I've been struggling for quite some time with what thermodynamics, cooling machine is all about. Thank you for providing such valuable different perspective. Love you work man, bravo!
thank you this is really helpful! I finally made the connection between taking energy out of the cold reservoir via work, and dumping it to maintain a Tc in the fridge. you made it clear! :)
Thanks for your kind words! I've added a comment at 3:45 explaining that it's not quite vertical and linking to an explanation of how crazy steep adiabats actually are. Thanks for watching, yo.
Hahah this is coolest physics lesson ever, you're funny but damn helped so much! Really useful! Though I'm a little confused at why the vertical line you draw at 3:45 is an adiabatic process, I thought because there is no change in volume then it is just iso-volumetric, therefore no work being done, not an adiabatic process where there is no heat transfer. Well anyway this video was very informative!
Connect an engine and a heat pump in parallel, then explain the performance of the system. The part of the work which was obtained by the engine is used for the heat pump. The temperatures of hot and cold reservoirs are maintained constant. Then think of the second law.
Great work homie, but I'd like to point out that @2:30 you wrote [1 - (Tc/Th)] as the simplified expression for efficiency of 'any' heat engine. This ONLY applies to the Carnot cycle and hence it would be the maximum achievable efficiency by an engine. Unless I'm tripping out haha
Thanks! I had heard it was the efficiency of any reversible heat engine. However, I'm not an expert on the idea of reversibility. Is there a reversible engine that does not follow Carnot? I'll certainly agree that it is the ideal efficiency, so perhaps an inequality would be better suited.
For refrigerator and AC, can someone explain to me why when we want cold temperature but its leaving from cold to hot with work put in. I thought we want hot to cold. I don't understand what's our desired state I guess. Thanks
In a real life application of the heat pump, what temperature do we consider as that of the hot reservoir TH. In a heat pump for example, lets say we circulate water at 298K in the hot side(condenser of heat pump) which heats it up to 333K for home use. I guess Tc in this case would be the outside temperature, but which temperature in this case should we consider as TH?
Considered your question about efficiency being more dependent on cold or hot reservoir: Mathematically, it looks like both have an equal effect on the efficiency but you mentioned that according the the 3rd law, it becomes nearly impossible to be 'OK' therefore I want to say that the temp of the cold reservoir has a bigger effect on the efficiency since, in chemistry terms, the cold reservoir would be the "limiting reactant". Though please correct me if I'm wrong.
But then I guess I could also say that because we have more control over the hot reservoir than the cold reservoir, then the hot reservoir has a larger effect on the efficiency since we can affect it more. Too bad I can't just talk to engines. That'd make this a lot easier.
Study, for instance, a full Carnot cycle on a PV diagram. I don't go into that depth yet. For my intro class, I just need my students to see conservation of energy for these engines.
Would a refrigerator as a heat engine, where the refrigerator uses a gas (refrigerant) that circulates around a loop with an expander and a compressor, be considered a open or closed system? I'm having issues determining whether it is open or closed because I don't know if mass transfer is involved with the use of a gas.
it's closed system because the gas inside the fridge doesn't disappear. it circulates . the compressor and evaporator do the job of heating and cooling the gas.
It means the process is done slowly and with the ability to move the engine the other direction in the cycle on the PV diagram at any time. It is the maximally efficient way to go!
So Qh = Qc + W This makes all the energy neatly accounted for right? What if your Cold Res. and Hot Res were 100 lb blocks of granite and they both started out at room temp. You plugged in your fridge and ran it until Qh - Qc = 50F and then unplugged the fridge. You could let them sit until they re-assumed room temp or you could connect a small peltier device between them and produce a small amount of electricity, for a little while as their temps equalized. The power produced would fade away. The act of doing that would cause the two blocks to equalize faster. The amount of power is very small. But where did this secondary power come from?
cop is Q devided by W while efficiency uses both either Q or W. its like work desired devided by work needed. efficiency means if you spend 100 J of energy, the machine will only spend 70 . but with that 70 , it does 5-6 cop of work.
I have question ! , we know that energy within the universe is constant (conserved) according to the first law. The second law says entropy is increasing ! but how come? why energy in the universe shouldn't increase with the increasing of entropy ?
