Question

An electron is placed midway between two parallel horizontal conducting plates 2.0 cm apart in air. What must be

the potential difference between the plates for the electrons to remain stationary? What

must be the sign of the charges in each plate?

Answer 1

The equilibrium condition

$$F_g=F_e\\ mg=qE\\ mg=qV/d$$

Hence, the potential difference

$$V=\frac{mgd}{q}\\ =\frac{9.1*10^{-31}*9.8*0.02}{1.6*10^{-19}}=1.11*10^{-12}\:\rm V$$

The top plate should be positive.