Response of a DC RC circuit in series embedded with a rotating varco

Response in electric potensial $U$ from a DC RC circuit in series has been obtained by varying the capacitance of the capacitor $C$. Variation of the capacitance begins at $t_0>0$. Alternating response is observed when the periode of change of capacitance in the same order as $\tau =RC$, which is held constant for $t<t_0$ and varied in triangle form for $t>t_0$.