# Reply To: Thermal & Fluid Mechanics Queries…

All About Education Forums Mechanical Engineering Thermal & Fluid Mechanics Queries… Reply To: Thermal & Fluid Mechanics Queries…

1. @Ritesh,
According to PK Nag,
“TdS= dU+pdV this equation can be applied in all the processes undergone by closed system” is true provided that there is no other forms of major energies (like KE, PE, viscuos, Electical) involved in the process.
You will get the answer after you can understands how actually this equation comes.
The original equation is,
dQ-dW=dE …………1 … (from First law of thermodynamics)
where dQ=small change in heat
dW=small change in work
dE=small change in total energy
now, dE=dU+dKE+dPE+dE+…..
where, dU,dKE,dPE,dE are small change in internal energy, K.E., P.E., Electrical energy resp.
we no for many processes we can neglect dKE,dPE&dE & other minor energies.
Also we know dS=dQ/T
& dW=pdV…. this is the work done on the system or by the system
so equ 1 becomes,
TdS= dU+pdV

So the term “pdV” represents the work done on the system or by the system & it have no mean whether the process is reversible or irreversible.

Hope you understands it. 🙂