Electric charge Electric charge, often referred to as charge, is a physical quantity (symbol \(Q\)) that indicates the way in which a particle is influenced by electric and magnetic fields. The magnitude of charge is expressed in the unit of coulomb ( \(\mathbf{C}\)).

There are two kinds of electric charge, namely positive and negative charge. An electron is an elementary particle with the smallest possible negative charge, the so-called negative unit charge. Ions are electrically charged atoms, molecules, or other groups of bonded atoms, and they may have positive and negative charge. Negatively charged ions, such as \(\mathrm{Cl}^{-}\) and \(\mathrm{S}^{2-}\), are called anions; positively charged ions such as \(\mathrm{K}^{+}\), \(\mathrm{Na}^{+}\) and \(\mathrm{Ca}^{2+}\) are called cations.

Similar charges repel each other, while differently charged particles attract each other with a force that increases quadratically with decreasing distance between the particles (Coulomb's law).

Electric field An electric field is the space (around a charge or a few charges), in which a force is excerted on an electric charge. The electric field strength at a point of an electric field is the strength of the force excerted on a positive unit charge at that point of the field. Like any force it is a vector quantity, in other words, it not only has a magnitude but also a direction. The direction of the field at a given point is the same as the direction in which a positive charge would move starting at that point. You can trace the path for any starting position that a positive charge would follow from that starting point in the electric field. Such a path is called a field line and the direction in which the positive test charge would move is indicated with arrows. When you place multiple field lines in one figure you get a good impression of the electric field. Below, the field lines are drawn around

1. a single positive point charge
2. one negative point charge
3. a combination of a positive and negative point charge of equal magnitude
4. a combination of two positive point charges of equal magnitude
5. a combination of a positive and negative point charge of different magnitude
6. two parallel plates with equal but opposite charges.