\(P^2 V=K\)
or \(P V^{-2}=K\)
\(N=-2\)
\(C=C_V+\frac{R}{1-N}=\) positive quantity
\(C > 0\)
\(W > 0 \quad\) [gas is expanding]
\(\because P V^{-2}=K\) so \(T V^{-3}=\) constant
\(\Rightarrow T\) will increases if \(V\) increases.
\(\Rightarrow \Delta T > 0\)
So \(\Delta U=\eta C \Delta T > 0\)
It will increase continuously.