Electric charge is a fundamental physical property possessed by subatomic particles that determines their electromagnetic interactions. Like mass, length, and time, electric charge is one of the basic quantities of the universe. It defines how an object is influenced by electric fields and how it generates its own electric field. The interaction between charge and field is the source of the electromagnetic force, one of the four fundamental forces of nature. Electric charges exist in two types: positive (+) and negative (–). Like charges repel each other, while opposite charges attract. This fundamental principle underlies many phenomena, from the formation of molecules through atomic bonding to everyday occurrences of static electricity.

The branch of physics that studies stationary electric charges and their interactions is called electrostatics. The existence of electric charge was first observed in ancient times when it was noticed that amber, when rubbed with wool, could attract small, lightweight objects. This observation marked the beginning of the scientific study of electricity and related concepts.

Atomic Structure and the Origin of Charge

According to modern physics, all matter is composed of fundamental units called atoms. Atoms, in turn, consist of smaller subatomic particles: protons, neutrons, and electrons. At the center of the atom lies the nucleus, which contains positively charged protons and neutral neutrons. Surrounding the nucleus are negatively charged electrons that move in specific orbits or energy levels. Protons and neutrons are bound together within the nucleus by the strong nuclear force, which is significantly stronger than the electromagnetic force.

Under normal conditions, an atom contains an equal number of protons and electrons. Thus, the total positive charge equals the total negative charge, and the atom is electrically neutral. A material becomes electrically charged when the balance between the number of protons and electrons in its atoms is disturbed. Because electrons are much lighter than protons and occupy the atom’s outer regions, they can be transferred from one atom to another through various interactions. When an atom loses electrons, it becomes a positively charged ion (cation); when it gains electrons, it becomes a negatively charged ion (anion). The total charge of an object is equal to the algebraic sum of the charges of the atoms that compose it.

Unit

In the International System of Units (SI), the unit of electric charge is the coulomb (C). One coulomb is defined as the amount of charge carried by a current of one ampere in one second. Expressed in terms of the elementary charge (e), one coulomb corresponds to approximately 6.242 × 10¹⁸ charges of either electrons or protons.

Electrical Concepts

Electric Field

An electric charge produces an electric field in the surrounding space. This field represents the region in which another charge would experience a force. The electric field is a vector quantity; it has both magnitude and direction at each point. By convention, the direction of the field at a point is the direction of the force that would act on a positive test charge placed there.

Potential Difference (Voltage)

The potential difference, or voltage, between two points in an electric field is defined as the work required to move a unit positive charge from one point to the other. In the SI system, the unit of voltage is the volt (V). One volt equals one joule of work per coulomb of charge moved. For an electric current to flow, a potential difference must exist between the ends of a conductor.

Grounding

Grounding is the process of connecting a charged object to the Earth using a conductive wire to neutralize it. The Earth, being an immense conductor, can act as an almost unlimited reservoir of charge, capable of accepting or supplying electrons. When a negatively charged object is grounded, excess electrons flow into the Earth; when a positively charged object is grounded, electrons flow from the Earth into the object until it becomes neutral.

Properties of Electric Charge

Electric charge is a physical quantity observable in nature and is considered one of the fundamental properties of matter. From the microscopic structure of matter to large-scale physical phenomena, electric charge operates under universal laws and is measurable. The principles governing charge are central not only to the understanding of electromagnetic force but also to many fields of modern physics.

Types of Charge and Interaction Behavior

In nature, there are two kinds of electric charge: positive and negative. This classification, derived from historical observations of electromagnetic interactions, also serves as a convenient convention for expressing physical processes mathematically. The basic rule of interaction states that like charges (two positives or two negatives) repel each other, while opposite charges attract. The magnitude of this force depends on the sizes of the charges and the distance between them. This relationship was quantified by Charles-Augustin de Coulomb in the 18th century and is expressed mathematically by Coulomb’s Law, which forms the foundation for calculating electrostatic forces.

Law of Conservation of Charge

Electric charge is one of the most fundamental conserved quantities in the universe. According to the law of conservation of charge, the total electric charge of an isolated system remains constant over time. This means that charge can neither be created nor destroyed but can only be transferred from one body or particle to another. For example, when two neutral objects are rubbed together, one becomes negatively charged and the other positively charged due to electron transfer. Although the charge distribution changes, the total charge of the system remains zero, as the gains and losses balance out.

Quantization of Electric Charge

Electric charge is a discrete quantity; it exists only in integer multiples of a fundamental unit known as the elementary charge. This property is called quantization. All observable charges are integral multiples of this elementary charge, which equals the magnitude of the charge carried by an electron or a proton, approximately 1.602 × 10⁻¹⁹ coulomb. The total charge of any particle or object can thus be expressed as q = n·e, where q is the total charge, n is an integer (positive or negative), and e is the elementary charge. This principle is essential for understanding the electrical structure of matter and has significant applications in modern particle physics.

Types of Electrification

Frictional Electrification

Commonly observed between insulators, this process occurs when two different materials are rubbed together, causing electron transfer. One material tends to give up electrons more readily, while the other tends to gain them. As a result, the electron donor becomes positively charged, and the receiver becomes negatively charged. For instance, an ebonite rod rubbed with wool becomes negatively charged, while a glass rod rubbed with silk becomes positively charged.

Contact Electrification

When a charged conductor touches another conductor that is neutral or carries a different charge, the total charge is redistributed in proportion to their capacities (which depend on size and geometry). The process continues until electrical potentials equalize, after which both objects may carry the same sign of charge or become neutral.

Induction (Electrification by Influence)

When a charged object is brought near a neutral conductor without touching it, it causes a redistribution of charges within the conductor. Charges opposite in sign to the approaching object are attracted toward it, while like charges are repelled to the far side, resulting in a temporary charge separation or polarization. No net transfer of charge occurs, but the distribution of charge within the conductor changes.

Coulomb’s Law

The magnitude of the force between two point charges was formulated by the French physicist Charles-Augustin de Coulomb in 1785. According to Coulomb’s Law: The electric force between two point charges is directly proportional to the product of the magnitudes of the charges and inversely proportional to the square of the distance between them. This force acts along the line joining the two charges and is expressed mathematically as:

$F=kx\big(\frac{|q_1xq_2|}{r^2}\big)$

Where:

• F is the magnitude of the electrostatic force [newton, N],
• q₁ and q₂ are the magnitudes of the charges [coulomb, C],
• r is the distance between them [meter, m], and
• k is the proportionality constant known as Coulomb’s constant, which depends on the medium. In a vacuum, its approximate value is 8.99 × 10⁹ N·m²/C².

Conductors, Insulators, and Charge Distribution

Materials are classified according to their ability to conduct electric charge. Substances that allow free movement of charges due to the presence of mobile electrons are called conductors (e.g., metals, saline water). Materials that greatly restrict charge movement are called insulators or dielectrics (e.g., glass, plastic, rubber, pure water).

When an excess charge is placed on a conductor, it tends to spread uniformly across the outer surface due to mutual repulsion among like charges. On irregularly shaped conductors, charge density becomes greater at sharp points or edges. This principle underlies the operation of devices such as lightning rods. The Faraday cage, on the other hand, operates on the principle of shielding a volume from external electric fields by enclosing it within a conductive shell. The metal bodies of airplanes and automobiles serve as Faraday cages, protecting passengers from lightning and external electromagnetic effects.