Finding the Electric Field Needed to Suspend a Charged Bead

[rockhao]

1. Homework Statement
A 5.0×10^−2g plastic bead is charged by the addition of 1.0×10^10 excess electrons. What electric field (strength) will cause the bead to hang suspended in the air?


2. Homework Equations



3. The Attempt at a Solution
It has something to do with F = Eq but i don't know where to go about there

 

[LowlyPion]

Welcome to PF.

Let's see.

You figure F = E*q.

And if you draw a force diagram, what does E*q need to balance with?

They give you the mass ... hmm, what could you do with that?

 

[rowkem]

So..

F=Eq

E=F/q - we know q but we need F.

For it to hang suspended, there needs to be no net force acting on the bead. The electrostatic force is one of two forces acting on the bead; as well as being the F in our equation. So...this "other force" needs to be equal to the electrostatic force.

Find the magnitude of this other force acting on the bead (the mass of the bead will come in handy...hint hint). As said, this force will be equal to the electrostatic force. Plug that into the above equation and solve for E. Good luck!

 

[rockhao]

i'm assuming F= ma which leads to F = Mg but how does work into the equation

so mg = E*q

E = 4.9e-11 ?