Which of the following is always valid? choose only one answer.
1. w = - p ∫ dv, for the calculation of work in a mechanically-reversible piston-cylinder arrangement
2. for any fluid undergoing a change in pressure, cp = dh/dt.
3. δh = δu + δ(pv)
4. δu = q + w
5. δh = δu + p.δv

4. ΔU = Q + W

Explanation:

Equation 4 corresponds to the definition of internal energy of a substance as part of a system. This equation is always valid and is fulfilled in all types of systems and refers to the known phrase:

"energy is neither created nor destroyed, transformed"First Law of Thermodynamics

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