Physics 32.7 Thermodynamic Potentials (8 of 25) What is Enthalpy? Another Def.
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- Опубликовано: 4 окт 2024
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In this video graphically explain what is enthalpy where total heat content (enthalpy)=total internal energy+total work done by the system.
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• Physics 32.7 Thermody...
Love the way you disect things down to the core. I like to think of the PV-term in h = u + PV, as the potential energy of the matter (usually gas when discussing enthalpy, but in this case a solid) with respect to a pressure of zero, i.e, PV=0. Since the ice block is at P=1atm, then it has PV energy as compared to at no pressure. This also explains why enthalpy is always higher than internal energy at the same temperature - enthalpy is baselined to zero pressure, whereas internal energy presumes can not extract any additional energy by expanding to a lower pressure. Do you agree?
You stated a lot of things. What are you looking to find agreement to?
@@MichelvanBiezen So, I've been trying to clearly understand the difference between internal energy (U) and enthalpy (H). I know that h = u + PV, so the difference is the PV-term. This PV term is often referred to as "pressure energy", but this never felt right to me since the kinetic energy implicit within the u term could easily also be extracted as pressure energy from the gas via an expansion. So my revelation/conclusion to which I am seeking concurrence, is that: the real difference between internal energy (U) and enthalpy (H), is that H is a measure of the energy of the gas (or solid) using zero-pressure as the zero-basis reference point.. Note that dH = (cv + R)dT, and nRT = PV, so the R term is just the added PV potential from zero-pressure; whereas U is a measure of the energy using the current pressure as the zero-basis point. Since the "current pressure" can change, U does not allow for proper tracking of the relative energy of the gas, but since H is always referencing to zero-pressure, it allows one to track of the total energy.
When a substace is heated it gains internal energy. All things remaining the same, all the heat (Q) entering the substance is converted into internal energy. However, if the substance expands during the heating process it must do work (pushing the atmosphere away to make more room for itself). That requires energy. Enthalpy accounts for both the change in internal energy and the energy needed to push away the atmosphere.
We are in the process of making some new videos on this topic (thermodynamic potentials) to try and make that a little more understandable.
You should make it clear that the work within enthalpy is the boundary work only, it makes more sense that way. Shaft work, electrical work, paddle work all still need to be accounted for.
Thank you so much for your videos!
Your videos are so useful 🤩👍🏻
Still no definition of 'potential'. Sorry, Michael, I don't like it.
they're ALL potential, ideally we could turn 100% of these energies into work which is impossible. So, we say that these are the "potential" energy of which can never be fully realized.