Magnetohydrodynamics (MHD; also called magneto-fluid dynamics or hydro­magnetics) is the study of the magnetic properties and behaviour of electrically conducting fluids. Examples of such magneto­fluids include plasmas, liquid metals, salt water, and electrolytes. The word magneto­hydro­dynamics is derived from meaning magnetic field, meaning water, and meaning movement. The field of MHD was initiated by Hannes Alfvén, for which he received the Nobel Prize in Physics in 1970. The fundamental concept behind MHD is that magnetic fields can induce currents in a moving conductive fluid, which in turn polarizes the fluid and reciprocally changes the magnetic field itself. The set of equations that describe MHD is a combination of the Navier–Stokes equations of fluid dynamics and Maxwell’s equations of electro­magnetism. These differential equations must be solved simultaneously, either analytically or numerically. History The first recorded use of the word magnetohydrodynamics is by Hannes Alfvén in 1942: The ebbing salty water flowing past London's Waterloo Bridge interacts with the Earth's magnetic field to produce a potential difference between the two river banks. Michael Faraday called this effect "magneto-electric induction" and tried this experiment in 1832 but the current was too small to measure with the equipment at the time, and the river bed contributed to short-circuit the signal. However, by a similar process, the voltage induced by the tide in the English Channel was measured in 1851. Faraday carefully omitted the term of hydrodynamics in this work. By omission, an entire body of work regarding hydromagnetic power within dams has been excluded. Equations In MHD, motion in the fluid is described using linear combinations of the mean motions of the individual species: the current density and the center of mass velocity . In a given fluid, each species has a number density , mass , electric charge , and a mean velocity . The fluid's total mass density is then , and the motion of the fluid can be described by the current density expressed as and the center of mass velocity expressed as: MHD can be described by a set of equations consisting of a continuity equation, an equation of motion, an equation of state, Ampère's Law, Faraday's law, and Ohm's law. As with any fluid description to a kinetic system, a closure approximation must be applied to highest moment of the particle distribution equation. This is often accomplished with approximations to the heat flux through a condition of adiabaticity or