Magnet in conductive media...

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Externet
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Magnet in conductive media...

Post by Externet » Fri Jun 08, 2012 6:05 pm

Hi all.
A moving magnet does create electrical currents in surrounding looped conductors; we know that.

If the magnet is submerged in a conductive fluid, call it mercury, seawater... Will the fluid bear currents around the moving magnet ?
Try not to jump to magnetohydrodynamics, just to this simple topic. Thanks.
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Re: Magnet in conductive media...

Post by Robert Reed » Fri Jun 08, 2012 9:27 pm

I do recall several years ago reading about the USN having developed a vessel speed indicator using an underwater magnetic device. Sketchy on details but had no moving parts. Maybe it relates to your question.

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Re: Magnet in conductive media...

Post by MrAl » Sat Jun 09, 2012 6:09 am

Hi,


If you hold a bar magnet on the left and a bar magnet on the right and the north pole of the magnet on the left facing the south pole of the magnet of the right forming a gap between the north pole and the south pole, then spin the two magnets counter clockwise with the gap center as the axis of rotation, you'll get a current flowing past the north pole away from you and a current flowing past the south pole flowing toward you at any point in time. That's just the generator effect. Since the currents are connected by a partially conducting fluid, you'll see some interaction between currents. Since the magnets are separated the two currents wont cancel immediately but may partially cancel after a full rotation. There is going to be a time lag between when one current is generated in a given location and when the opposite current is generated in that same location. This would mean at any given point in space (in the medium) the current would look like AC.

The generator effect will take place whenever a magnetic field is moved from one location to another, and there is some conducting medium nearby. Normally a wire is moved past a magnet or a magnet is moved past a wire at right angles to the wire and that forces an electron current to flow in the direction depending on the direction of movement.

So for a conducting fluid it would probably be like moving a magnet through a tub of conducting buck shot BB's :smile:
The BB's conduct but the resistance between BB's gives them some degree of isolation from each other too.

A horizontal bar magnet with north on the left and south on the right, moving down the page, would produce a current into the page on the left and out of the page on the right, given some conducting medium exists near the magnet.
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Re: Magnet in conductive media...

Post by EPA III » Sat Jun 09, 2012 3:42 pm

The simple answer to your question is yes.

When a conductor is placed in a magnetic field and there is relative movement, a potential or EMF (Voltage) will be generated. If there is a path to support it, current will flow.

Now, exactly what will happen in a real world situation is more complex because the fluid, by definition, can flow and it will. This is because the very current you are generating and which is in the magnetic field will then cause a force that will move the liquid. But you wanted the discussion to be limited so I will skip the calculus.
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Re: Magnet in conductive media...

Post by Dean Huster » Sun Jun 10, 2012 6:36 pm

One would think that a rare-earth magnet in a volume of mercury would act similar to a dead-shorted generator and be very difficult to move around in that medium. I have maybe a pound of Hg, but that's not enough for an experiment like that. And who wants to risk spilling that crap?!
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Re: Magnet in conductive media...

Post by Externet » Sun Jun 10, 2012 6:53 pm

The test could be done instead in plain water, measuring the torque (or motor current) needed to spin the magnet.
Then, dissolving table salt into the water, if such effect of 'dead short' behavior is effective, the torque/current reading to spin the magnet would increase, wouldn't it ?

Or any other suggestion to do the evaluation/confirmation ?
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Re: Magnet in conductive media...

Post by MrAl » Mon Jun 11, 2012 6:27 pm

Hi,

Salt would make the water more dense so it should increase the required torque to maintain a given speed regardless of the other effects.

There would definitely have to be an electric field set up which would give rise to at least a small current.

A dead short is not a good description because everything has resistance, even short things and things that conduct very well. Thus, we'd have to analyze the conduction pattern. If you take a cube of copper and connect leads to two opposite corners and apply a current, how does that current travel though the cube. It's not going to be the same everywhere because there is resistance along the line of current flow but also at 90 degree angles to that current flow. It's like sheet resistance only in 3d. So this is a similar problem. It's just a matter of plotting the field and the current flow.

