Repulsive forces between two particles with the same charge. The electrostatic force, also known as the Coulomb force, acts between two-point electric charges and is directly proportional to the magnitude of the charges and inversely proportional to the square of their distance. It can be expressed by the formula:

F is the force,
k is the Coulomb constant (k ≈ 8.9875 × 10⁹ kg⋅m³⋅s⁻⁴⋅A⁻² ),
q is the size of the charges,
r is their distance.

Similar charges are repelled, opposite charges are attracted. An atomic nucleus made up of positively charged protons will not disintegrate due to electrostatic repulsive forces because the protons are held together by the stronger but short-range strong nuclear force. If the nuclei of the atoms manage to overcome the electrostatic repulsive forces (e.g. due to high temperature) and get close enough to each other, then fusion can occur.