http://www.ck12.org Chapter 15. Electrostatics
15.3 Electrostatic Fields
Objectives
The student will:
- Understand what a field is.
- Understand how to solve electrostatic field problems.
Vocabulary
- electric field:The field that surrounds a charged particle.
- electric field line:A simple way of representing an electric field, also known as electric lines of force.
- test charge:Used to measure an electric field. The electric field is defined as the ratio of the force acting on
the positive test charge, at some point in space, to the magnitude of the test charge.
Introduction
Even though Isaac Newton felt confident about the universal law of gravity, he wondered just how the Earth reached
across the “empty” space and exerted a force upon the Moon. It was one thing to speak of contact forces. Hitting a
baseball, kicking a can– these forces made intuitive sense. But gravity was a force that acted through a distance like
some invisible hand extending over a limitless space. The same is true of the electrical force. Both equations suggest
that there is no end to the reach of this “invisible hand.” (The denominators of the equations show that regardless of
the distance between two objects, a force remains present.)
It wasn’t until the 19th century that Michael Faraday (1791-1867), Figure15.16, proposed the field concept,
which stated that the space between two electrical charges (or two masses) was permeated by a field. This
field communicated each object’s presence. The field distorted the space around each object such that the forces
experienced by the objects obeyed an inverse square law. Faraday conducted experiments to measure the intensity
of the field surrounding charged objects, thus supporting the field concept.
The Electric Field
The field that surrounds a charged particle is called anelectric field.
The electric field is measured by observing the force on a small positive “test charge” placed within the field. (The
test charge has to be small so it would have no effect upon the charges which have created the field we wish to
measure.) The electric field is then defined as the ratio of the force acting on the positive test charge, at some point
in space, to the magnitude of the test charge.