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Q & A: flowing saltwater in magnets

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Most recent answer: 12/19/2012
Q:
i am doing an experiment "how magneticfield effect rate of flow of salt water ,distilled water and tap water".the results arethe flow rate of both salt and tap water are reduced when a magnetic field is applied at the bottom of the burette.the flow rate of salt solution is lower than that of tap water. can you please tell me the reason....
- nitika (age 18)
new delhi
A:
This is a tough question.

 One effect is certainly real. The saltwater has a higher viscosity (stickiness) than the distilled water, which makes it flow more slowly. It's also denser, which speeds the gravity-driven flow, but not quite by enough to make up for the viscosity, at least at a 1 M concentration. The tap water should be similar to the distilled water, just a tiny bit more viscous.


(5/9/13) A further update. Trying to be a little more quantitative, I've attempted the calculation in several ways. each gives an effective force density of very roughly B2vσ/c2 (in Gaussian cgs units), where B is the magnetic field strength, v is the speed of the flow, σ is the electrical conductivity, and c is the speed of light. (Note that in SI units, other messy constants appear). Now for reasonable values (B=1000 G, σ=1011/s, v= 100 cm/s) we end up with a force density of only 10-2 dyne/cm3. That's very low compared to plain old gravity (103 dyne/cm3 on water). So it's very hard to see the magnet having much effect. Maybe my first answer wasn't far off. If the flowing liquid were a very good conductor (say mercury) the answer would be quite a bit different.


New Answer here, notice major changes from my old answer below!: There should indeed be an effect of magnets on the flow. After trying to go through a calculation (see below) that implied almost no effect, I suddenly remembered (in the middle of the night, unfortunately) another major effect that I'd seen, not calculated. Old record turntables sometimes used non-synchronous induction motors allowing the user to adjust the speed by changing the friction on the turning platter. How did they do this? A magnet could be moved closer or farther from the metal platter. The electrical eddy currents it induced in the platter dissipated heat, just like any current, taking energy way from the mechanical system.

Similarly, there will be eddy currents induced in the moving saltwater, and these too will pull energy out of the falling water. In other words, they provide an extra friction source. I'll get back to this with a quantitative calculation but wanted to get this basic correction up right away.

How do I know this idea is right and my more specific calculation below isn't? Essential thermodynamics- friction always pulls energy out of mechanical systems- is extremely generally true. In contrast, detailed calculations often can drop some term or other.  So I know this has to work similarly to the old turntable.

Anyway, perhaps it will be philosophically instructive to leave in this old rambling.

Old Answer: "The magnetic effect is harder to understand. I believe that it will be small, but perhaps I've missed something important. Here's what should happen. Any vertical magnetic field should have very little effect. Then as the solution with positive and negative ions flows down in a horizontal magnetic field, the Lorentz force will push the positive (Na+) ions one way and the negative (Cl-) ions the other way. A very slight charge will build up so that the electric field counterbalances the Lorentz force.  The electric field magnitude will be small, e.g. if the magnetic field strength is 1000 G and the flow rate is 1000 cm/s, the electric field will be only 0.01 V/cm.

So far none of these forces are up or down so I don't see why they would affect the flow rate. At the bottom of the burette, however, there will be some vertical forces. The streams of slightly negative charge going down one side and of positive charge going down the other will have to converge to get out the narrow central opening. That makes a small horizontal current at right angles to the magnetic field. Unless I'm making a mistake, the Lorentz force on that current will be in the downward direction, which seems like it  might speed up the flow. (There would be a net upward force higher up, as the flow entered the strong field region, but since it sounds like the field is strongest near the bottom, I think the bottom force is stronger.) [Note: This part, where I was uncertain about the net sign, has to have been wrong.]However, I'd expect this effect to be extremely weak, not measurable in any home experiment.

Maybe the magnet has some other tricky effect slowing down the flow, but I don't see it. Do any readers have thoughts on this?"

Mike W.


(published on 12/19/2012)

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