An alternating current I = I_0 cos(ω t) 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?
(b) Assuming that the E-field is zero outside of the conducting tube find E(s,t).
(s: radial coordinate in cylindrical coordinates, distance from inner wire)
(c) Find the displacement current density
(d) Find the total displacement current Id
(e) Compare the ratio of Id and I. If a=1mm, how high would the frequency ω have to be for Id to be 1% of I?