U is microscopic energy; H=U+PV is convenient for flow processes.
U is the microscopic energy of molecules; H=U+PV includes flow work and is convenient for open systems and constant-pressure processes. For ideal gases both depend only on temperature: ΔU=mc_vΔT, ΔH=mc_pΔT.
Boiling 1 kg of water at 100 °C absorbs h_fg≈2257 kJ.
Enthalpy combines internal energy and flow work, so it gives the energy balance directly for flows such as steam in a pipe.
U is microscopic energy; H=U+PV is convenient for flow processes.
Enthalpy combines internal energy and flow work, so it gives the energy balance directly for flows such as steam in a pipe.
In a flow process the flow work is pressure times volume per unit mass, and enthalpy includes it automatically, so the energy balance is simpler.
Internal energy is the stored microscopic energy. Enthalpy adds the pressure-volume term, which is the energy needed to push fluid into a system.
Typical applications include finding the energy carried by steam into a turbine, calculating the heat added to a fluid at constant pressure and setting up the energy balance for a heat exchanger with flowing streams.
Study "Heat and work" first (topic 8 of 105 in Thermodynamics & Fluids). It covers the groundwork this topic builds on.
Enthalpy in steam tables. Latent heat h_fg. Reference states arbitrary.
Boiling 1 kg of water at 100 °C absorbs h_fg≈2257 kJ.
The next topic is "First law of thermodynamics", which builds directly on this one.
It is classed as Beginner in Thermodynamics & Fluids (topic 9 of 105). It is an entry-level topic. It relies only on basic school mathematics and the ideas introduced earlier in this discipline.
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