Magnetohydrodynamics
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Magnetohydrodynamics
Other names for this field include hydromagnetics, magnetogasdynamics, and magnetoplasmadynamics. However, all of these terms are somewhat lengthy, so the abbreviation MHD is commonly used.
In any case, magnetohydrodynamics is an academic discipline within the field of fluid mechanics. MHD investigates the interaction between electrically conductive fluids and electromagnetic fields, with a primary focus on the interaction between the medium and the magnetic field.
It's important to remember that the concept of a fluid in physics is much broader than that of a liquid. A fluid is any substance that can flow, meaning it can change its shape depending on the container it occupies. Liquids are fluids, but so are gases. Fluids also include multiphase mixtures such as foams, emulsions, suspensions, and pastes.
Magnetohydrodynamics investigates the interaction between electrically conductive liquids and ionized gases with magnetic fields. It also covers the design of plasma engines, magnetohydrodynamic generators, high-speed aerodynamics, and even the physics of controlled thermonuclear fusion and cosmic magnetism.
What Can We Do With It?
To conduct the experiment, you will need the following materials:
- Copper wire—approximately 0.3mm (29 AWG) in diameter
- Ring magnet—such as one taken from an old speaker
- Electrolyte—water with a small amount of table salt
- Container
Once you have all the necessary materials, you can start assembling the experiment.
After stripping the insulation from the wire, form it into a ring around the magnet:
Next, place the magnet with the ring electrode into the container filled with electrolyte. Insert a central electrode, also made of stripped wire, into the middle of the setup:
Now you're ready! Apply a voltage of a few to several volts to the electrodes. You will observe the liquid swirling around the central electrode. The direction of rotation depends on both the direction of the magnetic field induction and the direction of the current flow. The video below demonstrates the effect (a drop of ink was added to the water for better visualization).
Another variation of this experiment involves building a simple flow motor that operates on the MHD effect. Its cross-sectional view is shown below:
This motor is designed as a channel with two walls made of metal plates acting as electrodes. When voltage is applied, an electric field is generated in the electrolyte between the electrodes. Perpendicular to this electric field are the lines of magnetic field induction created by magnets. As a result, the liquid is accelerated along the channel, enabling the construction of electric motors without moving parts, suitable for powering watercraft models. The video below demonstrates a microscopic model of such a motor in action:
As you can see, the liquid is rapidly accelerated within the motor and expelled from the opposite side.
Enjoy experimenting, and have fun!
Further readings:
- Bansal J.L., Magnetofluiddynamics of Viscous Fluids, Jaipur Publishing House, Jaipur, India, 1994
- Barbu V., Exact controllability magneto-hydrodynamic equations, Communications on Pure and Applied Mathematics, 2003, 56, pp. 732–783
- Haverkort J.W., Magnetohydrodynamics short introduction for fluid dynamicists, Magnetohydrodynamics, 2009
Marek Ples