Magnetohydrodynamics - Wikiparency

# Magnetohydrodynamics **Magnetohydrodynamics** (**MHD**) is the study of how electrically conducting fluids move in magnetic fields. The fluid can be a liquid metal, salt water or an ionized gas, a [[Plasma_(physics)|plasma]]. When a conductor flows across a magnetic field, a [[Voltage|voltage]] is induced in it and an [[Electric_current|electric current]] flows. The current in turn feels a force from the field, so the flow and the field push on each other. Hannes Alfvén put the starting point in one sentence in 1942: "If a conducting liquid is placed in a constant magnetic field, every motion of the liquid gives rise to an E.M.F. which produces electric currents."[^alfven1942] MHD joins [[Fluid_dynamics|fluid dynamics]] to electromagnetism, and it describes systems from the liquid [[Iron|iron]] core of the [[Earth|Earth]] to the plasma in a fusion reactor. On the Thury spine, MHD is where fluid flow meets the noble branch. The plasma a fusion machine must hold is a fluid, and magnetic fields are what hold it. ## Etymology The name joins *magneto-* (magnetic field), *hydro-* (water, standing for fluids in general) and *dynamics* (motion). Alfvén's 1942 letter called the waves he predicted "electromagnetic-hydrodynamic waves."[^alfven1942] ## History Michael Faraday tried the first experiment in 1832. He stretched a wire across London's Waterloo Bridge, dipped its ends into the Thames, and tried to measure the current induced as the brackish river flowed through Earth's magnetic field. Small voltages from chemical effects at the electrodes hid the signal, but the idea was sound.[^stern][^velimsky2021] In 1942 Alfvén predicted the waves now named for him, and in 1970 he shared the Nobel Prize in Physics "for fundamental work and discoveries in magneto-hydrodynamics with fruitful applications in different parts of plasma physics."[^alfven1942][^nobel1970] ## Equations MHD combines the equations of fluid motion (the [[Navier–Stokes_equations|Navier–Stokes equations]] and the [[Continuity_equation|continuity equation]]) with the laws of electromagnetism. The link between them is the Lorentz force: charges moving through a magnetic field feel a force perpendicular to both their motion and the field. Andrea Mitofsky's text on direct energy conversion notes that the MHD effect rests on the same Lorentz force equation as the Hall effect; the difference is that the Hall effect happens in solid conductors and semiconductors, and the MHD effect in conducting liquids and plasmas.[^mitofsky53] ## Ideal MHD Ideal MHD treats the fluid as a perfect conductor. Alfvén's 1942 letter showed that in such a fluid a disturbance can travel as a wave, a combination of fluid motion and magnetic field.[^alfven1942] In a real fluid, electrical resistance lets the field slip through the conductor and dissipates currents as heat, and whether that matters depends on the size and speed of the flow. ## Structures in MHD systems The largest MHD structure most people meet is the Earth's magnetic field, generated by a dynamo in the planet's liquid iron outer core.[^stern] In 1995 Gary Glatzmaier and Paul Roberts produced a three-dimensional computer simulation of this geodynamo that sustained a magnetic field for more than 40,000 simulated years and then reversed its polarity.[^glatzmaier1995] ## Waves Alfvén waves are the waves Alfvén predicted in a conducting fluid threaded by a magnetic field.[^alfven1942] The University of Minnesota's space physics group studies Alfvén and ultra-low-frequency waves in near-Earth space.[^mifa] ## Extensions Where the motions of individual particles matter, the single-fluid description of ideal MHD is extended with kinetic models. Kinetic Alfvén waves are one example studied at Minnesota.[^mifa] Related effects follow from the same force law: Mitofsky groups MHD with the Hall effect and with the electrohydrodynamic effect, which uses electric rather than magnetic fields to move a fluid.[^mitofsky53] ## Limitations MHD devices face hard engineering limits. In the 1960s engineers tried to build MHD generators that passed hot plasma through a magnetic field, but the plasma had to be kept at 3,000–4,000 K and few materials could withstand those temperatures, so progress was limited.[^mitofsky53] ## Applications ### Geophysics The geodynamo is an MHD system, and simulations of it reproduce a self-sustaining field with reversals.[^glatzmaier1995] ### Space physics Spacecraft measure the electric and magnetic fields that MHD waves carry through near-Earth space. The electric field instrument on NASA's Van Allen Probes was led by John Wygant of the University of Minnesota.