Magnetic field by that loop. Ampère's law is always valid for steady currents and can be used to calculate the -field for certain highly symmetric situations such as an infinite wire or an infinite solenoid. In a modified form that accounts for time varying electric fields, Ampère's law is one of four Maxwell's equations that describe electricity and magnetism. A charged particle moving in a -field experiences a "sideways" force that is proportional to the strength of the magnetic field, the component of the velocity that is perpendicular to the magnetic field and the charge of the particle. This force is known
Current density law (one of Maxwell's equations), which relates current density to magnetic field. In special relativity theory, charge and current are combined into a 4-vector. Charge carriers which are free to move constitute a free current density, which are given by expressions such as those in this section. Electric current is a coarse, average quantity that tells what is happening in an entire wire. At position r at time "t", the "distribution" of charge flowing is described by the current density: where J(r, "t") is the current density vector, v(r, "t") is the particles' average drift velocity (SI unit: m∙s), and