21. If the electrostatic potential were Igiven by $$\phi = {\phi _0}$$ (x2 + y2 + z2), where $${\phi _0}$$ is constant, then the charge density giving rise to the above potential would be
22. Consider an infinitely long straight cylindrical conductor of radius R with its axis along the Z-direction, which carries a current of 1A uniformly distributed over its cross-section. Which of the following statements is correct?
(where, r is the radial distance from the axis of the cylinder)
(where, r is the radial distance from the axis of the cylinder)