A source of $e.m.f.$  $E = 15\,V$ and having negligible internal resistance is connected to a variable resistance so that the current in the circuit increases with time as $i = 1.2 t + 3$. Then, the total charge that will flow in first five second will be ............... $C$
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$i = \frac{{dQ}}{{dt}}$ $ \Rightarrow $  $dQ = idt$ $ \Rightarrow $ $Q = \int_{\,{t_1}}^{\,{t_2}} i \,dt = \int_0^5 {(1.2t + 3)\,} dt$

$ = \left[ {\frac{{1.2{t^2}}}{2} + 3t} \right]_0^5 = 30\,C$

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