Electromagnetism is the study of the electromagnetic force which is a type of physical interaction that occurs between electrically charged particles. The electromagnetic force usually manifests as electromagnetic fields, such as electric fields, magnetic fields and light. The electromagnetic force is one of the four fundamental interactions in nature. The other three are the strong interaction, the weak interaction, and gravitation. The word electromagnetism is a compound form of two Greek terms, ἢλεκτρον, ēlektron, “amber”, and μαγνήτης, magnetic, from “magnítis líthos” (μαγνήτης λίθος), which means “magnesian stone”, a type of iron ore. The science of electromagnetic phenomena is defined in terms of the electromagnetic force, sometimes called the Lorentz force, which includes both electricity and magnetism as elements of one phenomenon. The electromagnetic force plays a major role in determining the internal properties of most objects encountered in daily life. Ordinary matter takes its form as a result of intermolecular forces between individual molecules in matter. Electrons are bound by electromagnetic wave mechanics into orbitals around atomic nuclei to form atoms, which are the building blocks of molecules. This governs the processes involved in chemistry, which arise from interactions between the electrons of neighboring atoms, which are in turn determined by the interaction between electromagnetic force and the momentum of the electrons. There are numerous mathematical descriptions of the electromagnetic field. In classical electrodynamics, electric fields are described as electric potential and electric current in Ohm’s law, magnetic fields are associated with electromagnetic induction and magnetism, and Maxwell’s equations describe how electric and magnetic fields are generated and altered by each other and by charges and currents. The theoretical implications of electromagnetism, in particular the establishment of the speed of light based on properties of the “medium” of propagation (permeability and permittivity), led to the development of special relativity by Albert Einstein in 1905. Although electromagnetism is considered one of the four fundamental forces, at high energy the weak force and electromagnetism are unified. In the history of the universe, during the quark epoch, the electroweak force split into the electromagnetic and weak forces.