Common transparent tape becomes charged when pulled from a dispenser. If one pie

Question

Common transparent tape becomes charged when pulled from a dispenser. If one piece is placed above another, the repulsive force can be great enough to support the top piece's weight. Assuming equal point charges (only an approximation), calculate the magnitude of the charge if electrostatic force is great enough to support the weight of a 6.00 mg piece of tape held 0.50 cm above another. (The magnitude of this charge is consistent with what is typical of static electricity.) I used q=sqrt mgr^2/k I still can not get the correct answer

Explanation / Answer

Electrostatic force between two point charges Fe = kq1q2/r^2, where k = 8.98755E9 Nm^2/C.

Since q1=q2, we'll represent q1q2 as q^2.

Fe is in equilibrium with gravitational force Fg = mg.

Then q = sqrt(mgr^2/k).

Here,

r = 0.50 cm = 0.5 * 10^-2 m

m = 6 * 10^-6 kg

The,

q = sqrt(mgr^2/k).

or, q = sqrt((6 * 10^-6 * 9.81 * (0.5 * 10^-2)^2 / (9 * 10^9))

q = 4.04 * 10^-10 C

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