Magnetohydrodynamics and electrohydrodynamics?

[Shroom Man 777]

What's the difference between magnetohydrodynamics and electrohydrodynamics? I was wiki-ing about submarine stuff, since I'm writing a fictional article about giving the Yamato beam lasers and making it a hydrofoil of doom, and I wandered around into both topics. The descriptions, the short ones at least (since I won't understand the detailed explanations), of both are incredibly similar.

[Sea Skimmer]

First off, those two terms refer to very wide scientific fields. I’m assuming you want to know about the propulsion systems which are only tiny subsets of the fields.

A Magnetohydrodynamic Generator uses an open ended tube filled with seawater to provide propulsion. The tube is wrapped by a magnetic liquid and magnetic field generators, which create rapid and very intense magnetic pulsations. The net result being water is pumped through the tube. This method of propulsion will have an inherently VERY low speed limit and requires vast quantities of electrical power for the thrust it provides. The only advantage is no moving parts, making it damn near useless and irrelevant for a surface ship. This sort of system has been tested with prototypes, top speed achieved being a bit under 3kts.

Electrodynamic Thrust involves propulsion based purely on magnetism. The ship would have a line of electromagnetic down its centerline, and two flanking rows of electrodes down each side of the hull. By sending a current through the electrodes you generate a second magnetic field, and the interaction between the fields should result in forward motion. This is a purely conceptual method of propulsion as far as I’m aware. Expect it to be very slow and very uneconomical in terms of power consumption. It would be great for an ultra long range spacecraft though, as a nuclear pile could keep it powered for decades and no propellant is needed.

[metavac]

To be brief, electrohydrodynamics concerns itself with the study of how particles of a fluid interact with each other electrodynamically. Magnetohydrodynamics concerns itself with a conductive fluid and the currents induced in it by a magnetic field.

[Wyrm]

Both electrohydrodynamics and magnetohydrodynamics are the study of fluid dynamics under the influence of electromagnetic forces, to wit the dynamics of an electrically conductive fluid, like a plasma, or suspention of DNA. In the case of electrohydrodynamics, the fluids are moving under the influence of elecric fields, such as a DNA suspention in an electrophoretic gel. In the case of magnetohydrodynamics, the fluids move in response to a magnetic field, the movement of which produces electrical currents in the conductive fluid, and therefore applies a force to the fluid. MHD is behind the dynamo theory of the Earth's magnetic field.