5.2 Electric Charge

Learning Objectives 

By the end of this section, you will be able to:

  • Describe the concept of electric charge and its properties

In the two centuries since Dalton developed his ideas, scientists have made significant progress in advancing our understanding of atomic theory. Some of this development came from the results of several pioneering experiments that revealed details of electric charge and discovery of ions. Before you learn about the internal structure of an atom and the experiments that led to their discovery, it is important to outline key concepts in electric charge and about the discovery of ions.

Electric Charge

You are certainly familiar with electronic devices that you activate with the click of a switch, from computers to cell phones to television. And you have certainly seen electricity in a flash of lightning during a heavy thunderstorm. But you have also most likely experienced electrical effects in other ways, maybe without realizing that an electric force was involved. Let’s take a look at some of these activities and see what we can learn from them about electric charges and forces.

Discoveries

You have probably experienced the phenomenon of static electricity: When you first take clothes out of a dryer, many (not all) of them tend to stick together; for some fabrics, they can be very difficult to separate. Another example occurs if you take a woolen sweater off quickly—you can feel (and hear) the static electricity pulling on your clothes, and perhaps even your hair. If you comb your hair on a dry day and then put the comb close to a thin stream of water coming out of a faucet, you will find that the water stream bends toward (is attracted to) the comb (Figure 5.2a).

A photograph of a stream of water bending sideways as it is attracted to a comb.
Figure 5.2a: Electric charge real life example 1: an electrically charged comb attracts a stream of water from a distance. Note that the water is not touching the comb. (credit: Jane Whitney in University Physics Volume 2 (Open Stax), CC BY 4.0).
Suppose you bring the comb close to some small strips of paper; the strips of paper are attracted to the comb and even cling to it (Figure 5.2b). In the kitchen, quickly pull a length of plastic cling wrap off the roll; it will tend to cling to most any nonmetallic material (such as plastic, glass, or food). If you rub a balloon on a wall for a few seconds, it will stick to the wall. Probably the most annoying effect of static electricity is getting shocked by a doorknob (or a friend) after shuffling your feet on some types of carpeting.
A photograph of thin strips of paper stuck to a plastic comb.
Figure 5.2b: Electric charge real life example 2: after being used to comb hair, this comb attracts small strips of paper from a distance, without physical contact. Investigation of this behaviour helped lead to the concept of the electric force (credit: Jane Whitney in University Physics Volume 2 (Open Stax), CC BY 4.0).
Many of these phenomena have been known for centuries. The ancient Greek philosopher Thales of Miletus (624–546 BCE) recorded that when amber (a hard, translucent, fossilized resin from extinct trees) was vigorously rubbed with a piece of fur, a force was created that caused the fur and the amber to be attracted to each other. Additionally, he found that the rubbed amber (Figure 5.2c) would not only attract the fur, and the fur attract the amber, but they both could affect other (nonmetallic) objects, even if not in contact with those objects. (Figure 5.2d)
A photograph of a piece of gold-colored amber that has been rubbed and polished to a smooth, rounded shape.
Figure 5.2c: Image of Borneo amber:  Borneo amber is mined in Sabah, Malaysia, from shale-sandstone-mudstone veins. When a piece of amber is rubbed with a piece of fur, the amber gains more negative charge, giving it a net negative charge. At the same time, the fur, having lost what we now know to be called electrons, becomes positively charged. (credit: work by Sebakoamber, PD)
Schematic labelled ( a ) shows an amber with two plus signs and two negative signs, and an overall net charge of zero is written above the amber. To the right of the amber is a piece of a cloth, which has three plus signs and three negative signs, and an overall net charge of zero is indicated above the cloth. The middle image labelled ( b ) shows the amber and cloth being rubbed together and two arrows point from cloth to the amber. The right image labelled ( c ) pictures the amber with two positive charges, four negative charges and a net charge of negative two written above it. The cloth has three plus signs and one negative sign and a net charge of plus 2 written above it.
Figure 5.2d: Visual of attractive forces: when materials are rubbed together, charges can be separated, particularly if one material has a greater affinity for electrons than another. (a) Both the amber and cloth are originally neutral, with equal positive and negative charges. Only a tiny fraction of the charges are involved, and only a few of them are shown here. (b) When rubbed together, some negative charge is transferred to the amber, leaving the cloth with a net positive charge. (c) When separated, the amber and cloth now have net charges, but the absolute value of the net positive and negative charges will be equal (credit: University Physics Volume 2 (Open Stax), CC BY 4.0).
The English physicist William Gilbert (1544–1603) also studied this attractive force, using various substances. He worked with amber, and, in addition, he experimented with rock crystal and various precious and semi-precious gemstones. He also experimented with several metals. He found that the metals never exhibited this force, whereas the minerals did. Moreover, although an electrified amber rod would attract a piece of fur, it would repel another electrified amber rod; similarly, two electrified pieces of fur would repel each other.

