A magnetohydrodynamic device converts magnetic energy to or from electrical energy through the use of a conductive liquid or plasma. Similar to the Hall effect, the fundamental physics of the magnetohydrodynamic effect is described by the Lorentz force equation, Equation 5.1.1. The difference is that the magnetohydrodynamic effect occurs in conductive liquids or plasmas while the Hall effect occurs in solid conductors or solid semiconductors. Another related effect, which is also described by the Lorentz force equation, is the electrohydrodynamic effect, discussed in Sec. 10.5. The difference is that the magnetohydrodynamic effect involves magnetic fields while the electrohydrodynamic effect involves electric fields.
Matter can be found in solid, liquid, or gas state. A plasma is another possible state of matter. A plasma is composed of charged particles, but a plasma has no net charge. When a solid is heated, it melts into a liquid. When a liquid is heated, it evaporates into a gas. When a gas is heated, the particles will collide with each other so often that the gas becomes ionized. This ionized gas is a plasma . When ions in either a conductive liquid or a plasma flow in the presence of a magnetic field perpendicular to the flow of ions, a voltage is produced.
This magnetohydrodynamic effect was first observed by Faraday in 1831 . In the 1960s, there was interest in building magnetohydrodynamic devices where the conducting medium was a plasma. These devices typically operated at high temperatures, in the range of 3000-4000 K . Progress was limited, however, because few materials can withstand such high temperatures. More recently, engineers have used this principle to build pumps, valves, and other devices for microfluidic systems  . These room temperature devices can control the flow of conducting liquids through the use of an external magnetic field.