A circular loop of wire of radius $r$ is in a uniform magnetic field, with the plane of the loop perpendicular to the direction of the field (Fig. P31.61). The magnetic field varies with time according to $B(t)=a+b t$, where $a$ and $b$ are constants. (a) Calculate the magnetic flux through the loop at $t=0 .$ (b) Calculate the emf induced in the loop. (c) If the resistance of the loop is $R$, what is the induced current? (d) At what rate is electrical energy being delivered to the resistance of the loop?