1. Electrostatics Electric Force

First review the gravitational force… Any two masses are attracted by equal and opposite gravitational forces: m1m1 m2m2 r F-F Newton’s Universal Law of Gravitation where…… G=Universal Gravitation Constant = 6.67x Nm 2 /kg 2  This is an Inverse-Square force  Gravity is a very weak force

r Charge (Q) Coulombs (C) 1 C = x e e = x 10  9 C Distance (m) Coulomb’s constant (k) k = x 10 9 N  m 2 /C 2 Force (N)

Notes on Coulomb’s Law 1) It has the same form as the Law of Gravitation: Inverse-Square Force 2) But… (can you spot the most basic difference between these two laws?) 3) The electrostatic constant (k) in this law is derived from a more fundamental constant:  0 = permittivity of free space = 8.85 x C 2 /Nm 2 4) Coulomb’s Law obeys the principle of superposition

1.Compare the electric force holding the electron in orbit (r = 0.53 × 10 -l0 m) around the proton nucleus of the hydrogen atom, with the gravitational force between the same electron and proton. What is the ratio of these two forces?

Vector addition review: Two forces acting on an object tail-to-tip method Components method

Coulomb’s law strictly applies only to point charges. Superposition: for multiple point charges, the forces on each charge from every other charge can be calculated and then added as vectors.

2.At each corner of a square of side L there are four point charges. Determine the force on the charge 2Q. Q 2Q 3Q4Q L

3.A  C and a  3.55  C charge are placed 18.5 cm apart. Where can a third charge be placed so that it experiences no net force? Q1Q1 Q2Q2 d x Q

[Extra] Two 6.7 kg bowling balls are placed in a vertical cylinder. Charge is added to each ball until they repel with enough force to separate the two balls by a distance of 43.7 cm. Assuming the two bowling balls are charged positively and equally, determine the amount of charge (to four significant digits) added to each individual ball.

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