- #1
ikjadoon
- 37
- 0
So, I'm studying for the MCAT. There..I've said it. :) But, it's not a homework question: I just don't understand the concept.
My study book's thermodynamics section includes a very rough sketch of a heat engine to describe the 2nd Law of Thermodynamics. A side note in the text says that if this section is confusing, just ignore the heat engine and simply know the 2nd Law of Thermodynamics (...that heat cannot be turned into work 100% in a cyclical process); however, I am interested in the heat engine (mainly because I don't understand how it works, haha).
My lapse in understanding: how can it take less work to compress a piston to its original state than how much work the piston did? It takes work to compress the piston, but how can that be any less than what work was done by the system through expansion?
I get the first "part" of the heat engine. We have a piston and it's in an isothermal environment. We remove some force holding the piston and thus the volume expands and pressure decreases. Since it is isothermal, however, heat transfers energy into the system to keep it at the same temperature (ΔE = q + w where ΔE = 0) via a hot reservoir. The heat we put into the system = the work we get out. Yes, this makes sense.
But, in all the websites and textbooks I've looked at, then somehow we can compress the piston back to its original state by doing work to the system--but we use less work!
Here's what I thought should be happening: So, let's say the gas expands and does 10J of work in the first "part" of the engine. To keep it isothermal, a hot reservoir transfers 10J of energy into the system. So, now we have an expanded cylinder and we've let some gas expand isothermally by adding some heat (producing a 100% conversion to work). I would think, though, that to "reset" the piston back to its normal state, we're going to do work to the system (10J of work) and a cold reservoir will heat up by 10J (to keep it isothermal).
No net work. We have to use just as much work to "reset" the piston. I don't see how we can reset the piston by somehow using less work (and thus have positive net work done by the system).
--
I realize this is pretty elementary. If you have any links I can peruse, let me know. I've looked at a fair few, but maybe if I look at them and tell you precisely where I don't understand what happened, we can sort through this conundrum. :(
My study book's thermodynamics section includes a very rough sketch of a heat engine to describe the 2nd Law of Thermodynamics. A side note in the text says that if this section is confusing, just ignore the heat engine and simply know the 2nd Law of Thermodynamics (...that heat cannot be turned into work 100% in a cyclical process); however, I am interested in the heat engine (mainly because I don't understand how it works, haha).
My lapse in understanding: how can it take less work to compress a piston to its original state than how much work the piston did? It takes work to compress the piston, but how can that be any less than what work was done by the system through expansion?
I get the first "part" of the heat engine. We have a piston and it's in an isothermal environment. We remove some force holding the piston and thus the volume expands and pressure decreases. Since it is isothermal, however, heat transfers energy into the system to keep it at the same temperature (ΔE = q + w where ΔE = 0) via a hot reservoir. The heat we put into the system = the work we get out. Yes, this makes sense.
But, in all the websites and textbooks I've looked at, then somehow we can compress the piston back to its original state by doing work to the system--but we use less work!
Here's what I thought should be happening: So, let's say the gas expands and does 10J of work in the first "part" of the engine. To keep it isothermal, a hot reservoir transfers 10J of energy into the system. So, now we have an expanded cylinder and we've let some gas expand isothermally by adding some heat (producing a 100% conversion to work). I would think, though, that to "reset" the piston back to its normal state, we're going to do work to the system (10J of work) and a cold reservoir will heat up by 10J (to keep it isothermal).
No net work. We have to use just as much work to "reset" the piston. I don't see how we can reset the piston by somehow using less work (and thus have positive net work done by the system).
--
I realize this is pretty elementary. If you have any links I can peruse, let me know. I've looked at a fair few, but maybe if I look at them and tell you precisely where I don't understand what happened, we can sort through this conundrum. :(