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Encyclopedia of Electrical Engineering
Encyclopedia of Electrical Engineering

# Electric Charge

Electric charge is the physical property of matter that causes it to experience a force when placed in an electromagnetic field. There are two types of electric charges; positive and negative (commonly carried by protons and electrons respectively). Electric charge is a basic property of electrons, protons and other subatomic particles. This makes electrons and protons stick together to form atoms. In atoms, the electron carries a negative elementary or unit charge; the proton carries a positive charge. Things that have the same charge push each other away (they repel each other). This is called the Law of Charges. It was discovered by Charles-Augustin de Coulomb. The force of attraction or repulsion between two charged bodies $Q_1$ and $Q_2$ can be determined by Coulomb's law:
Coulomb's law states that the strength, or magnitude, of the force between two point charges is proportional to the magnitudes of the charges and inversely proportional to the distance between the two charges.
Mathematically, this is given as: $$F= {{kQ_1 Q_2} \over {r^2}} (newtons, N) . . . . . Eq. (1)$$ where $F$ is in newtons, $k$ = a constant = $9.0 \times 10^{-9} Nm^{2}/C^{2}$, $Q_1$ and $Q_2$ are the charges in coulombs ($C$), and $r$ is the distance in meters between the two charges. In particular, note the squared $r$ term in the denominator, resulting in rapidly decreasing levels of $F$ for increasing values of $r$.
In the atom, therefore, electrons will repel each other, and protons and electrons will attract each other. Since the nucleus consists of many positive charges (protons), a strong attractive force exists for the electrons in orbits close to the nucleus [note the effects of a large charge $Q$ and a small distance $r$ in Eq. (1)]. As the distance between the nucleus and the orbital electrons increases, the binding force diminishes until it reaches its lowest level at the outermost subshell (largest $r$). Due to the weaker binding forces, less energy must be expended to remove an electron from an outer subshell than from an inner subshell. Also, it is generally true that electrons are more readily removed from atoms having outer subshells that are incomplete and, in addition, possess few electrons. These properties of the atom that permit the removal of electrons under certain conditions are essential if motion of charge is to be created.

#### Charge on a single electron

The charge on the electron is a fundamental property of a physical particle; it is the smallest unit of electric charge found free in nature. Knowledge of its numerical value is required in many areas of physics and chemistry.
The negative charge of each electron is found by experiment to have the same magnitude, which is also equal to that of the positive charge of each proton. Charge thus exists in natural units equal to the charge of an electron or a proton, a fundamental physical constant. A direct and convincing measurement of an electron's charge, as a natural unit of electric charge, was first made (1909) in the Millikan oil-drop experiment.

#### Unit of electric charge

The unit of electric charge in the SI systems is the coulomb, equivalent to the net amount of electric charge that flows through a cross section of a conductor in an electric circuit during each second when the current has a value of one ampere.
One coulomb consists of $6.24 \times 10^{18}$ natural units of electric charge, such as individual electrons or protons.
One electron itself has a negative charge of $1.6021766208 \times 10^{-19}$ coulomb.