertical force on a flat topped piston.

It does not matter if the piston top is conical or domed, or even asymmetric in some form to assist combustion, with calculus, the maths can always be solved and the result is always the same. 
When the vertical component of the force causes the piston to move, work is done by the gas on the piston, and this work is the mechanical work developed in engine.  Some is taken up in bearing friction, some in pushing the spent gases out the exhaust port, some absorbed by water pumps, cooling fans, valve gear etc and with some luck there is even some left to give a useful output.

I hope that answers the question.  Definitely not silly.

MJM460





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Offline Captain Jerry

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Re: Talking Thermodynamics
« Reply #1229 on: August 22, 2019, 02:56:09 PM »
Wiley's question was about force, not work,so the short answer is "yes."
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