1) The current density varies with radial distance r as J = ar2, in the cylindrical wire of radius R.The current passing through the wire between radial distance R/3 and R/2 is A) 65πaR42592 B) 25πaR472 C) 65πa2R32938 D) 81πa2R4144 Answer: Option AExplanation:Given , J= a r2 i=∫21J×2πrdr=∫R/2R/3ar2×2πrdr =2πa∫R/2R/3r3dr=2πa|r44|R/2R/3 =πa2[(R2)4−(R3)4] =πaR42×6581×16= 65πaR42592