Magnetohydrodynamics, reversed...

[Externet]

Passing a conductive fluid as seawater trough a permanent magnet pipe; will it create electrical current ?

How to 'collect' that current ?

[JonG]

Passing a conductive fluid as seawater trough a permanent magnet pipe; will it create electrical current ?

How to 'collect' that current ?

I would expect it to separate the positive ions (Na⁺) from the negative chloride ions (Cl^-) and these ions would move transverse to the direction of flow and also at right angles to the magnetic field. Sodium ions would go one way and chloride ions in the opposite direction. It is similar to, but not the same as, the situation observed with the Hall effect in a semiconductor. This would be expected to give rise to a potential difference at right angles to the direction of flow.

 

(note: the direction of the magnetic field should be perpendicular to the direction of flow)

Edited October 12, 2014 by JonG

[Enthalpy]

It does work, including with seawater, and is computed with the usual speed times induction gives electric field - with a vector product hence angles.

 

It is used on boats to measure the speed. Though:

- It gives the local water speed, as disturbed by the hull if measured nearby

- the voltage is small but metal cleanliness introduces important DC errors

- the standard answer is to have an AC magnetic field, at moderate frequency to limit the direct induction by the coil

 

0.1m/s=0.2knt resolution and 0.1T and 0.1m need 1mV resolution, which is very little for dirty electrochemistry where 0.1V unexplained variations are normal. AC field improves that; you could rotate a permanent magnet using a coil.

 

It looks simple, but a helix loch is still much in favour.

[JonG]

It is used on boats to measure the speed. Though:

- It gives the local water speed, as disturbed by the hull if measured nearby

- the voltage is small but metal cleanliness introduces important DC errors

- the standard answer is to have an AC magnetic field, at moderate frequency to limit the direct induction by the coil

 

I've read a little about it since responding to the original post and have been surprised at how the effect has been observed. For example:

 

The ebbing salty water flowing past London's Waterloo Bridge interacts with the Earth's magnetic field to produce a potential difference between the two river-banks.

 

from: https://en.wikipedia.org/wiki/Magnetohydrodynamics

 

I wouldn't have expected that to be measureable.