Page 44 - Engineering Electromagnetics, 8th Edition

2 CHAPTER

Coulomb’s Law and

Electric Field Intensity

aving formulated the language of vector analysis in the first chapter, we next
establish and describe a few basic principles of electricity. In this chapter,
Hwe introduce Coulomb’s electrostatic force law and then formulate this in
a general way using field theory. The tools that will be developed can be used to
solve any problem in which forces between static charges are to be evaluated or to
determine the electric field that is associated with any charge distribution. Initially,
we will restrict the study to fields in vacuum or free space; this would apply to media
such as air and other gases. Other materials are introduced in Chapters 5 and 6 and
time-varying fields are introduced in Chapter 9. ■

2.1 THE EXPERIMENTAL LAW OF COULOMB
Records from at least 600 B.C. show evidence of the knowledge of static electricity.
The Greeks were responsible for the term electricity, derived from their word for
amber, and they spent many leisure hours rubbing a small piece of amber on their
sleeves and observing how it would then attract pieces of fluff and stuff. However,
their main interest lay in philosophy and logic, not in experimental science, and it
was many centuries before the attracting effect was considered to be anything other
than magic or a “life force.”
Dr. Gilbert, physician to Her Majesty the Queen of England, was the first to do
any true experimental work with this effect, and in 1600 he stated that glass, sulfur,
amber, and other materials, which he named, would “not only draw to themselves
straws and chaff, but all metals, wood, leaves, stone, earths, even water and oil.”
Shortly thereafter, an officer in the French Army Engineers, Colonel Charles
Coulomb, performed an elaborate series of experiments using a delicate torsion bal-
ance, invented by himself, to determine quantitatively the force exerted between two
objects, each having a static charge of electricity. His published result bears a great
similarity to Newton’s gravitational law (discovered about a hundred years earlier).

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