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Questions 17-21 A long coaxial cable consists of two concentric thin walled cylinderical shells with radii a and b. Their length is l. The inner and outer cylinders carry equal currents I in opposite directions as shown in the figure.(r is the distance from the axis of the cylinder) 17. Find the magnetic field for r <a (inside the inner cylinder). (a) B = μοΙ0/2π(6? - a?) (b) Β = μο1/2πα (c) B = μο1/2πη2 (d) Β = μοΙ/4πι (e) B=0 b 18. Find the magnetic field for a <r<b (between the cylinders). (a) B = μοΙ/2πη (b) Β = μοΙ /2πη2 (c) B = μοΙ /2πα (d) B = MOI/47 (e) B = MOI/21 (b-a) 19. Calculate the energy density in the region between the cylinders (a <r<b). (a) HO12/87²m2 (b) 12/47²(a+b)HO (c) 1/2mol(a+b) (d) 12/4n²r2H0 (e) (H012/47² l(62 – a)) In (b/a) 20. What is the flux between the cylinders in a section of this system of length l? (a) 0 = MOICA (62 - a?)/(a + b) (b) 0 = 2 ICT (b + a) (c) 0 = (voll/27) In (b/a) (d) 0 = (uola/27) In (6/a) (e) 0 = (uoI/T) In (6/a) 21. What is the inductance of the cable? (a) L = (Hol/4nl) In (b/a) (b) L = 240I (62 - a) (c) L = (Hol/27) In (b/a) (d) L = MOIT (62 – a)/(a + b) (e) L = (Mol/l) ln (b/a)