# 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).

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.
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)
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

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.

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
definition