This suggested there were two types of an electric property, which eventually came to be called electric charge. It was concluded there were two types of electric charge – positive and negative. The difference between the two types of electric charge is in the directions of the electric forces that each type of charge causes:

  • These forces are repulsive when the same type of charge exists on two interacting objects
  • These forces are attractive when the charges are of opposite types

The most peculiar aspect of this new force is that it does not require physical contact between the two objects in order to cause an acceleration. This is an example of a so-called “long-range” force,  (or, as James Clerk Maxwell later phrased it, “action at a distance”), which later became known as a form of induction.

The properties of electric charge are as follows:

  • Charges can be positive and negative
  • Electric force can be either attractive or repulsive
    • If two interacting objects carry the same sign of charge, the force is repulsive.
    • If the charges are of opposite sign, the force is attractive.
  • The magnitude of the force decreases (rapidly) with increasing separation distance between objects. The magnitude of the force increases (rapidly) with decreasing separation distance between the objects.
  • The force acts by contact or induction (without physical contact between the two objects)
  • Not all objects are affected by this force

Exercise 5.2a

Check Your Learning Exercise (Text Version)

Read the following statement about electric charge and determine whether the statement is True OR False.

  1. Charges can be positive and neutral
  2. Electric force can be either attractive or equal
  3. If two interacting objects carry the same sign of charge, it results in electrostatic repulsion
  4. If the charges are of opposite sign, the force is attractive.
  5. If a balloon is rubbed on hair to gain charge and then is placed against a wall and sticks to the wall, the two objects have opposite charges
  6. The magnitude of the force decreases (rapidly) with decreasing separation distance between objects
  7. When two objects of similar charge repel each other without contact it is called induction.
  8. All objects are affected by electric force

Check Your Answer[1]

Source: “Exercise 5.2a” by Jackie MacDonald, licensed under CC BY-NC-SA 4.0

The discovery that matter (and its atoms) has properties of electric charge and contain both positive and negative charges led to the theory that a given neutral atom may be able to lose or gain such charges and become positively or negatively charged atoms, respectively. These charged atoms were later defined as positive ions – cations and negative ions – anions. This concept will be discussed in more detail in upcoming sections and chapters.

Attribution & References

Except where otherwise noted, this page is adapted by Jackie MacDonald from “Electric charge” In University Physics Volume 2 (Open Stax) by Samuel J. Ling, William Moebs, Jeff Sanny is licensed under CC BY 4.0. Access for free at University Physics Volume 2 (OpenStax)

  1. For the following answers, any false answers, have been rewritten to show the correct statement. The bolded words (also noted with an *) were changed from the original false statement to make the statement true.
    1. False - Charges can be positive and *negative;
    2. False - Electric force can be either attractive or *repulsive;
    3. True;
    4. True;
    5. True;
    6. False - The magnitude of the force decreases (rapidly) with *increasing separation distance between objects OR The magnitude of the force *increases (rapidly) with decreasing separation distance between objects;
    7. True;
    8. False – *Not all objects are affected by electric force
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Enhanced Introductory College Chemistry Copyright © 2023 by Gregory Anderson; Caryn Fahey; Jackie MacDonald; Adrienne Richards; Samantha Sullivan Sauer; J.R. van Haarlem; and David Wegman is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted.

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