Heat and Work

The energy of a system can be changed in two ways: by changing the quantum mechanical energy levels directly, or by changing which of those energy levels are occupied. These two different mechanisms for modifying the energy are called work and heat.

[@philliph.7542]

To me physical chemistry is the most fascinating of all subjects - and I have interest in a lot of things. But the chemical description of the the world/life (biology, geology, ...) through physical models in the language of maths in order to gain a deep understanding is just pure beauty!! I am myself a teacher (maths/chemistry) and I've seen quite some good physical chemistry content over the years. But what this channel presents, outshines everything I've come across so far, it excels in: the logical progression of the series, the clarity of the explanations, the emphasis on the essence, the mastering of the blackboard and - maybe most importantly - the choice of good examples! I can't believe I'm watching these lectures for free! Thank your Prof. Stuart, you can really be proud of your work! This is truly a gift for mankind. 🙂

[@PhysicalChemistry]

Thanks so much for your generous comment. I completely agree that there is nothing like arriving at a deeper understanding of the world by viewing it through the lens of some beautiful mathematics

[@pawannaithani99]

i just can't express my gratitude towards you.... for me you are solving life long doubts of mine through your video.. Thank you very much for your efforts.

[@PhysicalChemistry]

That's wonderful to hear, thanks for taking the time to comment

[@ombie9439]

🙏Sir, I really liked your lecture..for years I have been thinking how this equation is valid as we were told to mug up this equation..and now after so many years I finally get it..Thank you

[@PhysicalChemistry]

I'm glad it eventually clicked for you. Sometimes all it takes is seeing it from another perspective.

[@maths--maniac]

Sir teachers makes fun of me & insult me whenever i ask the questions of applied mathematics in their teaching what is actual sense of it..thank you so much sir for giving me the sense of mathematics.

[@PhysicalChemistry]

Don't be discouraged! It's true that not everyone sees the value in understanding the math behind a topic. But keep going with your education and you will outgrow those who hold you back

[@nillchen]

Love it! Thank you very much for those lectures!

[@PhysicalChemistry]

You're welcome. Thanks for the comment

[@mortezakhoshbin]

Hello dear dr.stuart If the box is rigid and you agitate (shake) the box, can it be interpreted as heat or work? It doesnt change the cube length but is not also changing the temp. Can it change the nx,ny,nz values? Thank you for such amazing lectures

[@PhysicalChemistry]

Interesting question! If the box is rigid, then you're right that no PV work is done. (Quantum mechanically, the energy levels are not changed) If the agitation does increase the energy of the gas, then that energy transfer shows up as heat. It *would* change the temperature. Quantum mechanically, the molecules would occupy higher energy levels (the nx,ny,nz values would change). If the temperature doesn't change, as you assume in your question, then there is no energy transfer into the system

[@denismitov1933]

Very good and interesting explanation. I never seen explanation of heat and work in this way. Do you use any book for this explanation?

[@PhysicalChemistry]

I would recommend the discussion in the book by McQuarrie and Simon (Physical Chemistry: A Molecular Approach). There are other options as well -- primarily in books on statistical mechanics, rather than books that take a more purely thermodynamic approach.

[@luukvandenakker7838]

Wow, very cool introduction to Heat and Work :) How would one explain Work due to a voltage in a similar manner?

[@PhysicalChemistry]

Great question; I like that you're thinking of how to apply the idea to different forms of energy. The concept of work is the similar for electrostatic energy as it is for other forms of energy. If you move an electron or other charged particle across a voltage difference, you are changing its energy. We call this "work", instead of heat, because you're changing the energy of the particle.

[@user-kr5mg6sf6n]

sir the energy ' U ' dercribed by the summation of energy levels times the probablities associated wit them is the average energy per molecule or the total energy of the system

[@PhysicalChemistry]

Whichever you like, as long as you're careful to keep it consistent! If the Eᵢ are the energies of individual molecules, then U is the average energy per molecule. If the Eᵢ are the energy states of the whole system (as I intend in this video), then U is the average energy of the whole system.

[@zulqarnainchaughtai]

Which book(s) are you following for thermodynamics?

[@PhysicalChemistry]

For my own class, I use a textbook that I am writing. For published textbooks, I like the one by McQuarrie and Simon

[@koyelsarkar6295]

Sir, is there any way to physically differentiate between heat and work? Both the way, either by changing the energy level itself or by promoting molecules to a different energy level, the molecule's internal energy changes....am a bit confused about this...