D11.7. A steel pipe is constructed of a material for which μ, = 180 and σ = 4 × 106 S/m. The two radii are 5 and 7 mm, and the length is 75 m. If the total current I(t) carried by the pipe is 8 cos cot A, where w = 1200л rad/s, find: (a) the skin depth; (b) the effective resistance; (c) the dc resistance; (d) the time-average power loss. Ans. 0.766 mm; 0.557 2; 0.249 2; 17.82 W