JEE Main · 2005 · 08 May 2005 · Offline · Alternating Current

A phase difference of exactly $\frac{\pi }{2}$ needs a purely reactive circuit: $L$ alone makes the current lag by $90^{\circ }$, $C$ alone makes it lead by $90^{\circ }$, and an ideal $L$-$C$ pair also gives $90^{\circ }$. An $R$-$L$ circuit always has resistance supplying an in-phase part, so its phase angle lies strictly between $0$ and $\frac{\pi }{2}$ and can never be $90^{\circ }$; it is the combination that cannot give the stated phase difference.