Consider a uniform electric field `E=3xx10^(3)hat(i)` N/C.
What is the flux of this field through a square of 10 cm on side whose plane is parallel to the YZ-plane ?

Explanation

Here, `E=3xx10^(3)hat(i)` N/C, i.e. field is along positive direction of X-axis.
Surface area, `S=(10" cm")^(2)=10^(-2)m^(2)`
When plane is parallel to YZ-plane, then `theta=0^(@)`.
`:.phi_(E)=EScos theta=3xx10^(3)xx10^(-2)cos0^(@)=30" N-m"^(2)//C`