
#1
Jan1308, 07:36 AM

P: 605

1. The problem statement, all variables and given/known data
Two 3.52 g point charges on 7.49mlong threads repel each other after being equally charged. What is the charge q? (θ=31°.) 2. Relevant equations F = KQQ/R^2 3. The attempt at a solution How do I find Q without having a distance r??? 



#2
Jan1308, 07:54 AM

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#3
Jan1308, 08:32 AM

P: 605

Oh yea...so the length from the center to one of the masses is 3.858 m.
So I can do... F = K*Q*Q/r^2 F = K*Q^2/ r^2 An wouldn't F = 0 since they cancel each other out? 



#4
Jan1308, 08:40 AM

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P: 40,874

Charges...please help!!
Since the masses are in equilibrium, the net force on each is zero. What forces act on each mass?




#5
Jan1308, 08:41 AM

P: 605

Tension




#6
Jan1308, 08:43 AM

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P: 40,874





#7
Jan1308, 08:46 AM

P: 605

Tension and weight are the forces




#8
Jan1308, 08:48 AM

P: 605

So do I include the electric force? Like...
T  W + Fel = 0 



#10
Jan1308, 08:50 AM

P: 605

Isn't that Fel?




#11
Jan1308, 08:50 AM

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#12
Jan1308, 08:58 AM

P: 605

X.) TSINθ + qE = 0
y.) TCOSθ  mg = 0 T = mg/COSθ mg/COSθ * SINθ + qE q = mg*TANθ/E 



#13
Jan1308, 09:16 AM

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P: 40,874

Tsinθ  qE = 0 (since the force components are in opposite directions) Realize that E is also a function of q, so rewrite that in terms of k, q, and r (which you figured out). Otherwise, you are on the right track. 



#14
Jan1308, 09:22 AM

P: 605

Okay so I end up with...
q = (mg*tanθ) / (kq/r^2) q = (mg*tanθ) / (kq/r^2) and the q's cancel? = (mg*tanθ) / (k/r^2) 


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