Carnot Cycle

A sequence of isothermal and adabatic expansions and compressions of an ideal gas, called the Carnot Cycle, provides a way to turn heat into work.

[@spuble]

You are the sole reason I survived this class despite having no previous exposure to basic thermodynamics

[@ayodapizzahere504]

Hehehehaw

[@PhysicalChemistry]

Glad to have been helpful

[@wouter8533]

I've spent hours trying to intuitively comprehend this concept and this was a huge leap in that direction, thanks.

[@PhysicalChemistry]

You're welcome. Happy to have helped

[@alvaroriquelme6667]

Amazing ... Thanks very much Professor

[@shriraamm8638]

Classic style. Detailed explantion

[@gelomelo3426]

Professor Steven stuart thank you for this beautiful professional unique presentation and I appreciate you for answering ppl questions and replying their questions. You're such a great person and incredible scientist ❤.

[@PhysicalChemistry]

That's very kind of you, thanks for the comment

[@olafobidowski4143]

absolutely great, revising for my IB exams. Subscribed after 2 minutes of this video ;))

[@PhysicalChemistry]

I'm glad to hear the videos are useful for you; thanks for the subscription

[@gelomelo3426]

Wonderful 👏 👏 👏 👏

[@parterintalamiajoshua8148]

thank you for the great good work

[@PhysicalChemistry]

I'm glad you like it. Thanks for the comment

[@RSS18]

Thank you. 🙂

[@PhysicalChemistry]

You're welcome

[@yigitcanbaysal824]

Professor I have a question here. Is Carnot cycle totally isothermal? I mean there are two isothermal processes, which are isothermal compression and isothermal expansion. Others are adiabatic compression and adiabatic expansion. These adiabatics are also isothermal, at the same time ?

[@PhysicalChemistry]

No, definitely not. The adiabatic steps are not isothermal. The temperature decreases during an adiabatic expansion, and increases during an adiabatic compression. Notice also that the adiabatic steps on the PV graph move the system from one temperature (one isotherm) to another.

[@bongumachelfred]

what app are you using to teach

[@neetchemistry5387]

👌👌👌👌

[@projectester8112]

You proved the equation V2/V3 = V1/V4 for adiabatic processes but you apply it to the equation for the isothermal process. Are you allowed to do that?

[@user-kr5mg6sf6n]

sir simple isothermal process also converts heat into work so what benefit do we get by such multistep process if instead, we can just expand or compress a gas isothermally because in the end the carnot cycle is also equivalent to an isothermal process

[@PhysicalChemistry]

Yes, you're right, an isothermal expansion also converts heat into work. But to make an engine, with repeated cycles of heat -> work conversion, you have to get back to your starting point. If you just isothermally compress back to your starting point, it costs you (at least) as much work to compress as you gained from the expansion. The purpose of the adiabatic steps in the Carnot cycle are to get to a lower temperature so that the isothermal compression won't require as much work, and then to heat back up to the starting point. There are certainly other choices other than adiabatic expansion that could be used instead. The Carnot cycle is just the name we give to the cycle that uses isothermal + adiabatic volume changes. A Stirling cycle uses isochoric (constant-volume) steps to do the heating/cooling. Engineers learn about Otto, and Brayton, and Diesel, and other types of engine cycles. But physical chemists usually just care about the general concept of a heat engine, and Carnot is just the specific type that usually gets taught. The Carnot cycle was important, historically, because it allowed some thermodynamic proofs of the efficiency of heat engines. That's probably the main reason it is still taught as the first example of a heat engine. It's also helpful at this point in the course to review / reinforce the features of adiabatic volume changes.

[@zulqarnainchaughtai]

In step two, which is adiabatic expansion, why we take heat capacity at constant volume in "del U" , although volume changes during that process?

[@PhysicalChemistry]

The internal energy of an ideal gas depends only on its temperature, not on its volume. So the internal energy change is ΔU = n C_V ΔT, even when the volume is changing.

[@zulqarnainchaughtai]

@@PhysicalChemistry thanks

[@gelomelo3426]

@@PhysicalChemistry sir thank you for answering his question, bc it was exactly the same question for me.🌹❤ I got the answer. 👌👌💚💚

[@RealMonkeyDKirby]

Wait...yeah how is it Cv is there is work done and a dv?

[@PhysicalChemistry]

Yes, this constant-V subscript often causes unnecessary confusion (including for me, momentarily, in this video, as you noticed). For this ideal gas, U is proportional to T. Perhaps U = 3/2 nRT or U = 5/2 nRT or whatever. Generally speaking, U = n C_V T. Note that there is no volume dependence in this equation. So it turns out that, even when the volume is not constant, ΔU = n C_V ΔT.

[@rashakhaleel4337]

Thank you so much,, is there alesson about pi t

[@PhysicalChemistry]

I'm not sure what you mean by "pi t", sorry

[@rashakhaleel4337]

@@PhysicalChemistry I think it is mean intrnal energy for non ideal gas Thats called , idont know how to type pi t in my keyboard ..

[@PhysicalChemistry]

@@rashakhaleel4337 I see. You mean the internal pressure, Π_T. I don't use that term, so I didn't recognize it right away, sorry. The internal pressure is (∂U/∂V)_T. I derive a thermodynamic equation for this derivative as an example in the video on the change of constraint rule: kzbin.info/www/bejne/rXesYo2hqdqEm68 . But I don't have a video that goes into any depth on the meaning or significance of the internal pressure for non-ideal gases.

[@ProfFeinman]

This is not an explanation. To do that, it might be good to have a picture of a Carnot device.