Properties of Electric Charge and Atomic Structure

In this class, we will review the concept and origin of electric charge as the fundamental cause of electrical phenomena. We will begin with a historical analysis where the Greeks observed electrical properties by rubbing amber with wool, leading to the law of signs to model electrical attraction and repulsion. At the microscopic level, subatomic particles are explored, and how electric charge is inherent to the structure of matter, with special emphasis on the composition of atoms and how they determine the electrical properties of matter. Lastly, it is emphasized that even though an atom may be neutral, it still participates in electrical phenomena due to atomic and molecular interactions and the transfer of electrons.

Learning Objectives:
By the end of this class, the student will be able to:

1. List subatomic particles and their respective charges and masses.
2. Explain why certain materials acquire an electric charge when rubbed.
3. Describe the relationship between the behavior observed by the Greeks (charged amber) and the modern notion of electricity.
4. Differentiate between neutral atoms, cations, and anions.

CONTENT INDEX
What is Electric Charge?
Electrical Phenomena in Antiquity
Law of Signs
Electric Charge and the Structure of Matter
Electrical Properties of Matter

What is Electric Charge?

To understand electrical phenomena, it is essential to address their underlying cause. Just as mass is responsible for gravity, electric charge is the cause of electrical phenomena.

As we begin our study of electromagnetism, it is difficult to define exactly what electric charge is. This understanding requires more advanced analysis than we will initially cover. However, we can study its behavior and properties to start familiarizing ourselves with electricity.

Electrical Phenomena in Antiquity

In ancient times, the Greeks noticed that rubbing amber with wool caused it to attract certain objects. This effect is referred to as the amber “becoming charged” or “acquiring electric charge.” In fact, the word “electricity” comes from the Greek “ήλεκτρον (élektron),” which means “amber.” Since ancient times, it was observed that “electrically charged” objects exhibited the following properties and behaviors:

1. If two initially neutral materials (free of charge), A and B, are rubbed together, they may acquire an electric charge. These charges acquired by A and B are of equal magnitude.
2. If, after being charged as described above, objects A and B are brought into contact, their charges cancel each other out.

Based on the observations described above, it is said that objects acquire equal but complementary charges. For this reason, these charges are designated as “positive” and “negative.” Furthermore, the behavior of charged objects can be modeled using the law of signs:

Law of Signs

Charged objects exhibit the following behavior:

• Positive x Positive = Positive → REPULSION
• Positive x Negative = Negative → ATTRACTION
• Negative x Positive = Negative → ATTRACTION
• Negative x Negative = Positive → REPULSION

Charges of opposite signs attract, and charges of the same sign repel.

Electric Charge and the Structure of Matter

Electric charge, like mass, is a fundamental property of the particles that constitute matter. The unit of measurement for electric charge is the Coulomb, denoted by the letter C. Atoms are composed of subatomic entities with specific charges and masses, as shown in the following table:

Commonly, the symbols p^+, e^-, and n_0 represent the proton, electron, and neutron, respectively. Protons and neutrons are found in the atom’s nucleus, while electrons orbit around it. More than 99.9% of an atom’s mass is concentrated in its nucleus.

The number of protons in the nucleus defines the element to which an atom belongs. The number of neutrons defines the isotope of that element, and the number of electrons indicates its ionization state. Traditionally, an atom is considered neutral, and if it has excess or deficient electrons, it is referred to as an ion. Ions can be negative (anions) or positive (cations).

Electrical Properties of Matter

The electrical properties of matter depend on its internal structure and how atoms or molecules interact with each other. Although a neutral atom may appear to have no charge, this does not mean it cannot participate in electrical phenomena. It is important to remember that the nature of electric charge depends on the interaction between atoms and molecules and their ability to transfer or share electrons, forming bonds.