Problem 4.209

A source of ac voltage $V=V_{0} \cos \omega t$ delivers energy to a consumer by means of a long straight coaxial cable with negligible active resistance. The current in the circuit varies as $I=$ $\left.=I_{0} \cos \omega t-\varphi\right) .$ Find the time-averaged energy flux through the cross-section of the cable. The sheath is thin.