Careful measurement of the electric field at the surface of a black box indicates that the net outward flux through the surface of the box is `8.0xx10^(3)"N-m"^(2)C^(-1)`.
What is the net charge inside the box ?

Explanation

Here, `phi=8.0xx10^(3)" N-m"^(2)C^(-1)`
Suppose that the net charge inside the box is q, then according to Gauss. theorem,
`phi=(q)/(epsilon_(0))` or `q=epsilon_(0)phi`
`because" "epsilon_(0)=8.854xx10^(-12)C^(2)N^(-1)m^(-1)`
`:." "q=8.854xx10^(-12)xx8.0xx10^(3)`
`=70.832xx10^(-9)C`
`rArr" "q=0.07muC`.