Answer
(a) $5.0\times 10^{-17}W$
(b) $5.0\times 10^{-5}W$
Work Step by Step
(a) The area of an eardrum is given as
$A=\pi r^2=\pi(4.0\times 10^{-3})^2=5.0\times 10^{-3}m^2$
Now we can find the energy per second at the threshold of hearing
$P=(10^{-12}\frac{W}{m^2})(5.0\times 10^{-3}m^2)=5.0\times 10^{-17}W$
(b) The energy per second received at the threshold of pain is given as
$P=(1\frac{W}{m^2})(5.0\times 10^{-5}m^2)=5.0\times 10^{-5}W$
