Need help to find intensity of waves.

[aldam3a]

Homework Statement

A tiny vibrating source sends waves uniformly in all directions. An area of 3.25 cm2 on a
sphere of radius 2.50 m centered on the source receives energy at a rate of 4.20 J/s.

(a) What is the intensity of the waves at 2.50 m from the source and at 10.0 m from the
source?

(b) At what rate is energy leaving the vibrating source of the waves?



2. The attempt at a solution

I attemped the first part of the question using this equation :

I = P / A
A = 4 Pi r^2

so I = 4.2 / 4 * Pi * (2.5)^2 = 0.053

I asked my supervisor and he told me that my answer is wrong.

Any one who knows how i can go through this question ?
Thanks in advance

 

[technician]

The power of 4.2J/s is passing through an area of 3.25cm2...not the surface area of the sphere.
I think you should also give intensity as W/m^2 so you will need to convert units to do that.