What is the magnitude of a the vertical electric field that will balance the weight of a plastic sphere of mass that has been charged to - 3.0 nC? g

Let E be the strength of electric field and m be the mass of plastic sphere.

charge on plastic sphere, q = - 3 n C = - 3 x 10^-9 C

The force due to electric field on the charged plastic sphere = q E

The weight of the plastic sphere = m g

where, g is the acceleration due to gravity.

Now the force due to electrostatic field is balanced by teh weight of the plastic sphere.

q E = m g

E = m g / q

E = m x 9.8 / (3 x 10^-9) = 3.26 x 10^9 x m

Substitute the value of m and get the value E.