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

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SUMMARY

The discussion centers on the application of Coulomb's Law in calculating the charge on a bead in an electrostatic equilibrium scenario. The user attempted to balance the gravitational force acting on the bead, which has a mass of 250g, with the electrostatic force derived from Coulomb's Law. The calculation involved using the formula F = (KQa * Qb) / r^2, where Qa is -7 µC, and the distance r is 0.011 m. The user initially calculated Qb to be 4.71 x 10^-9 C, but identified an error in their conversion of units, leading to an incorrect result.

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  • Understanding of Coulomb's Law and electrostatic forces
  • Knowledge of gravitational force calculations
  • Familiarity with unit conversions, particularly between centimeters and meters
  • Basic principles of electrostatics and charge interactions
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  • Review the derivation and application of Coulomb's Law in electrostatic problems
  • Study the concept of electrostatic equilibrium and forces acting on charged objects
  • Practice unit conversion techniques, especially in physics contexts
  • Explore the implications of charge polarity in electrostatic interactions
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Students studying physics, particularly those focusing on electrostatics, as well as educators looking for practical examples of Coulomb's Law applications in real-world scenarios.

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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!
 
Physics news on Phys.org
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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