How would you design this test/experiment ?

[Externet]

Regards.

A magnet moving immersed in seawater. How to sense/detect if creates some voltage/current presence by placing oscilloscope probes in the seawater ?

 

[Externet]

Correcting; meant to probe electrodes with an oscilloscope.

 

[BradtheRad]

Interesting concept. Salt water is a conductor. Perhaps eddy currents are formed near the magnet. Not easy to detect.

What if the seawater is in thin tubes, wrapped around the magnet? Spin the magnet to create current.
However there may be little or no voltage differential. That makes it hard to detect with an oscilloscope.

The old-fashioned compass might serve as an ammeter.

 

[betwixt]

Two solutions I can think of:

1. sense the change in magnetic field using insulated pick-up coils. Any change in field will induce a voltage in the coils.
2. use hall effect sensors in a watertight container.

As far as I'm aware, there is no significant attenuation of magnetic field in sea water so both methods should work.

Or am I getting the question wrong and you have to sense a voltage actually generated in the water itself?

Brian.

 

[danadakk]

How Sensitive Are Sharks to Electric Fields?

A recent show tried to explain how sensitive sharks are to electric fields...they almost got it right. Wired Science blogger Rhett Allain looks at the physics of sharks' sixth sense.

www.wired.com

Electric eels ability to stun

Electric eel - Wikipedia

en.wikipedia.org

Regards, Dana.

 

[Externet]

Thanks, gentlemen.
The test is to perceive on a bench the reality of what would happen 30m under open sea, on a magnet motion similar to a flag which may be very different from spinning. Adding tubes would not represent reality.

The electrical behavior of the seawater is what needs measurement. The change of the magnetic field is not of interest. There is no attenuation of magnetic field undersea. My diving compass works equally well at depths. A Hall sensor would sniff the magnetic field, not electrical potential as am after in submerged electrodes.

Did a couple of tests, moving a magnet in and out of a seawater container; and moving the water while exposed to a very strong magnetic field. Got no readings on the most sensitive scale connected to electrodes, not corresponding to intuition... Perhaps doing it wrong ?

As of eels, learned once they do shut their eyes tightly when 'discharging' to avoid self electrocution
and electrocution does not happen in seawater because it is much more conductive than the tissues of a fish and the preferred electrical current path stays largely in the much more conductive water.

 

[betwixt]

Thank you for clarification.

This is rather like working out the voltage induced in a wire if a magnet is moved INSIDE it rather than the field intersecting the wire.

Intuitively, I would say it is possible because like wire, the sea water is a conductor but the distribution of the voltage would be hard to define and lack of insulation would disperse the voltage widely making a differential hard to measure. There is also the issue of exactly how conductive the water is because salinity varies around the World and maybe even at different depths. My try would be to take two different measurements, each from a pair of electrodes but the pairs being spaced apart as much as possible. My reasoning is that if they are in the same environment it should be possible to subtract their measurements to get a theoretical zero. Rather like one being the 'control' sample and the other the actual one. Any discrepancy must therefore come from an outside influence, however small.

Brian.

 

[KlausST]

Hi,

an idea:
If you move a magnet in sea water... and it creates voltage and current, then this also means "power".
If it causes power (loss) , then one should be able to see this as mechanical force.
But I´m not sure if one can measure this "additional force" when one moves the magnett.

Next step.
If the moving magnet causes power ... does it cause something like eddy current? does it also cause force to the water?
In other words: if I have a box made of glass with sea water inside. And now make magnets outside (but close to) the box rotate, so that the magnetic field surely enters the sea water. .... does the sea water begin to rotate, too?

I don´t know the answer(s).

Klaus

 

[danadakk]

If you move a magnet in sea water... and it creates voltage and current, then this also means "power".
If it causes power (loss) , then one should be able to see this as mechanical force.

Or heat.....


Regards, Dana.

 

[Externet]

With magnetohydrodynamics;
Magnetism + electric current produces seawater motion.
Does seawater motion in an electric field produces magnetic field ? I believe yes.
Does seawater motion in magnetic field produces electric current ? I believe yes.

From intuition, if you have two interacting, the third develops.

Conceptually, the grey "insulating walls" below avoids short-circuiting the +,- electrodes by the conducting fluid. Without it, seawater would pose nearly a short circuit inside and outside the 'tunnel'. There will be current; but voltage would collapse greatly. Probing may be the hurdle.

Now, with the presence of the grey insulators, the fluid is still conductive. The current path can be anywhere around the insulators. Do not get that part .

What if the yellow electrodes do not touch the conductive seawater because they are fully wrapped in insulation... There will be no current. But will there be an electric field between the yellow electrodes as in a capacitor dielectric ?
If there is an insulated electric field; does it mean the voltage cannot be measured, probed nor its presence confirmed with an instrument, but it is theorically there ?

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Note to myself: Next: resistor in series to one electrode. Electric field is not electric current for MHD to happen

--- Updated Jul 21, 2022 ---

Thank you.

This is rather like working out the voltage induced in a wire if a magnet is moved INSIDE it rather than the field intersecting the wire.

Sort of the magnet descending inside of a copper pipe ? Where to measure voltage ?

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[FvM]

What if the yellow electrodes do not touch the conductive seawater because they are fully wrapped in insulation... There will be no current. But will there be an electric field between the yellow electrodes as in a capacitor dielectric ?

The setup is used for magnetic inductive flowmeters, isolated (capacitive) electrodes work well with AC magnetic field.

I don't see a direct relation to the original moving magnet question. It deals with a different issue, due to the rotational symmetry, it generates circular current but no voltage difference.

 

[BradtheRad]

Sort of the magnet descending inside of a copper pipe ? Where to measure voltage ?

---->

I did this trick and it really works. It's a demonstration of eddy currents. Power is generated however I've never seen a way to measure voltage of an eddy current inside the metal mass. Resistance is very low, thus voltage is very low. Nevertheless sufficient power is generated so it slows the magnet's drop inside the pipe.

Additional demonstrations of this principle:
* a magnet floating above a spinning metal platter (non-ferrous metal)
* a magnet halts a metal block just before impact.

Youtube has videos showing these effects.

The above can serve as thought experiments.

As for salt water, we might picture magnet motion producing eddy currents, yet for some reason too weak to get our attention and become cited by early experimenters as spectacular phenomena.