Suppose a radio signal has a 3 [N/C] E-field , pointing downward at time t=0. If

Question

Suppose a radio signal has a 3 [N/C] E-field , pointing downward at time t=0. If the tip of an old-fashioned "whip" antenna is 7 mm diameter, how much charge would be spread over that antenna tip? (antenna is a good metal conductor, so the E-field inside it is zero) { start-out: should they be positive, or negative? } { follow-up: where did they come from? what charge is there, now? } a) ?0.3253 [fC] = 3.253 x10-16 [C] b) ?1.022 [fC] = 1.022 x10-15 [C] c) ?4.088 [fC] d) +4.088 [fC] e) ?102.2 [fC] f) ?1.168 [pC] g) +1.168 [pC]

Explanation / Answer

the electric field is E = 3 N/C antenna diameter is d = 7 mm = 7 * 10^-3 m radius is r = d/2 = 3.5 * 10^-3 m let the charge on antenna tip be q we know that E = k * q/r^2 or q = E * r^2/k where k = 1/4pie_o = 9 * 10^9 Nm^2/C^2

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