Magnet in seawater...

[Externet]

Hi.

A plain magnet, fully isolated from the seawater it is immersed in; as inside a plastic hermetic box.

 

The magnetic field is present in the seawater. Now one of these things happen: Either the magnet is made to spin or move, or the seawater is put to flow around it.

 

Does the electrically conductive seawater experiences an electric current in the vicinity of the magnet ?

 

If there is no motion at all relative to each other, will there be no electric current ?

[Anders Hoveland]

There will indeed be electrical current (or in this case movement of ions) induced in the water.

A static magnetic field can only induce eddy currents, and no useful electrical energy can be extracted.

There are actually quantum mechanical reasons that these eddy currents are not subject to the type resistance relating to common electric current which produces heat. These eddy currents have very physical effects (Lenz's law, diamagnetism), but in some ways they do not behave like normal electric current.

 

But so far, no useful energy has been derived from a stationary magnet.

 

As incredible as it sounds, a stationary magnet will indeed induce the movement of ions in salt water, and this will essentially exhibit superconducting/superfluid-like phenomena. The specific reasons involve some calculations and science that are too complex for me to get into here. But the basic explanation is that, as frustrating as you may find this, "work cannot be extracted from nothing".

[Mosheh Thezion]

A magnetohydrodymamic engine uses this process to drive silent submarines as used in that movie "hunt for Red october."

neat stuff.. and very real.... http://en.wikipedia.org/wiki/Magnetohydrodynamic_drive

 

-Mosheh Thezion

[Externet]

Thanks, gentlemen.

 

... a stationary magnet will indeed induce the movement of ions in salt water...

 

Anders... would that be the case for an insulated magnet not contacting the seawater ? If yes, where from/to would the ions move ?

 

Am trying to discern if the magnets in seawater work in this situation only if there is motion that causes electrical current, and not because of the magnetism :

http://science.howstuffworks.com/environmental/life/zoology/marine-life/shark-magnet1.htm

[Enthalpy]

...an insulated magnet not contacting the seawater ?

No difference. A varying magnetic field needs conductivity along the path it is to induce a current, that's all.

In nearly any electric machine, the magnets or electromagnets are carefully insulated from the circuit where current is induced.

 

That is, a magnetic field needs no material medium at all and works in vacuum as well.

A static or slow magnetic field decreases steeply over distance so the magnet should be kept near.

An electromagntic field - which varies quickly - can cross huge distances, like billions of light-years for light coming from stars.

 

A static magnetic field can only induce eddy currents

Hi Anders, nice to see you!

 

The expression "eddy current" is more commonly used for a very real macroscopic current, resulting from a varying magnetic field, that flows in a closed loop within some conductor and not through an external circuit, and does have power to produce heat: unwanted losses in transformer cores, desired braking force...

 

But if the magnetic field is static, you get only effects like para and diamagnetism as you describe. Plus, in the case of type 1 superconductors, induced currents (see Meissner effect) which are macroscopic but give no power for free, sure.

 

Fun: in the Meissner effect, currents repel the induction out of the material even if the induction existed before the superconductor was cooled, so this is not a simple consequence of zero resistance and induced voltage.

 

A magnetohydrodymamic engine uses this process to drive silent submarines... and very real...

I've seen no single piece of evidence that MHD has been used to propel a submarine.

 

I've only heard of the Yamato, which did navigate, but had the very bad efficiency expected from any MHD boat, which must discourage any submarine designer.

 

Plus: submarine design puts a huge effort in minimizing any magnetic field, and there MHD is the wrong direction.

 

Add the detectable trailing heat produced by huge losses: my bet is that designers improve screw propellers, or have non-screw mechanical propellers.

 

Anyway, pictures of recent submarines are public, and they hide only the propellers, whose position and size are compatible with screws.