1.
P134 Homework 31 (#10) Problem 31.48 10 of 10 In reading the instruction manual that came
with your garagedoor opener, you see that the transmitter unit in your car produces a 250 mW
350 MHz signal and that the receiver unit is supposed to respond to a radio wave at this
frequency if the electric field amplitude exceeds 0.10 V/m You wonder if this is really true. To
find out, you put fresh batteries in the transmitter and start walking away from your garage while
opening and closing the door. Your garage door finally fails to respond when you're 42 m away
Part A What is the electric field amplitude at the receiver when it first fails? Express your answer
with the appropriate units Value SubmitPre vious Answers Request Answer X Incorrect, One
attempt remaining; Try Again
Solution
(A) I = P / (4 pi r^2)
I = (250 x 10^-3 W) / (4 pi (42 m)^2)
I = 1.128 x 10^-5 W/m^2
and I = Eo^2 / 2 u0 c
1.128 x 10^-5 = Eo^2 / (2 x 4pi x 10^-7 x 3 x 10^8)
Eo = 0.0922 V/m