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Influence of magnetic fluid in magnetic force

  1. Apr 16, 2014 #1
    Looking for a way to increase magnetic forces in electrical devices, an option is reduce the air gap, but some times it is not a practicable solution.

    The question is what happend if we put a ferrofluid between the magnets?

    Imagine the following situation: a closed volume composed by a non magnetic tube and two magnets in N-S orientation at the ends. First case, air between magnets. Second case, ferrofluid filling the gap.

    Is the force in the second case bigger that force in first case?

    The ferrofluid has more magnetic permeability, but my doubt is about pressure gradients inside the fluid . I do not know if the pressure is constant along the tube or we have a higher pressure at the ends. If it is the case, do it balance the increment in force due to a higher magnetic permeability in the medium?

    I did not make the experiment. What do think about that?
    please remember that a ferrofluid has magnetic particles inside, and it experiment forces due to magnetic fields.
     
  2. jcsd
  3. Apr 18, 2014 #2

    Simon Bridge

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    What sort of device are you thinking of?

    ... the fluid would respond to the magnetic field.
    Since a ferro-fluid is just a bunch of iron filings (or similar) suspended in an oil or a jelly, the result on the field would be much the same as just putting iron between the poles.

    ... force where? Inside the fluid?

    So you are imagining quite a large quantity of the fluid then?

    Adding more iron does increase the flux through the iron - concentrates it.
    But you can also shape the poles of the magnets.

    The details will be important.
    What sort of device are you thinking of?
    What do you need it to do?
     
  4. Apr 18, 2014 #3
    Simon, thanks for your time. In order with your comments:

    In my work as engineer I found many times problems where efficiency in electromagnetic devices like contactors, motors, magnetic cranes (little or big) is needed due to restrictions in available space, power consuption or weight. In some applications like specific kinds DC motors, you need a high torque with very low RPM. The air gap between rotor and estator is one of the main parameters that drives the power consumption due to the poor air's magnetic permeability.

    At least in the info that I have, permeability in a ferrofluid is higher than air but lower that iron. The idea is raise the permeability allowing a concentrated magnetic flux. I want approach as much as possible to the effect of a piece of iron between poles, but a deformable one that allows pole's movement.

    Resultant force over the poles. In the experiment that I set (not neccesary a real device, it is just for a simple presentation of the phenomenom), I would like to know if the resultant force, sum of magnetic plus pressure over pole wet area, increases. It depends on pressure distribution inside the fluid. If you put a piece of iron between poles, in contact with both, the magnetic force increases, but the resultant force become zero due to contact pressure against the iron piece. Contact pressure exactly balance magnetic force. It happens with a solid.

    In a real device, the ammount of ferrofluid could be little. It is just an experiment.

    If you increases the efficiency of magnetic circuits, you can increase the power to weight ratio, for example in a servo motor for aerospace applications. But you still need allow movement, then, a ferrofluid with low viscosity seems to be an interesting option.
     
  5. Apr 18, 2014 #4

    Simon Bridge

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  6. Apr 18, 2014 #5
    Simon:

    thank you for the info. You are right, there are some developments and experiments related with devices like high torque motors, but in general I can't find info about what happens with the ferrofluid in terms of change in viscosity, pressure gradients, heat generation due to internal friction, etc.
    Probably I must develop some experiments, but before to do it, I would understand the physic. Always I prefer that an experiment verify a prediction, I would make some calculus before.

    thanks again.
     
  7. Apr 18, 2014 #6

    Simon Bridge

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    I've had to learn about them due to some of the more out-there ideas.
    There are different formulations - so the exact behavior will depend on the particular fluid - I'd expect increased drag with the applied field strength for instance.

    They are still a bit too new - a developing field - you'd need to go to the journals to learn in more detail, or find a manufacturer and ask them.
    http://iopscience.iop.org/0953-8984/18/38/E01
    http://www.nature.com/nature/journal/v504/n7479/full/504229a.html
    http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0103-97332005000400018

    ... it is possible there are college courses in them at the senior undergrad or post-grad level.
     
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