How does magnetohydrodynamics work?
Magnetohydrodynamics (MHD) is the study of the magnetic properties of electrically conductive liquids.
Its most popular application is the MHD drive as described in fiction such as Clive Cussler's Oregon adventures.
A MHD drive for a ship requires that the ship has a tube which is open at both ends running from front to rear. To give an example of a sea going ship, the tube would be below the waterline. Sea water conducts electricity and can flow through the pipe. The MHD drive then requires a strong magnetic field to be place around the pipe. Then a large electric current is passed through the liquid in the pipe. The magnetic field and the electric current interact to produce an electromotive force in the liquid which propels it along the pipe towards the rear of the ship.
The MHD drive is not really practical at our current level of technology. The problem is that the equipment required to produce the magnetic field and electric current would be too bulky and expensive.
It is also possible to use an MHD drive to power a spacecraft by replacing the water with plasma or ionised gas which are electrically conductive.