Why Is Coulomb's Law Not Working in My Calculation?

AI Thread Summary
Coulomb's Law is not yielding correct results in the calculation of the charge on a bead suspended above a fixed charge due to a misunderstanding of the distance unit conversion. The user attempted to balance the gravitational force acting on the bead with the electrostatic force calculated using Coulomb's Law but initially miscalculated the distance. After correcting the distance from centimeters to meters, the calculated charge on the bead was still incorrect. Additionally, there is confusion regarding the necessity for the opposing charge to be negative, which is essential for the forces to balance given the negative charge of the fixed charge. Accurate unit conversion and understanding of charge interactions are crucial for solving the problem correctly.
mujadeo
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Homework Statement



A rigid, insulating fiber runs along a portion of the y-axis; the fiber isnot free to move. Gravity acts downward (g = 9.81 m/s2).A charge Qa = -7 µC is fixed to the fiber at the origin. A bead with a hole drilled through its center is slipped over the fiber andis free to move along the fiber without friction. The mass of the bead is m = 250g and its charge is Qb. At equilibrium, the bead floats a distanceyb = 11 cm above the origin.


Calculate the charge on the bead.


Homework Equations



Why isn't coulonmbs law working for me?

The Attempt at a Solution



OK here's what i did

F = (KQa * Qb) / r^2

(absolute values for the charges)
(i got F by realizing that it must balance the opposing downward force (drew FBD)

so,

(.245)9.81 = (9.0*10^9) (7*10^-6)Qb / ((.011)^2)


So Qb comes to 4.71*10-9
=wrong for sum reason

Please help and THANKS!
 
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UPDATE:
i was saying that 11cm = .011m that was prob.

One other question though.
Why does the opposing charge have to be negative??
 
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