A microphone of cross sectional area `0.80 cm^2`is placed in front of a small speaker emitting 3.0 W of sound output. If the distance between the microphone and the speaker is 2.0 m how much energy falls on the microphone in 5.0 s?

Explanation

The energy emitted by the speaker in one second is3.0 J. Let us consider a sphere of radius 2.0 m centred at the speaker. The energy 3.0 J falls fals normaly on the total surface of this sphere in one second. The energy falling on the area `0.8 cm^2` of the micorphone in one second.
`=(0.8cm^2)/(4pi(2.0m)^2)xx3.0J=4.8xx10^-6J`
The energy falling on the micorphone is 5.0 is
`4.8xx10^-6Jxx5=24muJ`.