Electromagnetism is the study of the physical phenomena associated with a physical property of the matter called the electric charge. There are two types of electric charge (from now on we will call them charge): positive charge and negative charge. These charges are quantified as a multiple of a fundamental unit of charge (the absolute value of the charge of one electron), that, in SI units is:
Where C is the abbreviation of Coulomb, the SI unit of charge.
Matter consists of electrically neutral atoms: each atom has the same number of negative and positive charges. The positive charges are called protons and they are found in the atom nucleus. The nucleus is surrounded by a number of negative charges (electrons) equal to the number of protons. The following figure is a schematic diagram of an atom structure.
The heavier an atom is, the more protons are contained in its nucleus, and therefore the larger the number of electrons.
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An object is said to be positively charged if its atoms have lost some of their electrons (for instance when you rub an object against another one) and there is a charge imbalance. Conversely, an object is said to be negatively charged if its atoms have more electrons than protons.
Since the number of atoms in an object is generally very large (to give you an idea of the order of magnitude, the Avogadro number is 6.02214076×10²³), on the macroscopic scale it would seem like the charge of an object is continuous. But in reality, its charge consists of the charge imbalance for a huge number of atoms. Nevertheless, the microscopic scale of the matter will not be taken into account to study the electrical phenomena and we will consider the interactions between charges that we will simply name q.
The physical interaction between charges is called the electromagnetic force and it is one of the fundamental forces of Nature .
The interaction between charges at rest is called the electrostatic force and it is mathematically described by Coulomb’s law. Magnetism is the study of the phenomena associated with charges in motion. The Scottish scientist James Clerk Maxwell brought together both phenomena when he formulated his equations at the end of the 19th-century. He discovered that electricity and magnetism are two different aspects of the same phenomenon: the interaction between electric charges.