a point charge q is located on axis of disc of radius r at distance b from the plane of disc.if one-fourth of electric flux from charge passes through disc then.......
(a)r = b (b)r = 2b


ans:(a)

Dear shefali

now lets Draw a sphere whose center is at a distance b from the disk and which cover the disk

we know that flux throug a segment which makes an angel  θ at the center is

Φ=Q/2ε (1-cosθ)

where cosθ = b/(b²+r²)^1/2

given flux passes in 1/4 th

so Q/2ε (1-cosθ)  =Q/4ε

    1-cosθ = 1/2

     cosθ =1/2

  b/(b²+r²)^1/2 =1/2

  r =√3 b

This question is also in Resnick Halliday problem number 64 chapter 24 .and the answer given is r=√3 b

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