Electric Field question?

1. Jan 26, 2010

TKstreaker13

What is the strength of an electric field that will balance the weight of a 2.5g plastic sphere that has been charged to -2.9nC ?

I know that F= ma and that E= Kq/r^2 and also that F= Kq1q2/(r^2). I just don't know how to relate an electric field equation using only mass and a charge?

2. Jan 26, 2010

Staff: Mentor

OK. What would 'a' be in this case?
Those are expressions for the field from a point charge and the force between two point charges. You won't need them here. Hint: If the field is E, and the change is q, what's the electric force on the charge?

What forces act on the plastic sphere? What's the net force?

3. Jan 26, 2010

TKstreaker13

a= acceleration

and i will need the F=qE equation

if the sphere is being balanced then the net force will be zero?

4. Jan 27, 2010

Staff: Mentor

I meant, what would be the value of the acceleration in this case.

Right. That's the one you need.

Exactly. What two forces act on the sphere?