(adsbygoogle = window.adsbygoogle || []).push({}); 1. The problem statement, all variables and given/known data

Two identical, small spheres of mass 2.0g are fastened to the ends of a 0.6m long light, flexible, insulating fishline. The fishline is suspended by a hook in the ceiling at it's exact center. The spheres are each given the same electric charge. They are in static equilibrium, creating an angle of 30° between the halves of the string. Calculate the magnitude of the charge on each sphere.

Here's a sketch of the picture given on the worksheet:

Red lines are what I drew over the diagram to make a right triangle.

This being a bonus type question, I was given three possible answers and had to find out which one was the real one:

q =

3.7 x 10^-7 C or

1.2 x 10^3 C or

1.2 x 10^-3 C.

2. Relevant equations

Coulombs Law: [tex]F = k \ \frac{q_1 q_2}{r^2}[/tex]

3. The attempt at a solution

First I converted the mass into kilograms: m = 0.002kg

Then I split the angle into two right triangles to get the distance between the spheres.

r/2 = Sin15 x 0.3 m

r = approx. 0.16m

Then I figured since the spheres are at rest, then a = 0 and Fg = Fe...so I tried coulombs law to solve for q:

[tex]F_{g} = F_{e}[/tex]

[tex]mg = k \ \frac{q_1 q_2}{r^2}[/tex]

[tex]\frac{mgr^2}{k} = q_1 q_2[/tex]

I ended up with 2.6 x 10^-11...obviously way off any of the true answers.

Thanks for taking the time to read over all this. =D

Cheers - Krunklehorn

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# Homework Help: Two spheres hanging, find charge.

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