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Homework Help: Theoretical generator

  1. Aug 7, 2008 #1
    This is not a homework problem, just the musings of a bored mind.

    Suppose I plumbed a loop from some non-conductive pipe, say PVC. In this loop I placed a pump, also non-conductive. Let's say I place the pipe in a powerful magnetic field, perpendicular to the flow of fluid in the pipe. If I fill the plumbing with mercury and run the pump, a voltage should appear as the mercury passes through the field.

    My question is does the potential created only drive a current through the entire length of mercury, thereby short circuiting the whole thing, or if I place a couple of electrodes in the flow, perpendicular to the field and the flow, will I get a usable current?

    Part of my mind wants to say that the voltage appears at right angles to the flow and field, and should be extractable there. But another part says that because the mercury is a conductor, the whole shebang would be shorted. What do you think?
  2. jcsd
  3. Aug 8, 2008 #2


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    Staff: Mentor

    Not sure I'm visualizing this correctly, because I'm not seeing any voltage generated. If you change the amount of flux going through the loop, that will induce an EMF and thus an electrical current in the mercury loop. Having metal move through a uniform B-field doesn't induce anything that I can see.
  4. Aug 11, 2008 #3
    I thought that a conductor moving perpendicular to the flux induces an EMF in the conductor? Changing the flux or moving the conductor is equivalent, no? I figured this principle would be basically the same as an MHD generator.
  5. Aug 11, 2008 #4


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    Staff: Mentor

    Moving a conductor through a uniform magnetic field will not do anything. And an MHD generator is dealing with charges, not with balanced charges like in a metal.
  6. Aug 11, 2008 #5
    Forgive my ignorance, but if a conductor cutting across lines of force doesn't induce an EMF, how do generators work?
  7. Aug 11, 2008 #6


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    A generator works by varying the flux through the coils of wire. That's why the coils move past opposite polarities of magnet poles as the rotor spins around.

    If you have a big flat field (doesn't vary much with position), moving the coil does not vary the flux cutting through the coil. Spinning the coil inside a flat field so that the flux reverses every half turn does induce an EMF. Does that make sense?
  8. Aug 11, 2008 #7
    I see what you are saying now. I can picture the coils cutting sort of diagonally (from 0 to 90 degrees, anyway) across the lines as the armature spins, instead of perpendicular. OK, so if the pipe with the mercury flow cut a path at an angle 0<x<90 to the field, would this induce an EMF?
  9. Aug 18, 2008 #8
    You must have been reading my mind-- I've been wondering about a similar 'thought experiment'. Actually, the previous explanation was good, as far as it goes, for conventional generators, but ANY conductor moving within a field will provide a current: ANY field moving within a conductor (say, a hollow coil) will create a current.

    Simple toy mercury-pool motors are easy to construct- do a net search for "Barlow mercury motor", "Faraday mercury motor", or "mercury pool motor". You could even do your 'thought experiment' for real-- there are loads of cheap, powerful permanent magnets out there.

    Try it with salt-water-- pump salt water through a hula-hoop. Tape a couple of magnets to one side, and wrap a few dozen turns of wire (coil)around the other side. When you turn on the pump, I'll bet you see voltage on the ends of the coil-- it should vary with pump speed. There are also patents out there on mercury-pool dynamos.

    I'm betting you like the 'closed loop' idea, because your 'theoretical generator' doesn't use a pump at all-- Mercury boils at 357 degrees C (about 675 degrees F)-- well within the range of a relatively small parabolic dish (say, an old-style satellite dish?) Am I, as they say, 'getting warm?'

    I'd love to have you do the salt-water version, and post your results here-- should only be a couple of hours of work to Mickey-Mouse something that's testable, if you have a pump and a good VOM, scope, or galvanometer!

  10. Aug 18, 2008 #9
    It sounds like your describing some type of cold magnetohydrodynamic (MHD) generator.

    EDIT: Woops, it seems this was already mentioned up top. Ignore this.
    Last edited: Aug 18, 2008
  11. Aug 18, 2008 #10
    simple mercury motor

    Probably the worlds simplest motor, with no SOLID moving parts-- I was doing some research (as a teen, over 30 years ago) in my parent's cellar. I took a piece of vinyl tubing (about 1/4" diameter-- as I recall, it was aquarium tubing) about 3" long, and sealed one end of the tube with a bolt. Poured about 1 3/4" of saturated saline solution into the tube, followed by a small glob (about 1/4" long) of mercury, and then filled the remaining portion of the tube with saline. I then plugged the opening with another bolt.

    I mounted the tubing like a smile, with the mercury drop at the bottom of the "U", connected to the bolts by the conductive saline. Ran a current throught the bolts. That little drop of mercury spun around like it was on crack!

    DIGRESSION TO FOLLOW..... Well, after watching this thing in amazement for a few minutes, I noticed a little green precipitate beginning to form around the boundaries of the mercury and the saline solution. Did a little thinking, and realized I'd probably created mercuric chloride (This was 30 years ago, so forgive me if I'm a little hazy....).

    I knew this was bad stuff, but I remembered that the local Fire Department had "hazardous waste days" where they would accept your toxic stuff. Figuring I could get rid of it before my parents came home, I called the fire department business number to inquire how to deliver it to them. (I was just gonna ride my bike down, and drop it off. ) They said they couldn't handle it, and gave me a phone number of 'the proper people' to call.

    I'm pretty sure this predated the EPA, but I called the number the FD gave me, and the people I talked with absolutely freaked! They wanted to come down and cordon off the house, bringing trucks, and guys in hazmat suits. I tried to explain I had only made a couple of milligrams, but they let me know this was a Very Serious Incident, and kept demanding my address. I didn't want my parents coming home to a scene like that, so (in those pre-caller-ID days) I played the 'chicken card', and hung up the phone.

    I was terrified! I kept thinking the FBI was gonna raid the house with a squad of guys wearing aluminum-foil suits -- and in my few years as a young amateur scientist, I'd already caused my parents to accumulate more than a couple of gray hairs! I brought the sealed tube to school with me the next day (something you wouldn't wanna do nowadays!), and explained my predicament to Dr. Gregory, the chairman of the science department. He took it off my hands and promised to rendre it harmless and dispose of it through a local college's chem department-- after a stern warning to seek the advice of a qualified teacher before conducting further experiments.

    In retrospect, I'm sure he found the whole thing hilarious, but it kept a lid on my experimenting for a few weeks!!

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