Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Magnetic field due to flowing charged fluid

  1. Apr 10, 2015 #1
    Hey, I've been wondering about the behavior of flowing charged particles lately. In particular, imagine that I have a tank of charged fluid.

    Lets assume this charge density is 1 C/cm^3. I then allow the fluid to flow through a pipe of 1 cm^2 cross-sectional area at a rate of 1 m/s. Thus there is a charge of 100 C flowing past a fixed point on the pipe per second. My question is, would this flow produce a magnetic field equivalent to a 100A current in a solid conductor?

    I've read up on magnetohydrodynamics but it doesn't seem to answer this question on any of the sources I've found.
  2. jcsd
  3. Apr 10, 2015 #2

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper

    Why not work it out?

    Note: moving charges = current so 100C/s of fluid is the same as 100C/s from any process.
    Only the geometry of the charge makes a difference ...
  4. Apr 10, 2015 #3
    Well yes this is what I thought would be the case but I was not sure so I asked. Follow up question, what would be the best way to charge a fluid? Would a static electricity charging source work best, or is it possible to create a charge imbalance of the type I mentioned using a battery?
  5. Apr 10, 2015 #4

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper

    Typically you would use a fluid composed of ionized stuff... you know: chemistry.
    Some people make a suspension if statically charged metal dust in something else.
    What is this for?
  6. Apr 10, 2015 #5
    The idea is to try to create a strong magnetic field using a fluid rather than a solid conductor so as to magnetize a central region.

    The question is how can I create a net excess of electrons in the fluid so that the fluid itself has a net charge.
  7. Apr 10, 2015 #6

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper

    You don't have to tell me if you don't want, but that limits how much I can help you.

    I gave you two typical approaches to make a charged fluid in post #4 ... extrapolate from there.

    You can usually get stronger magnetic fields using solid conductors - they hold more charge for their volume, the charge is well behaved, and you can get bigger currents for the available charge density. There is a reason electromagnets are made out of copper wire. What is the problem you are trying to solve?
  8. Apr 10, 2015 #7
    No I'm not hiding anything lol that's what I'm doing. The central region could be and iron core or a permanent magnet, the idea here is to see if it is possible to create a magnetic field capable of saturating (or magnetizing) a core by essentially using physical forces to drive an electrostatic charge.

    From what I can tell, such a method would not be prone to the same I^2 R heating losses as a current driven by a potential difference (though there would be mechanical losses and back EMF still).

    I am not saying this is more efficient than just pulsing a current, I just want to try this out and see what happens. In the case of the fluid, the idea was to drive the charges using a pressure differential. In a solid, I suppose one could consider a hollow copper cylinder, electrostatically charged below the breakdown voltage of its containing medium, being spun around its central axis by an external motor. The iron or magnet would be placed in the core of the cylinder. Wouldn't this be an equivalent situation?
    Last edited: Apr 10, 2015
  9. Apr 10, 2015 #8

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper

    OK- since it's a wide open "lets see what happens" type setup, your best bet is just to use whatever ionizable or chargeable material you have to hand.
    You want the early attempts to be cheap because they are probably going to be unsatisfying but you will learn from the results.
    Welcome to science.

    You can work out the magnetic field due to a spinning cylinder of charge.
    Generally you want to charge an insulator rather than a conductor because you want mechanical forces on the material to move the charges around.
    There are lots of geometries - you could charge a graphite ball on a string and set it in motion as a conical pendulum.

    hat you really want here, though, is a target action: not all magnetic fields are equal so what do you want the field to do? Just saturate an iron core? You can do that by making a small enough core ... so there is still stuff left out that will stop you from doing a good experiment. You need to write a well defined aim down before you start. This is what is missing. Once you have that, a great deal will follow automatically.
  10. Apr 10, 2015 #9
    Hi Simon,

    You are right that since this is an experiment it would be best to set up proper controls and define a clear aim.

    I have attached a sketch of my proposed setup to help with my description of the experiment. Since the amount of charge I can accumulate will be a function of the material's surface area, I think that a powdered material would be best so as to maximize the surface area/volume ratio. Powders are also well known to easily pick up static charges during handling. The powder will be held in the walls of a hollow cylindrical case. The diagram shows the charging apparatus (the belt and pulley system used in a van de graff setup) which induces a charge on an electrode connected to the powder through a hole in the case. It occurred to me as well that as long I was using a solid core material I could achieve the same relative motion by spinning the core rather than the case, which I think may be more stable as vibrations induced by spinning the case may cause some charge loss (I have no idea if its true but it still feels like a cleaner solution). A hall sensor placed at a fixed position above the core will measure the B field.

    The aim is to determine the magnetic field induced within a fixed volume by physically moving an electrostatic charge, I will keep the following constant between runs:

    - size of the core
    - size of the case
    - belt and pulley RPM

    I will then measure the relationships between the core RPM and induced B field.

    I have read, as you mentioned, that insulating materials are better at holding charge, but I'm not sure if I can charge an insulator in the way I described. Could you clarify this for me, as well as give me any opinions you have on my setup and how I may improve it? It seems fairly low cost all in all, as I have a hall sensor already as well as some small motors.

    PS: The diagram says 'Electrode used to transfer charge into the powder core' but I shouldn't have used the word core there as it may be a bit confusing. For clarity, the powder is in the cylindrical case, and the core is a piece of soft iron.

    Attached Files:

  11. Apr 11, 2015 #10

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper

    Insulators are easily charged - i.e. amber and glass can be charged by rubbing.
    Just a note - is rotating an iron rod by a charge the same as rotating the charge about the iron rod?
    How does this symmetry work in electromagnetism?

    Anyway, it looks like you are going to find out :)
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook