# Coulomb's Law and insulating fiber

1. Apr 2, 2006

### nahya

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 = -3 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 = 170 gand its charge is Qb. At equilibrium, the bead floats a distanceyb = 16 cm above the origin.
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the coulomb force must balance the gravitational force, so..
the answer is in C, not µC, so Q(a) = 0.003 C.
K * 0.003* Q(b) / 0.16^2 = 9.81 * 0.17
solving for Q(b), i get 1.581226667e-9.
i checked my solution, and it did equal to the gravitational force.
what am i doing wrong?
am i miscomprehending the problem, or the coulomb's law?

Last edited: Apr 2, 2006
2. Apr 2, 2006

### nocturnal

$$0.003C \neq -3 \mu C$$

(there are two things that need to be changed to make this an equallity)

Last edited: Apr 2, 2006
3. Apr 2, 2006

### nahya

doh!
thanks. hehe.