Power of EM Wave Absorbed by Circular Disk of Radius 2m

[NA19]

Homework Statement

A plane polarized electromagnetic wave propagates
with Erms = 30 V/m. What is the power transmitted to
a circular disk of radius r = 2m, if all of the light is
absorbed by the disk and S is perpendicular to the
disk?

Homework Equations

There is a passage that goes along with this that states that I =Erms^2/(cμ0), and that I = P/A. u0 = 4 × 10–7 N•s^2/C^2.

The Attempt at a Solution

Basically, I plugged in the values of Erms and μ0 into the Intensity equation and then multiplied by A (pi*4) to get 94.2.

But the answer key says the answer is 30 and that instead of multiplying by pi*r^2, you multiply by r^2. Please help!

 

[Simon Bridge]

Looks like you tried to apply the formulas without understanding them.
(You also left out your units.)

See also:
http://farside.ph.utexas.edu/teaching/302l/lectures/node119.html

You could lose a pi if it cancels out someplace, or it could be a mistake.

 

[NA19]

Where is the other pi?

 

[Simon Bridge]

You'll just have to try to understand the problem to see.