Summary:
In this lesson, we will review the concept and origin of electric charge as the fundamental cause of electrical phenomena. We will start 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. Finally, it is emphasized that although an atom may be neutral, it still participates in electrical phenomena due to atomic and molecular interactions and electron transfer.

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

1. List the 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.




TABLE OF CONTENTS
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 its 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 precisely define what electric charge is. This understanding requires more advanced analysis than what we will cover initially. 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 when amber was rubbed with wool, it could attract certain objects. This effect was described as the amber “becoming charged” or “acquiring an electric charge.” In fact, the word “electricity” comes from the Greek “ήλεκτρον (élektron),” meaning “amber.”
Since ancient times, it was observed that “electrically charged” bodies acquired the following properties and behaviors:

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

Based on the above observations, it is said that bodies acquire equal but complementary charges, which is why we designate these charges as “positive” and “negative.” Additionally, the behavior of charged bodies can be modeled using the law of signs:

Law of signs

Charged bodies 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 each other, and charges of the same sign repel each other.

Electric charge and the structure of matter

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

Particle	Mass [kg]	Electric charge [C]
Proton	1.6726219 \times 10^{-27}	+1.602176634 \times 10^{-19}
Neutron	1.6759271 \times 10^{-27}	0 (neutral)
Electron	9.10938356 \times 10^{-31}	-1.602176634 \times 10^{-19}

Typically, the symbols p^+, e^-, and n_0 represent the proton, electron, and neutron, respectively. Protons and neutrons are found in the nucleus of the atom, 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 missing electrons, it is called 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 seem 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 to form bonds.

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