Chapter 21, Problem 79PP

Coaxial cables are widely used with audio-visual technology, electronic instrumentation, and radio broadcasting, because they minimize interference with or from signals traveling on the cable. Coaxial cables consist of a wire inner conductor surrounded by a thin cylindrical conducting shield, usually of braided copper (Fig. 21.39). Flexible insulation separates the conductors. A straight length of coaxial cable can be approximated as an infinitely long wire surrounded by a cylindrical shell. Normally the two conductors carry charges of equal magnitude but opposite sign. (Charge actually varies with lime and position as signals travel down the cable, but for these problems consider the charge to be fixed and spread uniformly.)

FIGURE 21.39 A coaxial cable (Passage Problems 76-79)

  79. A coaxial cable in electrostatic equilibrium carries charge –Q on its inner conductor and +Q on its shield. If the charge on the shield only is doubled.

1. a. the magnitude of the electric field between the conductors will double.
2. b. the magnitude of the electric field outside the shield will double.
3. c. the magnitude of the electric field at the outer surface of the shield will become twice the magnitude of the field at the shield's inner surface.
4. d. the magnitude of the electric field at the outer surface of the shield will equal the magnitude of the field at the shield's inner surface.