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Two magnetic fields interacting in single core?

  1. Jun 21, 2015 #1
    tank-coil-large.jpg I have a question that I can't figure out , there is a toroidal transformer , and the core has an attached part on which another coil is wound. the main coil of the toroidal transformer is connected to a DC source. which creates a static magnetic field in the torus core. the other coil at the side is also attached to DC source. it now has one pole going into the torus while the other pole flux lines are directed with a metal path towards a turning needle in the center of the torus , theoretically the flux through the needle should find its way through the airgap into the torus where the other pole of the side coil is located.
    Will it do so?

    in other words can these two magnetic fields interact with each other , I mean what would happen with the needle in the middle would it start to turn around because as it would try to align with the other pole of the side coil the current in the main torus coils would set up a magnetic field that would push the needle sideways ?

    I would be thankful if someone could explain this situation to me , especially about the case I presented here .
    I attached a picture for better understanding.

    thank you. View attachment 85038
  2. jcsd
  3. Jun 21, 2015 #2


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    Staff: Mentor

    Which flux through the needle? The core will confine the magnetic field mainly to the core.
    The second circuit is closed only if you attach the needle to the core somehow. There is nothing that would interact with each other.
  4. Jun 21, 2015 #3
    let me put it like this,
    well when the yellow coil is energized it would create two poles one would enter the torus the other is given path to the needle , the needle should then align pointing towards the yellow coil as the magnetic flux is trying to align where there is least resistance , like in reluctance motors ,
    now once the needle would align towards the part where the yellow coil is , I switch on the other DC source to the big coil wrapped around the torus itself , what happens now?
    to my mind since the wires on the torus all go one direction in the inside window , shouldn't those two magnetic fields interact? , since the yellow coil has one of its poles attached to the torus the whole torus now is basically one pole and the other extends through the needle and points to the torus but since the wires around the torus have a perpendicular field isnt there now a force exerted on the needle that kinda pushes it around now, like in a motor ?
  5. Jun 21, 2015 #4
    I think the fields are static and they just "combine" forces, so the needle will not "spin around in circles" it will just align to the combined field.
  6. Jun 21, 2015 #5
    surely it would align if there was just the yellow side coil, but since there is also the main coil on the torus , how could it align , i searched and came up with the fact that toridal solenoids/windings don't have clear N-S poles rather it resembles a continuous stream of flux lines pointing in one direction (DC case) so the inner side of the torus has these wires going in one direction so the needle should be continously pushed around ? Im kinda puzzled , please give your feedback, thank you.
  7. Jun 23, 2015 #6
  8. Jun 23, 2015 #7
    Yes, the built in AI of this forum must have figured you were up to doing something devious! :nb)
    (Most likely what really happened is you accidentally hit some combination of keys and the blue text is the result.)

    On your original post I'm no expert, but I think things would settle into some overall shaped magnetic field, and the needle would end up aligned with the direction in which the field is most dense.
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