# If capacitor is changed from 0 to 20 v using constant current. Variation in energy U of capacitor with time t will be

At any instant,
$q= q_0 (1-e^{-t/RC})$
and $V= V_0 (1-e^{t/RC})$
$U= \large\frac{1}{2} $$CV^2 \quad=> \large\frac{1}{2} q_0$$ V_0^2 (1-e^{-t/RC})^2$
Hence U varies exponentially with time.
Hence A is the correct answer.
edited Apr 19, 2016 by meena.p