Electric Charge
Electric charge is a fundamental intrinsic property of matter that causes particles to experience a force when placed in an electromagnetic field, existing as either positive (carried by protons) or negative (carried by electrons) with an elementary charge unit of e = 1.602 × 10⁻¹⁹ coulombs. Charge is conserved in all physical processes (the total charge of an isolated system remains constant), and it is quantised, meaning any observable charge is an integer multiple of the elementary charge. Electric charge is the source of the electric force, which is described by Coulomb's Law and governs all electromagnetic interactions in nature and technology.
Key Formula
q = n × e
LaTeX: q = ne
| Symbol | Meaning | Unit |
|---|---|---|
| q | Total electric charge | C (coulombs) |
| n | Number of elementary charge units (integer) | dimensionless |
| e | Elementary charge (1.602 × 10⁻¹⁹) | C |
Worked Example
Problem
A glass rod loses 5 × 10¹² electrons when rubbed with silk. Calculate the charge acquired by the glass rod.
Solution
Step 1: When electrons are removed, the rod becomes positively charged. Charge = number of electrons × elementary charge Step 2: Substitute values. q = n × e = 5 × 10¹² × 1.602 × 10⁻¹⁹ q = 8.01 × 10⁻⁷ C Step 3: The glass rod carries a positive charge (electrons have been removed).
Answer
q = +8.01 × 10⁻⁷ C ≈ +0.801 μC
Properties of Electric Charge
| Property | Description | Example | Significance |
|---|---|---|---|
| Conservation | Total charge in isolated system is constant | Charge transfer between objects | Foundation of circuit analysis |
| Quantisation | Charge exists in multiples of e = 1.6 × 10⁻¹⁹ C | Electron, proton charge | Explains discrete atomic structure |
| Positive charge | Carried by protons | Proton: +e | Attracts negative charges |
| Negative charge | Carried by electrons | Electron: −e | Attracts positive charges |
| Additive nature | Total charge = algebraic sum | Neutral atom has equal +/− | Net charge determines force |
Interactive Tools
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Related Terms
Coulomb's Law
Coulomb's Law states that the magnitude of the electrostatic 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, with the force acting along the line joining the charges. Formulated by Charles-Augustin de Coulomb in 1785 through careful torsion balance experiments, it is the electrostatic analogue of Newton's Law of Universal Gravitation and forms the cornerstone of classical electrostatics. The law governs the forces responsible for atomic bonding, molecular structure, and the operation of capacitors and electrostatic devices.
Electric Field
An electric field is a vector field that exists in the region around an electric charge or a changing magnetic field, representing the electrostatic force that would be exerted per unit positive charge placed at any point in space. The field lines emanate outward from positive charges and point inward toward negative charges, with the density of field lines indicating field strength. Electric fields are central to understanding capacitors, electromagnetic waves, semiconductor devices, and the operation of all electrical equipment from simple circuits to complex communication systems.
Electric Potential
Electric potential at a point in space is the amount of electric potential energy per unit positive test charge at that location, representing the work done per unit charge to bring a positive test charge from infinity to that point against the electric field. It is a scalar quantity measured in volts (V), where 1 volt equals 1 joule per coulomb. Electric potential is fundamental to understanding capacitors, batteries, and electrical circuits, and the difference in electric potential between two points (voltage) drives the flow of electric current.
From the Latin "electrum" and Greek "elektron" (amber), because the ancient Greeks observed that rubbing amber attracted small objects. The word "charge" is from Old French "charger" (to load). The unit coulomb is named after Charles-Augustin de Coulomb (1736–1806).