Question

An oil drop of mass 1.0mg and having a charge of 2.0 x 10^-6 is suspended in a box. What is the magnitude of the electric field in the box?

Answer 1

Mass of oil drop, $m=1\space mg=1\times10^{-6}\space kg$

Charge, $q=2\times10^{-6}\space C$

$F=qE=mg$

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

$E=\dfrac{10^{-6}\times 9.8}{2\times10^{-6}}=4.9\space N/C(upwards)$