Hello ,

Flux - Consider some surface in an electric field E. Let us select a small area element dA on this surface. The electric flux of the field over the area element is given by dΦE=E⋅dA .

Let the area of the square be A = l² . The square is in YZ plane and therefore parallel to it.

So for total  flux passing through an area  Φ _E = E⋅A= EAcosθ,

Therefore  flux of electric field passing through the area = l² , of a square parallel to YZ plane is : ΦYZ= (a i + b j).(A i) = aA .