CH21.Problems
Coulomb’s Law
JH
Checkpoint 3:
The figure here shows three arrangements of an electron e and two protons p.
a) Rank the arrangements according to the magnitude of the net electrostatic force
on the electron due to the protons largest first.
b) In situation c, is the angle between the ne force on the electron and the line
labeled d less than or more than 45 degree?
In-between q1 and q2 cannot contain an equilibrium position
Right of q2 also cannot. Q2 is the stronger charge and the
closer to such position
Left of q1 may contain such position.
Call the new charge q3 and put it L0 left of q1:
k q3 (+1)/(L0)^2 + q3 (-3)/(L+L0)^2 = 0
Sqrt(3) L0 = L +L0  L0 = L /(sqrt(3) – 1) = 13.7
X = 3.00 cm to left of +q charge. It can be shown that the net force for the other
Two charges that the net force is zero as well.
q3/q = -4/9 by computing net force on +q then you will find that net force on 4q is zero
F = k q^2/r^2
n = Q /e
Answer: (a) 8.99×1019 N ; (b) 625 electrons
66. An electron is in a vacuum near Earth’s surface and located at y = 0 on a vertical y axis. At
what value of y should a second electron be placed such that its electrostatic force on the first
electron balances the gravitational force on the first electron?
Answer: 5.077 m  5.1 m (bellow)