ED II. Homework 1. (due 2/11/02)

- Show
that the
**minimum wavelength**of radio waves that can be received at a depth of 10m below the surface of the sea is of the order of 10^{5}m. The conductivity of sea water is approximately 4W^{-1}m^{-1}( the units are also called Siemens).

For an interesting reading, see this page:

http://server5550.itd.nrl.navy.mil/projects/haarp/elf.html

- A copper rod is 10m long and 6 mm in diameter. Show that its resistance at 100Hz, 1 Mz and 10 GHz is approximately 6 mW, 0.15W and 15 W respectively.

- Calculate
the total force due to the radiation pressure
on a plane which is perpendicular to the direction of the sunlight. The
plane has an area of 1 km
^{2}and the intensity of the sun at the surface of Earth is 1350 W/m^{2}.

- A
Ti.Sapphire laser has a peak intensity of 10
^{20}W/cm^{2}, calculate the peak electric field in units of MV/m. What is the electric field on an electron which is at 1 Bohr radius from the center of the hydrogen atom? The intensity of the sun at the surface of Earth is 1350 W/m^{2}. What is the total power received on the surface of Earth, and compare the number with the total power received in 1 cm^{2}of the Ti:Sapphire laser.