You can actually plot the field in a pan (non conducting) of water. Tap water conducts a little even without additives, distilled water a bit less. I've done experiments where a power resistor is submerged in water to see how much better it's power handling ability becomes. It conducts a little but not much. A little salt would make it a little more conductive.
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Re: Magnet in conductive media...

Post by haklesup » Thu Jun 14, 2012 5:18 pm

I expect that currents would be induced but they would immedietly be shorted within the surrounding fluid probably causing some circulation of the fluid particles in a complex and virtually unpredictable manner. A reaction force probably would not be created since the fluid would not stay stationary long enough for the force to be applied. The fluid would move before the magnet would feel a significant reaction to the back EMF. If constrained in a container with sufficient friction, a reaction force might be felt.

A bit different but you might look to the way ferrofluids are used in electronically adjustable shock absorbers. I suppose this is not unlike the reverse of how the earth's magnetic field is formed by a rotating conductive liquid at the core and the complex churning this induces.

In most systems where a magnetically induced current is useful, the media which that current is induced is restricted such as a coil or wire or a metal plane nearby. How would one employ the right hand rule for example in a non structured fluid. How would the magnetic force from various magnetic domains in the fluid add up if attempting to add them moves them apart or rotates them away due to magnetic repulsion. There would ne a net force of Zero unless the fluid were constrained

I suspect the effect is mainly only useful in solid metals. A cool demonstration is dropping a rare earth magnetic ball down a copper pipe, it will move in slow motion on the way down. The salt water experiment would probably work better if the water was seeded with either a drop of food coloring or some plastic particles so you could see if the magnet induces unusual fluid dynamics as compared to a similar shaped non magnetic object. I doubt measuring torque would be easy because any change would be small even in salt saturated water. Salty jello might be interesting (if it can even solidify with that much salt). Switching an insulated electromagnet on and off in still salty water might also reveal if any effect is significant.

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Re: Magnet in conductive media...

Post by Dean Huster » Tue Jun 19, 2012 6:48 am

A dead short is not a good description because everything has resistance, even short things and things that conduct very well.
Oh, gimme a break, Al. I'm obviously comparing an unloaded generator to one where you've strapped a 0.00247 ohm load to the output terminals.
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Re: Magnet in conductive media...

Post by haklesup » Tue Jun 19, 2012 6:01 pm

A Dead Short only has exactly 0 ohms resistance in textbooks (IMO it's better called a Perfect Short ). A Dead Short is generally regarded by any engineer as the lowest practical resistance in a real circuit and is understood to be near zero but non zero. By and large, engineers have a good sense of "Close enough" unlike scientists. There is no exact value for close enough, it depends on the accuracy of the information you need.

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Re: Magnet in conductive media...

Post by MrAl » Thu Jun 21, 2012 4:06 pm

Hi again,


I did not try to state that a dead short is not close to some small value of resistance, i meant to try to draw attention to the voltage drop as being non zero. If we have a zero voltage drop then we have no field to analyze, but if we assume at least some voltage drop then we have a field and thus a field pattern that can be examined. We can not get a true dead short at any time (zero resistance) because of nature, and that's a good thing because with a little resistance we can check out the voltage drops and see what is going on.
So it's just from the point of being able to analyze the effects, not that the resistance is not close to some low value.
A real resistive load for example might be 100 ohms, so 1 ohm would be considered a short on the output. A real normal load of 10 ohms might be ok even if dropped to 1 ohm, so maybe 0.1 ohms would be considered a short. But a real normal load of 1 ohms might need as low as 0.01 ohms to be considered a short. We can take this as far as we want to illustrate that a dead short is a relative thing. It can be relative to the application at hand or even to the theory in mind.

