Question

A ball of radius `R=50 cm` is located in a non`-` magnetic medium with permittivity ` epsilon=4.0 `. In that medium a plane electromagnetic wave propagates, the strength amplitude of whose electri component is equal to `E_(m) = 200 V //m`. What amount of energy reaches the ball during a time interval `t=1.0 `min ?

Answer 1

For the Poynting vector we can derive as
`lt S gt = (1)/(2) sqrt((epsilon epsilon_(0))/(mu_(0))) E_(m)^(2)` along the direction of propagation.
Hence in time `t`(which is much longer than the time period `I` of the wave, the enegry reaching the ball is
`piR^(2) xx (1)/(2) sqrt((epsilon epsilon_(0))/(mu_(0))) E_(m)^(2)xxt = 5 kJ`.