Recent Posts

Pages: 1 ... 6 7 [8] 9 10
71
Chatterbox / Re: Simplicity hit and miss...going to a new home.....probably.
« Last post by kvom on August 17, 2019, 03:48:36 PM »
Vixen's post reminded me of the final voyage in "Around the World in 80 Days",  where the wooden parts of the steamer were ripped off to feed the boiler so Fogg could land in time.
72
Chatterbox / Re: Mach4 Bummer
« Last post by gldavison on August 17, 2019, 01:51:52 PM »
Well, don't clap your hands and stomp your feet just yet, but maybe I have found the problem.. more testing in progress. Will let you know.
73
Chatterbox / Re: Talking Thermodynamics
« Last post by Captain Jerry 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!
74
Chatterbox / Re: Talking Thermodynamics
« Last post by MJM460 on August 17, 2019, 12:20:13 PM »
Hi Paul, just to formulate the question in that way shows a high level understanding of the issue.  Even my trusty Thermodynamics text book dodges the issue of providing a definition of energy, simply stating that it is such a well understood concept that a definition is not really necessary.  It then goes on to define work and heat as energy in transit.  Work crosses a boundary when a force acts through a distance, while heat is energy crossing a boundary due to a temperature difference.  Energy is not a substance that can be identified moving around.  So energy can be stored as potential energy (height), kinetic energy (velocity) or internal energy (which is evident as temperature).  And it can move either as work due to work being done, or as heat due to a temperature difference.  And energy can be changed between the various forms.  It should also be mentioned that energy is not created or destroyed, conservation of energy is one of the fundamental laws of physics.  Mind you, it can sometimes be quite a puzzle to understand where all the energy went, or came from, but if we look hard enough it can be found.

Perhaps not a very good explanation, but that is the nature of the question.  So what does it mean in the context of your question?

If we consider a block of metal at some high temperature, the atoms all have energy, which if it was possible to see them, would be evident in their vigorous motion.  In a solid it is more or less like vibration around a point.  In a cooler block, the atoms are still in motion, but the motion is smaller amplitude or velocity.  The atoms do not all have the same energy, but the temperature is the result of the average. Presumably larger numbers of the atoms are closer to the average, while small numbers depart from average by a larger amount.  Think of the bell curve of a normal distribution.

If one surface of a block is heated to some temperature perhaps by a flame, or by contact with another block already at a high temperature, then the atoms near the hot surface bouncing against the slower moving atoms deeper in the block results in some energy exchange between those atoms, with the higher velocity atoms slowing and the slower moving ones increasing in velocity.  Thus some energy is transferred by the collisions, and while the temperature gradient is maintained the process continues.  If the whole block is at the same temperature, all the atoms have on average, the same energy level, and there is no transfer of energy.

If the material is relatively simple atoms in some sort of crystal array, like most metals, the process of energy transfer proceeds quote rapidly.  Think of silver, copper or iron and so on.  But if the block is a more complex compound, it is much harder to get those molecules vibrating in a pattern that results in predictable collisions.  It still happens, but proceeds more slowly.  Or if the material has a cross linked structure, like a plastic, similar resistance to heat transfer.

So the actual rate of transfer is determined to a large degree by the molecular structure of the material.  And not so easy to regulate in the way would regulate steam or water flow for example.  It is essentially controlled by the material molecular structure and the temperature difference.

Another example where the molecular theory of matter can help our understanding.  I hope it helps answering the question.

Hi Willy, brown sugar and honey are both quite different sugars.  The brown sugar is mostly sucrose, the ordinary table sugar, and a disaccharide, but the last of the molasses has not been washed out in the final crystallisation process in its manufacture.  It also traps some moisture which sort of sticks it together.  So it needs a bit of help by stirring to mix it up well with the water. 

Honey is mostly fructose and glucose, both simple monosaccharides, of which the glucose can chain together into long chains.  But there are over 100 other compounds in honey which contribute to its unique qualities.

Sugar just gets more and more complicated as soon as you look at it all closely.  Definitely well outside my comfort zone.  I prefer the simple things.

Your block of copper is much easier to discuss.

When you heat a metal, it expands in all three directions.  You can look up the coefficient of expansion many text books, or just search the web.  If you constrain this expansion as you have described, you get very high stresses which can be calculated by Hookes law and the modulus of elasticity.  These stresses will soon exceed the yield stress of the copper, and the atoms will be pushed around in the lattice, just as if you had squashed it is a vice, or hit it with a hammer.  Each atom effectively occupies the same volume, but the atoms are pushed around and the lattice distorted.  Similarly, with steel or other metals.  This is the source of thermal expansion stresses.  A major consideration in designing pipes for refineries and gas plants. If a metal object is heated unevenly, the colder parts expand less and effectively constrain the hotter parts which are trying to expand.

I suppose if those pioneers had actually bathed in tea or coffee they would have had scold injuries in places very embarrassing to explain, and it might have held back progress considerably.

MJM460

75
From Kits/Castings / Re: 15cc Seal and 30cc Seal Major
« Last post by Bluechip on August 17, 2019, 11:41:45 AM »
 Hi Jo

:whoohoo:  :cartwheel:   :wine1: etc. etc.

Dave
76
From Kits/Castings / Re: 15cc Seal and 30cc Seal Major
« Last post by Jo on August 17, 2019, 11:33:37 AM »
I heard a startled noise and found a delivery driver being hassled by Surus    :facepalm2: He clearly was holding an engineering parcel as it was held together by the duct tape on the outside  :thinking:..

Closer examination found it contained a set of castings with that patented special coating so I did not need to read the further clue on the base (which read "Chuk Alyn Foundry" ) to know where it originally came from  :pinkelephant:


Lets hope Surus is feeling generous and we can have a fondling session later  :naughty:

Jo
77
From Kits/Castings / Re: 15cc Seal and 30cc Seal Major
« Last post by Jo on August 17, 2019, 07:04:52 AM »
Thank you Jim, nice to see someone else following along on these builds :)

Jo
78
Chatterbox / Re: Talking Thermodynamics
« Last post by steam guy willy on August 17, 2019, 03:40:47 AM »
Hi MJM, Just a reply to the honey question... I tried putting a large brown sugar cube in some tea hot  water and the cube sank to the bottom and fell apart...1 Hour later it was still visually the same and did not taste sweet. once it was stirred the colour was dissipated and it tasted sweet . ...thinking about Einstein and Archimedes i wonder how quickly the advancement of science would have progressed if they had actually bathed in tea/coffee !!! :lolb: :lolb:

Thinking about Pauls question ..we know that materials expand when heated...however if a piece of a copper cube were heated between two very solid low thermal imouveable  objects  how would the copper try to expand?? would the molecules become oval on the restricted faces/plane or would the copper expand a larger/different amount on the un restricted faces ??

willy.
79
Tooling & Machines / Re: Die Filer attachment for the lathe
« Last post by Don1966 on August 17, 2019, 03:13:03 AM »
Looking great Dave and yeah buddy piccys would be nice... :ThumbsUp:


 :cheers:
Don
80
Tooling & Machines / Re: Dial indicator repair
« Last post by Mcgyver on August 17, 2019, 02:40:23 AM »
yup, ebay, very inexpensive.  SKF also makes this size, but don't know their price.
Pages: 1 ... 6 7 [8] 9 10