The Law Posted November 10, 2009 Report Share Posted November 10, 2009 Never really understood this section. Does anyone wanna take a crack at explaining the main points of heat engines in "heat engines for a dummy" format? Link to comment Share on other sites More sharing options...
beng Posted November 10, 2009 Report Share Posted November 10, 2009 i'm not sure what it is specifically that you need to know, or what you don't understand, but here is a start: A heat engine extracts work from the movement of heat from a high temp source to a low temp sink. It's very much like if you are on top of a platform and jump down onto a lever. Because you are up on the platform you are at a higher relative potential energy than the ground, so when you jump down you can create work (pushing down the lever) as you return to the ground. So you being on the platform is like the gas at a high temp, the mechanical work or lever is likely something being spun, and the low potential energy is low temperature gas. Feel free to ask more specific questions. It hasn't been toooo long since I took my thermo courses. Link to comment Share on other sites More sharing options...
changster Posted November 11, 2009 Report Share Posted November 11, 2009 ^ please explain why heat engines can't be 100% efficient, thanks! Link to comment Share on other sites More sharing options...
The Law Posted November 11, 2009 Author Report Share Posted November 11, 2009 Thanks for your help, I kind of get them now after reviewing. But I'll definitely post here if I find something confusing. Link to comment Share on other sites More sharing options...
beng Posted November 12, 2009 Report Share Posted November 12, 2009 ^ please explain why heat engines can't be 100% efficient, thanks! Well the answer you won't like is that if you go through the math to calculate efficiency it becomes impossible. For any thermodynamic process you start calculating efficiency by saying "efficiency = what I get out/what I put in". Like anything, there is no free lunch, and that's how efficiency is (usually) defined. Anyway the derivation results in the efficiency being 1 - Tcold/Thot, which makes sense, because the engine will obviously work better the larger the temp discrepancy. So you can see that in order for the efficiency to actually reach 100%, either Tcold has to go to 0 kelvin (impossible), or Thot has to reach infinity. If you look at the case of a real life heat engine, it's pretty easy to see that T cold is usually just the ambient temperature and that it would cost energy to make it cooler. I think though, there is another upper limit just because it is a carnot cycle. I don't remember anymore about it unfortunately. blah that's the best I can come up with offhand. Link to comment Share on other sites More sharing options...
Recommended Posts
Archived
This topic is now archived and is closed to further replies.