Question

Figure shows a charge $q$ placed at the centre of a hemisphere. A second charge $Q$ is placed at one of the positions A,B,C and D. In which position(s) of this second charge, the flux of the electric field through the hemisphere remains unchanged?

• A. A
• B. C
• C. B
• D. D

Answer 1

Answer: (A), (B)

$A$ and $C$ are correct. For flux, there is no effect when incoming lines of forces have way to go out from a closed surface i.e outgoing lines of forces are present . Here, when we draw lines of force from $A$ and $C$ they will intersect the curve at two points i.e. incoming & outgoing lines of forces are present i.e. no effect on flux ultimately. while when we draw line from $B$ and $D$ they will intersect curve at one point i.e. incoming lines of forces are present but outgoing lines of forces are absent means it affects flux.