15. (5 %, 本題計算過程須計分) Fig. 7 shows a very simple RC circuit with a resistor \( R \) ohms, a capacitor \( C \) farads, and a battery of \( V \) volts. Prove that the current \( I(t) \) after switch is on (\( t = 0^+ \)) is \( I(t) = (V / R)e^{-t/(RC)} \). Your proof:__ (15)__