Q.9
Write the mathematical equation of first law of thermodynamics for following processes : (a) Isochoric process (b) Adiabatic process
Best answer
Answer: (a) \(\Delta U = Q_v\) (b) \(\Delta U = W\)
First law: \(\Delta U = Q + W\), with \(W = -P_{\text{ext}}\Delta V\).
(a) Isochoric process (\(\Delta V = 0\)): \(W = 0\), so \(\Delta U = Q_v\). The heat absorbed at constant volume is all used to increase the internal energy.
(b) Adiabatic process (\(Q = 0\)): \(\Delta U = W\). Work done on the system increases its internal energy; in an adiabatic expansion the system does work at the cost of its internal energy (\(\Delta U\) negative).
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