Phenomena involving both electric and magnetic fields, and the study of such phenomena. The first indication of a link between electricity and magnetism was shown by Hans Christian Ørsted, who demonstrated that an electrical current caused the deflection of a compass needle (1819). This established that magnetic effects are produced by a moving electrical charge. Ørsted's observation was interpreted by Michael Faraday in terms of lines of magnetic influence circulating around the wire. Ampère deduced an expression for the magnetic force between two current-carrying wires to give the original form of what is now called Ampère's law (1827). Faraday demonstrated that switching off a current in a circuit produced a momentary current in a nearby circuit, and that moving a magnet close to a circuit also produced momentary currents (1831). This established that electrical charge can be made to flow by changing magnetic fields, the basis of electromagnetic induction (expressed as Faraday's law). Similar work was performed by Joseph Henry (1829). The first generator was built by Faraday in 1831.
The unification of electricity and magnetism into a single theory of electromagnetism is due to James Clerk Maxwell, who first expressed the laws of Faraday and Ampère in their modern form as two of the four Maxwell's equations. Using his equations of electromagnetism, Maxwell postulated that light is electromagnetic disturbance with velocity 
, where ?0 and ?0 are the permittivity and permeability of empty space, respectively (1864). Heinrich Hertz used oscillating electrical circuits to produce radio waves which travelled at the velocity of light (1887), thereby providing experimental support for Maxwell's work. The expression of the velocity of light in terms of fundamental constants suggested to Einstein that it should always be the same for all observers. This conclusion is central to his theory of special relativity (1905), which in turn explains more fully the relationship between electric and magnetic effects. While an observer stationary with respect to an electric charge will see it as a source of electric field only, a second observer moving relative to the first will see the same charge as a source of both electric and magnetic fields in a way dictated by special relativity.
| Electromagnetism | |
| Magnetism | |
| Electrostatics | |
|---|---|
| Electric charge | |
| Coulomb's law | |
| Electric field | |
| Gauss's law | |
| Electric potential | |
| Magnetostatics | |
| Ampere's law | |
| Magnetic field | |
| Magnetic moment | |
| Electrodynamics | |
| Electric current | |
| Lorentz force law | |
| Electromotive force | |
| Electromagnetic induction | |
| Faraday-Lenz law | |
| Displacement current | |
| Maxwell's equations | |
| Electromagnetic field | |
| Electromagnetic radiation | |
| Electrical circuits | |
| Electrical conduction | |
| Electrical resistance | |
| Capacitance | |
| Inductance | |
| Impedance | |
| Resonant cavities | |
| Waveguides | |
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Electromagnetism is the force observed as static electricity, and causes the flow of electric charge (electric current) in electrical conductors.
SI electricity units
| edit SI electromagnetism units | ||||
|---|---|---|---|---|
| Symbol | Name of Quantity | Derived Units | Unit | Base Units |
| I | Magnitude of current | ampere (SI base unit) | A | A = W/V = C/s |
| q | Electric charge, Quantity of electricity | coulomb | C | A·s |
| V | Potential difference or Electromotive force | volt | V | J/C = kg·m·A−1 |
| R, Z, X | Resistance, Impedance, Reactance | ohm | Ω | V/A = kg·m·A−2 |
| ρ | Resistivity | ohm metre | Ω·m | kg·m·A−2 |
| P | Power, Electrical | watt | W | V·A = kg·m |
| C | Capacitance | farad | F | C/V = kg·A |
| Elastance | reciprocal farad | F−1 | V/C = kg·m·s−4 | |
| ε | Permittivity | farad per metre | F/m | kg·A |
| χe | Electric susceptibility | (dimensionless) | - | - |
| G, Y, B | Conductance, Admittance, Susceptance | siemens | S | Ω·m·A2 |
| σ | Conductivity | siemens per metre | S/m | kg·s |
| H | Magnetic field, magnetic field intensity | ampere per metre | A/m | A·m−1 |
| Φm | Magnetic flux | weber | Wb | V·s = kg·m·A−1 |
| B | Magnetic flux density, magnetic induction, magnetic field strength | tesla | T | Wb/m·A−1 |
| Reluctance | ampere-turn per weber | A/Wb | kg·s | |
| L | Inductance | henry | H | Wb/A = V·s/A = kg·m·A−2 |
| μ | Permeability | henry per metre | H/m | kg·m·s |
| χm | Magnetic susceptibility | (dimensionless) | - | - |
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