Electrostatics 17

17.1 Introduction

ESBHD

In Grade 10, you learntabout the force betweencharges. In this chapteryou will learn exactly how to

determine this force andabout a basic law of electrostatics.

See introductory video:VPlbc at http://www.everythingscience.co.za

17.2 Forces between charges - Coulomb’s Law

ESBHE

Like charges repel eachother while opposite charges attract each other.If the charges are at restthen

the force between themis known as the electrostatic force. The electrostatic forcebetween charges

increases when the magnitude of the charges increases or the distance between the charges decreases.

The electrostatic force was first studied in detailby Charles Coulomb around 1784. Through his

observations he was able to show that the electrostatic force between two point-like charges is

inversely proportional tothe square of the distance between the objects. He also discovered that the

force is proportional to the product of the charges on the two objects. That is:

F∝

Q 1 Q 2

r^2

,

where Q 1 is the charge on the onepoint-like object, Q 2 is the charge on the second, and r is the

distance between the two. The magnitude of theelectrostatic force between two point-like charges is

given by Coulomb’s Law.

DEFINITION: Coulomb’s Law

Coulomb’s Law states that the magnitude of theelectrostatic force between two point

charges is directly proportional to the magnitudes of each charge and inversely pro-

portional to the square of the distance between the charges:

F = k

Q 1 Q 2

r^2

The proportionality constant k is called the electrostatic constant and has the value:

k = 8, 99 × 109 N· m^2 · C−^2.

Similarity of Coulomb’sLaw to Newton’s Universal Law of Gravitation.

Notice how similar Coulomb’s Law is to the formof Newton’s Universal Law of Gravitation between

two point-like particles:

FG= G

m 1 m 2

r^2

,