A tiny particle of mass ` 4 mug ` is kept over a large horizontal sheet of charge density ` 4 xx 10 ^(-6) Cm ^(-1) ` What charge should be given to the particle so that if released it does not fall down?

Explanation

Let charge q be given to the particle. As particle close not fall, hence downward weight of particle mg = upward force due to electric field of charged plate ` q_E =q . (sigma)/( 2 in _0) `
` therefore " " q= ( 2 mg in_0)/( sigma ) `
Here , m ` mug = 5 xx 10 ^(-6) g = 5 xx 10 ^(-9) kg , sigma= 4 xx 10 ^(-6) C m^(-1) `
` therefore " " q= ( 2xx 5 xx 10 ^(-9) xx 9.81 xx 8.85 xx 10 ^(-12)) /( 4 xx 10 ^(-6)) = 2.17 xx 10 ^(-13) C . `