[^rbsp-efw] ### Magnetic confinement fusion A tokamak confines a plasma with magnetic fields in the shape of a torus, and the first tokamak, T-1, began operating in Russia in 1958.[^doe-tokamak] ### Engineering An MHD generator produces electric power from the interaction of a moving fluid, usually an ionized gas or plasma, with a magnetic field.[^britannica-mhd] Run in reverse, the same interaction becomes a pump or a thruster, working like an [[Electric_motor|electric motor]] whose moving part is the fluid itself. Japan's *Yamato-1* became the world's first superconducting MHD-propelled ship on its trial runs in June 1992.[^yamato] At small scale, room-temperature MHD pumps and valves now move conducting liquids in microfluidic systems.[^mitofsky53] ## Minnesota *This section is specific to Wikitube.* The Minnesota Institute for Astrophysics lists Sun–Earth interactions and space physics among its research areas, including heliospheric plasmas, radiation belts, magnetic reconnection and Alfvén waves.[^mifa] John Wygant of the University of Minnesota was principal investigator for the electric field instrument on NASA's Van Allen Probes, which measured the fields in Earth's radiation belts.[^rbsp-efw] ## See also - [[Plasma_(physics)]] - [[Fluid_dynamics]] - [[Navier–Stokes_equations]] - [[James_Clerk_Maxwell]] - [[Helium]] ### Further reading - Mitofsky, Andrea M. (2018). *Direct Energy Conversion*, version 1.0.0. — on the [[PORTAL_Energy]] and [[PORTAL_Hydrogen]] book shelves; §5.3 "Magnetohydrodynamics." ## References [^alfven1942]: Alfvén, H. (1942). "Existence of electromagnetic-hydrodynamic waves." *Nature* 150: 405–406. https://doi.org/10.1038/150405d0 [^nobel1970]: Nobel Prize Outreach. "The Nobel Prize in Physics 1970." https://www.nobelprize.org/prizes/physics/1970/summary/ [^stern]: Stern, David P. "The Dynamo Process." NASA Goddard Space Flight Center. https://pwg.gsfc.nasa.gov/Education/dynamos.htm [^velimsky2021]: Velímský, Jakub; Schnepf, Neesha R.; Nair, Manoj C.; Thomas, Natalie P. (2021). "Can seafloor voltage cables be used to study large-scale circulation? An investigation in the Pacific Ocean." *Ocean Science* 17: 383. https://doi.org/10.5194/os-17-383-2021 [^mitofsky53]: Mitofsky, Andrea M. (2018). *Direct Energy Conversion*, version 1.0.0. Trine University. §5.3 "Magnetohydrodynamics," pp. 96–97. https://open.umn.edu/opentextbooks/textbooks/direct-energy [^glatzmaier1995]: Glatzmaier, Gary A.; Roberts, Paul H. (1995). "A three-dimensional self-consistent computer simulation of a geomagnetic field reversal." *Nature* 377: 203–209. https://doi.org/10.1038/377203a0 [^mifa]: Minnesota Institute for Astrophysics. "Sun-Earth Interactions and Space Physics." University of Minnesota. https://cse.umn.edu/mifa/research/sun-earth-interactions-space-physics [^rbsp-efw]: University of Minnesota Space Physics. "RBSP-EFW." https://www.space.umn.edu/missions/rbspefw-home-university-of-minnesota/ [^doe-tokamak]: U.S. Department of Energy, Office of Science. "DOE Explains...Tokamaks." https://www.energy.gov/science/doe-explainstokamaks [^britannica-mhd]: Jackson, William D. "Magnetohydrodynamic power generator." *Encyclopaedia Britannica*. https://www.britannica.com/technology/magnetohydrodynamic-power-generator [^yamato]: Sasakawa, Y.; Takezawa, S.; Sugawara, Y.; Kyotani, Y. (1995). "The superconducting MHD-propelled ship YAMATO-1." In *Fourth World Congress on Superconductivity*, vol. 1. NASA. https://ntrs.nasa.gov/citations/19960000249  **Part of the [[WT!Thury_Hydrodynamics_Compendium|Thury Hydrodynamics Compendium]]** — main article for section 27, *Magnetohydrodynamics*. Related sections: [[Vorticity]] · [[Nuclear_fusion]] · [[Geophysical_fluid_dynamics]].   **Microsim — three.js (Wikitube framework):** *Magnetohydrodynamics* <div class="wt-sim" data-src="https://wikitube-3d-microsims.netlify.app/thury/Magnetohydrodynamics.html" data-title="Magnetohydrodynamics"></div> *Built from `MICROSIM_GUIDE/specs/sims/Magnetohydrodynamics.json`; part of the [[WT!Thury_Hydrodynamics_Compendium|Thury Hydrodynamics Compendium]] set.*  ## Wikipedia : Wikitube **Strict pair:** [Wikipedia](https://en.wikipedia.org/wiki/Magnetohydrodynamics) : [Wikitube](https://en.wikitube.io/wiki/Magnetohydrodynamics) · pinned revision [1360636092](https://en.wikipedia.org/w/index.php?oldid=1360636092) · 2026-09-10 ## Previous hub tags Hubs: `Life_Physics`, `Systems`. Portals: [[PORTAL_Thury_Hydrodynamics_Apex_Spine]], [[PORTAL_Physics]]. --- *Thury main articles, wave 2 · 2026-09-10 · drafted · Compendium section 27 · sim pending THY-036.*