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A point charge q is located on the axis ...

A point charge q is located on the axis of a disc of radius R at a distance b from the plane of the disc as shown in figure 1.295 What should be the radius of the disc if one fourth of the total electric flux form the charge passes through the disc.

A

`sqrt(3)b`

B

`b//sqrt(3)`

C

`b//sqrt(2)`

D

`sqrt(2)b`

Text Solution

Verified by Experts

The correct Answer is:
A
Flux of charge .Q. passing through the disc will be
`phi=(Q)/(2epsi_(0))(1-costheta)" "......(1)`
Total flux of charge .Q. `=(Q)/(epsi_(0))` and `phi=(1)/(4)((Q)/(epsi_(0)))" "......(2)`
From (1) and (2) `(Q)/(2epsi_(0))(1-costheta)=(1)/(4)(Q)/(epsi_(0))`
`impliescostheta(1)/(2)implies(b)/(sqrt(R^(2)+b^2))=(1)/(2)`
`implies4b^(2)=R^(2)+b^(2)impliessqrt3b=R` is option (A)
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