Question

Hi, I do not quite understand the first law of thermodynamics. My books says that in thermodynamics you consider the mechanical energy as constant. Therefore only the internal energy can vary and thus : dU = dQ + dW (simplified). But I don't get how this means that the total energy in the system remains constant, because if dU is anything different from 0, there's a change in total energy, isn't there? I must be having a wrong view on this, could anyone help me out please? Thanks!

Answer 1

we know energy can neither be created nor be destroyed.
It can be transformed from one form to another

Answer 2

Since energy keeps changing its form it remains constant

Answer 3

dQ is heat added. That heat either causes work W to be done by system or increases its internal energy U.