28.65 . CALC A long, straight wire with a circular cross section of radius R carries a current I. Assume that the current density is not constant across the cross section of the wire, but rather varies as J = ar, where a is a constant. (a) By the requirement that J integrated over the cross section of the wire gives the total current /, calculate the constant a in terms of I and R. (b) Use Ampere's law to calculate the magnetic field B(r) for (i) r