1.3.1 Electric Charge

An object may become electrically charged through friction or contact with another object.
This phenomenon is called electrification, and an object that carries an electric charge is called a charged body.

The quantity of electricity carried by a charged body is called electric charge.
There are two types of electric charge: positive (+) and negative (−). The atomic nucleus carries a positive charge (positive electric charge), while electrons outside the nucleus carry a negative charge (negative electric charge).
The positive charge +e carried by the nucleus of a hydrogen atom (a proton) is the smallest unit of electric charge and has the following value in the rationalized MKSA system of units.

This is called the elementary charge, and the magnitude of the charge carried by any substance is an integer multiple of e.
※When electromagnetic phenomena are considered from a macroscopic perspective, electric charge is treated as continuously divisible.

In an isolated system—a system with no exchange with its surroundings—the algebraic sum of the charges of all objects involved in an electrification process has been confirmed to remain constant. This principle is called the law of conservation of electric charge and is regarded as a fundamental law of physics.

1.3.2 Coulomb’s Law

The force exerted by a stationary electric charge on another charge is called the electrostatic force (or Coulomb force). In 1785, Charles-Augustin de Coulomb (France, 1738–1806) experimentally established the following properties.

The electrostatic forces exerted directly between two charged particles have the following properties:

 i )         The force acts along the straight line connecting the charged particles (central force).
 ii)         Charges with opposite signs attract each other, while charges with the same sign repel each other.
 iii)        The magnitude of the force is proportional to the product of the charges and inversely proportional to the square of the distance between them.

In physics, particularly electromagnetism, Coulomb’s law is initially explained using the idealized model of a point charge, in which the size and internal structure of a charged particle are ignored and only its electrical properties are considered.

Consider two point charges Q₁ and Q₂ separated by a distance r. The magnitude F of the electrostatic force is expressed by the following equation.

In the rationalized MKSA system of units, the proportionality constant k has the following value.

Here, ε₀ is called the permittivity of free space and is given by the following equation, where c is the speed of light in a vacuum (2.997 924 58 × 10⁸ m/s).

Here, F/m represents farads per metre.

When the force exerted by charge Q1 on charge Q2 is denoted by F21, it is expressed by the following equation.

Here, r/r is a unit vector.

About This Article

References
・Fundamentals of Electricity, Volume I, Corona Publishing Co., Ltd., by Toshio Utsunomiya, Hiroshi Takahashi, and Isao Izumi
・Electromagnetism, Corona Publishing Co., Ltd., by Mitsuto Matsubara, Koichi Sakaguchi, and Hiroaki Yamada
※This article was prepared with reference to the sources listed above and organized based on the author’s understanding.