Question 18

A point charge +10 μC is a distance 5 cm directly above the centre of a square of side 10 cm, as shown in Fig. 1.34. What is the magnitude of the electric flux through the square? (Hint: Think of the square as one face of a cube with edge 10 cm.)


The square can be considered as one face of a cube of edge 10 cm with a centre where charge q is placed. According to Gauss’s theorem for a cube, total electric flux is through all its six faces.

Hence, electric flux through one face of the cube i.e., through the square,

Where, ∈0 = Permittivity of free space

= 8.854 × 10−12 N−1C2 m−2 q = 10 μC = 10 × 10−6 C

= 1.88 × 105 N m2 C−1

Therefore, electric flux through the square is 1.88 × 105 N m2 C−1.

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