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