7. Calculate the current \( i(t) \) of series RL circuit shown in Fig. 3 if the initial current \( i(0) = 1\mathrm{A} \) and the impressed voltage is \( v_{i}(t) = V_{0}[u(t - a) - u(t - b)] \). (Assume \( V_{0} = 5\mathrm{V}, R = 2\Omega \), and \( L = 0.02\mathrm{H}, a = 2\mathrm{sec}, b = 4\mathrm{sec} \)). Roughly draw the waveform \( i(t) \). (10%)