# Electric field on two charged spheres

1. Feb 5, 2012

### Nivlac2425

1. The problem statement, all variables and given/known data
Two tiny spheres of mass m = 7.90 mg carry charges of equal magnitude, 72.0 nC, but opposite sign. They are tied to the same ceiling hook by light strings of length 0.530 m. When a horizontal uniform electric field E that is directed to the left is turned on, the spheres hang at rest with the angle θ between the strings equal to 50.0 degrees in the following figure.
What is the magnitude of E?

2. Relevant equations
qE = F

3. The attempt at a solution
I know that I need the total force acting on a sphere in order to calculate the electric field strength. I just can't seem to solve for that force.
Does the attraction force of the oppositely charged spheres need to be accounted for as well? Or is the angle separation only caused by the electric field force?
Thanks for helping me out here!

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Last edited: Feb 5, 2012
2. Feb 5, 2012

### Mindscrape

Yes, the attraction force needs to be accounted for. What will the extra electric force do? Well, actually, even for myself to answer that question I'd need to know which charge is on the left/right so hopefully you're given that.

3. Feb 5, 2012

### Nivlac2425

The charge on the left is positive, and the charge on the right is negative. We know this because the convention for my class is that electric fields come outward from positive charges.

Ok, so since there is an attractive force, then at the equilibrium shown, would it just be ƩF= $F_{E}$ - $F_{q}$ ?
Or: force due to electric field - force due to attraction = the total force causing the angle ?

4. Feb 5, 2012

### Mindscrape

Oh, yeah, duh, the electric field has to be repelling because the coulomb force is attractive in this case, which means positive on left and negative on right.. I was just checking to make sure you were paying attention. <_< >_>

You're getting there. You're missing another important force vector. What is the vector that the electric forces are giving? What else contributes to making the angle?

5. Feb 17, 2012