Question

1. A coaxial cable consists of a copper tube of radius 6.00 mm (rc ) surrounded by a thin jacket of polystyrene (with dielectric constant Kp) with an outer radius of 6.20 mm (rp) which is in turn surrounded by a thin steel mesh.

a. Derive the formula for the capacitance per meter between the copper core and the steel mesh of this cable in terms of rc , rp , Kp and physical constants. Because of the small thickness of the polystyrene, assume that when the capacitor is charged, E will be fairly constant between the copper and steel.

b. What is the value of the capacitance between the inner copper core and the steel mesh for each meter of cable?

2. You’re trapped on a Pacific atoll and you need to construct a radio out of the trash that has washed up on the shore. The resonant circuit in your radio requires a capacitor with a capacitance of 10 pF. You find a bunch of aluminum foil and plenty of dried out newspapers. Show how you would construct the capacitor including all your estimates and assumptions. Be sure to specify all dimensions and show all calculations.

Answer 1

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