Electricity and magnetism/Magnetism/Magnetic force by electric current

A magnet naturally orients itself perpendicular to an electric current. Its direction depends on the direction of the current. An electric current is therefore a source of magnetic force. This is a discovery made by Hans Christian Ørsted in 1820: 400px If we cut the current, this magnetic force disappears: 400px Ampère concluded that an electric current can behave like a magnet. This conclusion led him to the discovery of Ampère's law (1825): Two parallel electrical wires carrying a current attract each other if the currents go in the same direction and repel each other if the currents go in the opposite direction. 452px The force between two electrical wires carrying a current is the magnetic force. It cannot be an electric force since the two wires are electrically neutral. The unit of measurement of electric current, the ampere, was defined from the magnetic force between two wires carrying a current: "The ampere is that constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed one metre apart in vacuum, would produce between these conductors a force equal to 2×10−7 newtons per metre of length." (International Bureau of Weights and Measures, 1948) Since 1 A = 1 C/s, the definition of the ampere also defines the Coulomb. When Ampère discovered his law, electrons were not known, so the ampere could not be defined as a current of electrons. Now that the electrons are known and their charge -q = -1.602 176 487 ×10−19 C has been precisely measured, we can define the ampere by the number of electrons it takes to make 1 C: 1 ampere = 6.241509074×1018 electrons per second, roughly six billion billion electrons every second. Current loops behave like magnets. Two parallel loops attract each other if their currents go in the same direction and repel each other if they go in the opposite direction. A current loop is therefore a magnetic dipole: 640px The magnetic field produced by a current loop is similar to the electric field produced by an electric dipole: 640px Ampère assumed that the magnetic force of magnets is produced by microscopic current loops. We now know that it comes mainly from the spin of electrons. Electrons are magnetic dipoles because they behave like spinning tops. The rotation of electrons is not an electric current, but the effect is similar to that of a microscopic current loop. As with the electric field, the magnetic field produced by several sources is the sum of the fields produced by each of them separately. For two parallel wires carrying currents in opposite directions, we obtain the total field by making a vector sum: 500px The Earth's magnetic field is produced at the center of the Earth by its constantly moving liquid iron core: 640px