# Homework Help: EM waves -- Energy Calculation

1. May 22, 2017

### Arman777

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

2. Relevant equations

3. The attempt at a solution
I didnt understand much the question. Should I use, $υ=εE^2$ ? Then I ll take rms value of that E.But I am not sure that is that the max value.Is it max value ? I didnt understand part (B)

2. May 22, 2017

### kuruman

3. May 22, 2017

### Arman777

we didnt see this subject yet..So I am trying to solve the question using just book , and I didnt quite understand.We have energy density and we need to find rms value of E.I wrote like $u=εE^2$ cause it also contains the energy of B-field.So this value is actually used for a instant time.But I think I can take is as a max value of E then $E_{rms}=E_{max}/(\sqrt 2)$

I didnt understand part b cause I didnt understand what the question wants from us.

4. May 22, 2017

### kuruman

First, you missed the factor of $\frac{1}{2}$ in the energy density.
Part (b) asks you to find distance $r$ from a 7.5 kW radio transmitter so that the energy density in a sphere of radius $r$ has the value 4×10-14 J/m3.

5. May 22, 2017

### Arman777

I solved thanks