An alternating current I = I₀ cos (wt) flows down a long straight wire, and returns along a coaxial conducting tube of radius a. 

(a) In what direction does the induced electric field point (radial, circumferential, or longitudinal)? 

(b) Assuming that the field goes to zero as s → ∞, find E(s, t). [Incidentally, this is not at all the way electric fields actually behave in coaxial cables, for reasons suggested in footnote 10.