1. A comb collects charge **Q** Coulombs.
A small piece of paper, of mass **m = 0.01 g**,
collects charge **-Q**.
The comb approaches the piece of paper from above.
When it is **d = 1 cm** above the paper, the paper
flies up to the comb.

Write an equation for the size of the charge **Q**.

Answer: grav force on paper F = m*g k*Q*Q electric force on paper F = ----- d^2 When the forces are equal, k * Q^2 m * g = ---------- d^2 Solve for Q m * g * d^2 Q = sqrt ( ----------- ) k

2. Find the value of Q.

Answer: m * g * d^2 Q = sqrt ( ----------- ) k -5 2 [ 10 kg * (9.8 m/s^2) * (0.01 m) ] = sqrt [ ------------------------------- ] [ 8.99 x 10^9 N*m^2/C^2 ] = 1.0 x 10^(-9) Coulombs

3. What are the SI units of electric field?

Answer: Newtons per Coulomb. (N/C)

Copyright © Michael Richmond. This work is licensed under a Creative Commons License.