# Homework Help: Radiation RMS

1. Apr 13, 2010

### huntingrdr

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

At a distance of 25m from an isotropic 100 MHz radio source the intensity is 120W/m2.

A) What is the rms electric field at 25m?

The answer should be 213 V/m.

B) Over a 2 hour period, what is the momentum given to a 4cm x 4cm surface (at the 25m distance) perpendicular to the source that reflects 3/4 of the energy?

The answer should be 8.06*10^-6.

C) What is the total power transmitted by the source?

The answer should be 9.42*10^5.

2. Relevant equations

I=P/a = E^2max/2(u)(c)
3. The attempt at a solution

Emax = 274587.4
Erms=Emax/sqrt(2)

I can not get any of these parts right. I tried them but can't get the same results. Can someone show me how to get these? Thanks for the help!

2. Apr 13, 2010

### tiny-tim

Hi huntingrdr!

(have a mu: µ and an epsilon: ε and a square-root: √ and try using the X2 tag just above the Reply box )
You mean I=P/a = E2max/2(µ)(c)?

No, you're confusing electricity and magnetism

it's I = P/A = ε0E2max/2c …

see http://en.wikipedia.org/wiki/Energy_density#Energy_density_of_electric_and_magnetic_fields"

Last edited by a moderator: Apr 25, 2017
3. Apr 13, 2010

### huntingrdr

I figured out parts a &b. Can anyone help me with c?