Between two horizontal parallel plates, uniform electric field `E` directed vertically downward. A charged particle `(q,m)` remains in equilibrium in vaccum between the plates. Find shortage/excess of electrons in the charged particle .

Explanation

Since particle is in equilibrium i.e. net force on it is zero. The weight of particle is balanced by electric force. Since electric force is in upward direction , hence charge `q` is negative i.e. excess of electrons
`qE = mg rArr q = (mg)/(E )`
`n e = (mg)/(E ) rArr n = (mg)/(e E)`
where `n`: number of electrons in excess.