What is the Charge of Two Suspended Pith Balls at Equilibrium?

[bnwchbammer]

Homework Statement

The two pith balls below each have a mass of 4.9 g and equal charge. One pith ball is suspended by an insulating thread. The other is brought to x = 3.5 cm from the suspended ball. The suspended ball is now hanging with the thread forming an angle of 30.0° with the vertical. The ball is in equilibrium with FE, Fg, and FT. Calculate each of the following.

Fg (got that)

Fe (Havent gotten that)

Charge (Have no real clue how to get that)

Homework Equations

Fe=(K(qa)(qb))/r^2

The Attempt at a Solution

I'm assuming I'd be able to use Fe=(K(qa)(qb))/r^2 except I don't know how to find the charges.

Also, as an unrelated question, how do I find Voltage with the electric field and distance in a parallel plate?

 

[LowlyPion]

Welcome to PF.

Note that the charges are equal. Though there is no indication that they are + or -.

To determine the charge you need to determine the |Fe| from your force diagram. You know the weight and the angle the string is at, so you can figure the Tension and the Fe.

Then solve for |q|, because you don't know the sign of the charge - only that they repel.

 

[nlightner]

I would first clarify the context of this problem. It appears to be a problem in electrostatics, specifically dealing with the Coulomb's Law and equilibrium of charged objects. The two suspended pith balls are held in equilibrium by the balance of three forces: the gravitational force (Fg), the electrostatic force (Fe), and the tension force of the thread (FT).

To find the electrostatic force, we can use the Coulomb's Law equation, Fe = (Kqaqb)/r^2, where K is the Coulomb's constant, qa and qb are the charges of the two pith balls, and r is the distance between them. However, as you mentioned, we do not have enough information to find the charges. We would need to know either the value of K or the distance between the two balls to solve for the charges.

To find the voltage between two parallel plates, we can use the equation V = Ed, where V is the voltage, E is the electric field, and d is the distance between the plates. This equation assumes that the electric field is uniform between the plates. If the electric field is not uniform, we would need to use the integral form of the equation, V = -∫E•dl, where E is the electric field and dl is the differential length along the path between the two plates.

In summary, to solve for the charges of the suspended pith balls, we would need more information. And to find the voltage between two parallel plates, we can use the equation V = Ed, assuming a uniform electric field.