Case in illustration:
We take 12 resistors and connect them end to end forming a cube of resistors. That's one cube where each resistor makes up one edge of the cube.
Now we make 999999 more of these cubes (1 million total) and connect them side by side allowing those cubes that meet edge to edge share the same resistors rather than use two for one edge, and we do this forming a much larger cube of cubes of resistors with 100 cubes on each edge. That makes one big cube shaped 3d grid of resistors.
Now we energize this construction plus and minus at two of the farthest apart corners (distance wise).
The question is, what is the voltage at the node at the 39th cube corner in from all directions from the most negative node at the corner of the whole cube?
Here's where the difference comes in...
If we allow each resistor to be 1 ohms each, we can analyze for that voltage and figure out what it will be. If we allow each resistor to be 0.1 ohms each we can still analyze for that voltage and figure out what it will be. If we allow each to be 1e-6 (1 micro ohm) we can still analyze that circuit to find out what that voltage will be. Even if we allow the resistance of each resistor to be 0.00000000000000000000000000000000000001 ohms each, or even 1e-999 ohms, we can still figure out what that voltage will be. However, if we allow the resistors to go all the way to an exact zero ohms, then there is no voltage drop so we cant calculate what the voltage is because that means there is no voltage drop across the whole thing.
Now we might be tempted to say, "Well, it has to be zero then so that's the result". But what we are really after is the potential at each point in space in a REAL medium, and zero volts doesnt help at all with that problem, and that means we have to use at least some resistance in order to get an answer.
I hope this clears up what my point really was here :smile:
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Re: Magnet in conductive media...

Post by Bob Scott » Thu Jun 21, 2012 4:07 pm

Externet wrote:If the magnet is submerged in a conductive fluid, call it mercury, seawater... Will the fluid bear currents around the moving magnet ?
I remember the design of a turntable braking mechanism (1950?) for speed control, from one of those online old Popular Science magazines at books.google.com... Underneath the turntable was an aluminum disk that rotated with the turntable. The gap of a stationary magnet was inserted across the disk, causing braking action. That is, the rotating disk was between a small gap between the magnet poles. I guess that the free electrons in the metal disk would rather go around the gap than through it.

Charged particles tend to follow magnetic lines or they get diverted. Charged particles from the solar wind get caught in the Earth's magnetic field and (as theory goes) bounce back and forth between Earth's North and South poles, riding on the magnetic field lines, causing aurora displays.

It is one of the subjects that interests me lately. There is a rule that you learn in first year engineering . An electrical current in one dimension with a magnetic field 90 degrees to the current, causes force in the third dimension, in quadrature with the other two dimensions. It's how loudspeaker voice coils work. It's how a CRT's electron beam gets deflected. I find this not intuitive at all (that the force goes off in a completely different direction). I guess I just have to accept that it is one of the laws of nature.

I think that if a magnet was inside a pool of mercury and the mercury were a superconductor, the pool would tend to solidify. The particles would resist any movement across field lines. But since mercury is not a superconductor, it will just get viscous or like jelly? That's my guess.
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Re: Magnet in conductive media...

Post by Bob Scott » Thu Jun 21, 2012 4:42 pm

MrAl wrote:I did not try to state that a dead short is not close to some small value of resistance, i meant to try to draw attention to the voltage drop as being non zero. If we have a zero voltage drop then we have no field to analyze, but if we assume at least some voltage drop then we have a field and thus a field pattern that can be examined.
Maybe I'm misunderstanding, but there can indeed be a field or current with zero voltage.

Consider a copper ring. You move a bar magnet through the ring. as the bar passes, currents are induced in the ring. There is absolutely no voltage involved. If there were, where would you place the voltmeter probes?
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Re: Magnet in conductive media...

Post by Robert Reed » Thu Jun 21, 2012 7:59 pm

Which begs the question:
Can you actually have current without an electric field to cause it?
Can you actually have voltage without the current flow to produce it?

What came first - the chicken or the egg?

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Re: Magnet in conductive media...

Post by sofaspud » Fri Jun 22, 2012 3:50 am

Hmmm..... I'll have to ask Eli the ice man.

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