Electric charge is important in determining how a body or particle will behave and interact electromagnetically. It is also key for understanding how electric fields, electric potentials and electromagnetic waves come into existence. It starts with the atom and it’s number of protons and electrons.
Charges are positive or negative. In a neutral atom, the number of protons in a nucleus is equal to the number of electrons. When an atom loses or gains an electron from this state, it becomes a negatively or positively charged ion. When bodies or particles exhibit a net charge, either positive or negative, an electric force arises. Charges can be caused by friction or irradiation. Electrostatic force functions similar to the gravitational force – in fact the formulas look very similar! The difference between the two is most importantly that electrostatic force can be attraction or repulsion, but gravitational force is always attraction. However for small bodies, the electrostatic force is primary and the gravitational force is negligible.
Charles Coloumb conducted experiments around 1785 to understand how electric charges interact. He devised two main relations that would become Coulomb’s Law:
The magnitude of the force between two stationary point charges is
- proportional to the product of the magnitude of the charges and
- inversely proportional to the square of the distance between the two charges.
The following expression describes how one charge will exert a force on another:
The unit vector in the direction of charge 1 to charge 2 is written as e12 and the position of the two numbers indicates the direction of the force, moving from the first numbered position to the second. Reversing the direction of the force will result in a reversed polarity, F12 = -F21.
The coefficient ke will depend on the unit system and is related to the permittivity:
The permittivity of vacuum, ε0 = 8.85*10^(-12) C^2N*m^2.
Coulomb forces obey superposition, meaning that a series of charges may be added linearly without effecting their independent effects on it’s ‘target’ charge. Coulomb’s Law extends to bodies and non-point charges to describe an applied electrostatic force on an object; the same first equation may be used in this scenario.