Homework Help: Power and Intensity of Radio Waves

1. Oct 28, 2007

y_chen

1. The problem statement, all variables and given/known data

An AM radio station broadcasts with a power of 21.0 kW at a frequency of 910 kHz. Estimate the intensity of the radio wave at a point 18.0 km from the broadcast antenna.

2. Relevant equations

I= p/A I1/I2=r2^2/r1^2 w=2pi f

3. The attempt at a solution

From the given frequency, we can find omega (w). However, I am stuck on where to proceed afterwards. How would you find A? Thanks so much for helping.

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Last edited by a moderator: Mar 10, 2009
2. Oct 28, 2007

GTrax

I cannot quite decode the relevant equation. Maybe its a fonts thing.
w (omega) = 2*pi*f is just expressing frequency in radians per second instead of cycles per second.

In free space, a radio wave intensity decreases as the area of the wavefront expanding sphere increases. In Earth atmosphere, it is also frequency-dependent because of propagation and absorption losses, ground losses, and ionospheric conditions. Not that we need be concerned..because
You have a given expression for the intensity, so we dodge all that hard stuff.

I can see $$I =\frac{p}{A}$$ which makes sense. Not sure what the rest (I1/I2) is set equal to $$r_2^2/r_1^2$$

I am unclear how $$\omega=2\pi f$$ while true, actually affects anything here.

Sorry I don't quite get it yet. Have another try about what this intensity expression is.

3. Mar 10, 2009

flxwcc

just use the the amount of power and the distance. you dont need frequency.

I = P/(4*pi*r^2)