Q.

A disc of radius a/4 having a uniformly distributed charge 6C is placed in the x-y plane with its centre at (-a/2,0,0). A rod of length a carrying a uniformly distributed charge 8C is placed on the x-axis from x=a/4 to x=5a/4. Two point charges -7C and 3C are placed at (a/4,-a/4,0) and (-3a/4,3a/4,0), respectively. Consider a cubical surface formed by six surfaces x=±a2, y=±a2, z=±a2. The electric flux through this cubical surface is             [2009]

1 -2Cε0  
2 2Cε0  
3 10Cε0  
4 12Cε0  

Ans.

(1)

From the figure, total charge enclosed in the cubical surface is qin=3C+2C-7C=-2C

According to Gauss's theorem, the electric flux through the cube is

ϕ=qinε0=-2Cε0