Adam,
Carnot engines is not a topic that should be overstudied on the MCAT, but you are right in studying it because occassionally there will be passages about it (I had one last year :\). Here's what you need to know:
As you know, no matter how much energy you put into something, not all of the energy (unfortunately
) will be put into the final product. The Carnot engine is just a thermodynamic example of this: it represents what happens in the real world, not the ideal world.
Your car's engine is an example of a Carnot engine. The heat energy (qh) created by combusting gasoline cannot all be used to run the car (unfortunately). Some of the heat generates mechanical energy (w) that is used to drive the car, but most of the heat energy will run into the RADIATOR, the cold reservoir of your car (qc).
There is also the idea of the 'reverse Carnot engine'...the best example of this is a refrigerator. A refrigerator takes heat from the cold reservior (the inside of the fridge) and puts it into a hot reservoir (the external environment). As you know, in order to heat water you have to put work energy into it (a bunsen burner, pressure, etc.) So, in order for a refrigerator to work it has to take qc (cold) and add work to it (w) to make it qh (hot)...just like heating water.
I hope this helps...I want to make sure anyone that has a question gets it answered (the MCAT is stressful enough without confusing concepts)...so if you have a question please don't hesitate to PM me or reply. Good luck!
~Bryce
