A circular plate of area `0.4 cm^(2)` is kept at distance of `2m` source of power `piW`. Find the amount of energy received by plate in `5` secs.

Explanation

The energy emitted by the speaker in one second is `pi J`. Let us consider a sphere of radius `2.0 m` centered at the speaker. The energy `pi J` falls normally on the total surface of this sphere in one second. The energy fallinf on the area `0.4 cm^(2)` of the microphone of the second
`= (0.4 xx 10^(-4))/(4 pi 2^(2)) xx pi = 2.5 xx 10^(-6)J`
The energy falling on the microphone in one second
`= 2.5 xx 10^(-6)J xx 5 = 12.5 muJ`.