Two charged spheres repel (attached to strings)

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SUMMARY

The problem involves two small conducting spheres, each with a mass of 1.0 × 10-4 kg, suspended by nonconducting threads of length 0.2 m, charged with an equal charge Q. The spheres repel each other due to electric force, reaching equilibrium at an angle of 10 degrees. Using Coulomb's law and gravitational force calculations, the electric force of repulsion can be expressed as F12 = (k * Q2) / (0.16 * sin2(5°)). The solution confirms the approach to balance gravitational and electric forces to find the charge Q.

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It's been awhile since I've had physics I, so this problem is giving me a headache.

Q) Two very small conducting spheres, each of a mass [itex]1.0 \times 10^{-4}\,\,(kg)[/itex], are suspended at a common point by very thin nonconducting threads of a length [itex]0.2 \,\,(m)[/itex]. A charge [itex]Q[/itex] is placed on each sphere. The electric force of repulsion separates the spheres, and an equilibrium is reached when the suspending threads make an angle of [itex]10 \,\, (deg)[/itex]. Assuming a gravitational force of [itex]9.80 \,\, (N/kg)[/itex] and a negligible mass for the threads, find [itex]Q[/itex].

My Work)
We first will deal with:
[tex]\vec F_{12} = \frac{\hat R_{12} k q_1 q_2}{R^2_{12}}[/tex]
[tex]\sum \vec F_i = m \vec a[/tex]
Since equilibrium is reached, [itex]\vec a = \vec 0[/itex]. Thus,

[tex]\sum F_i = \vec T_2 + \vec F_G + \vec F_{12} = \vec 0[/tex]

We now find the forces.

[tex]\vec F_G = -\hat y (9.8 \times 10^{-4})[/tex]

Setting up the coordinate system we assume the orgin as at the point of interesection of the two threads. Thus, a vector that points to sphere-two is:
[tex]\vec S_2 = \hat x(0.2 \sin 5^{\circ}) - \hat y(0.2 \cos 5^{\circ})[/tex]

The vector quantities for coloumbs law are as follows:
[tex]\vec R_{12} = \hat x (2(0.2\sin 5^{\circ}))[/tex]
[tex]R = 0.4 \sin 5^{\circ}[/tex]
[tex]\hat R = \hat x[/tex]

Thus, since the spheres have an equal charge
[tex]\vec F_{12}=\frac{\hat x k Q^2}{0.16 \sin^2 5^{\circ}}[/tex]

Now I know the tension has to exert a force that holds the sphere in place, so gravity and the electric repulsion keep it from moving away. So do I just say that [itex]\vec T_2 = -\vec S_2[/itex]?

I'm not really sure what to do. Is what I'm doing even correct?

thanks in advance :smile:
 
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I figured it out
 

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