Question

Two metal plates having a potential difference of $800V$ are $2cm$ apart. It is found that a particle of mass $1.96\times {10}^{-15}Kg$ remain suspended in the region between the plates. The charge on the particle must be:

• A. $3e$
• B. $4e$
• C. $6e$
• D. $8e$

Answer 1

The correct option is A $3e$

Electric field between plates is given as,

$E=V/d$

$E=800/0.02=4000v/m$

As the charge particle suspended in equilibrium position then weight of the charge particle must be balanced by the electric force.

$mg=qE$

$q=\frac{mg}{E}$

$q=4.7\times {10}^{-19}C$

Elementary charge $e=1.6\times {10}^{-19}$

$\frac{q}{e}=2.94\approx 3$

$q=3e$