# Homework Help: Determing Energy Per Second

1. Dec 15, 2008

### Cheezay

1. The problem statement, all variables and given/known data
The radius of a typical human eardrum is about 4.10 mm. Calculate the energy per second received by an eardrum when it listens to sound that is at the threshold of hearing, assumed to be 1.04E-12 W/m^2.

2. Relevant equations
I=P/A

3. The attempt at a solution
First i start by converting 4.10mm to m, for which i get .0041m.

The rest seems simple:
Intensity= 1.04E-12 W/m^2
Area= 4(pi)(r^2)

plugging in my numbers i get 1.04E-12 W/m^2= P/.000211

Then I solve for P, because power=Energy per time and is expressed in W.
This problem seems like it should be simple. I really have no idea where i am making the mistake, any help would be greatly appreciated.

2. Dec 15, 2008

### cepheid

Staff Emeritus
I'm assuming the eardrum is roughly circular. Hence:

$$A = \pi r^2$$

3. Dec 15, 2008

### Cheezay

Thanks for the help, that's what i was doing wrong :)