A (hopefully) simple question about electro-magnetism

Discussion in 'General Electronics Chat' started by strantor, Jul 20, 2015.

  1. strantor

    Thread Starter AAC Fanatic!

    Oct 3, 2010
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    In the picture below, there are two iron cores. They have exactly the same number of turns on each, and both are exactly the same except for where they are cut (and # of turns on the cut-out piece vs on the main piece). Air gap is the same on both.

    If I were to pass the exact same current through both coils, and I tried to remove the cut-out piece from each, would one require more force than the other?

    magnetic attraction.png
     
  2. #12

    Expert

    Nov 30, 2010
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    The answer is in amp-turns. Magnetism is trying to use the rest of the iron to complete its path. More amp-turns make more magnetism.

    A method: Use some soft wire like un-fluxed solder braid to make connections to the movable segment. Use something like cotton string or fishing line to connect the movable segment to some weight. Find out how much weight is required to pull the segment out of line.
     
  3. strantor

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    Yes, amp turns. I guess the essence of my question is, focusing on the green core and then the blue core; is the decrease in amp-turns on the cut-out piece "made up for" by the increase in amp-turns on the base piece? Because overall, the amp-turn are the same between the two models.
    I plan to do some testing just like that, but I wanted to get an idea what to expect before I do.
     
  4. Kermit2

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    Feb 5, 2010
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    Identical in all respects including air gaps and amp turns, no difference would be found excepting the tiny amount of inertia to overcome in moving the one with more mass.
    Try making both sections same length but cutting one air gap to have a section making a 45 degree angle vs tye one with 90 degree angle. This technique used in solenoids to increase attractive force.
     
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  5. #12

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    Your drawings show less turns in the cut-out section and I couldn't understand how the whole coil was energized from looking at the drawings.
    I like the wedge effect described by Kermit. It's a valid way to let the air gap approach zero.

    ps, Variac the current instead of altering the weights. I assume a Variac is more convenient than trying to use calibrated weights or weigh a stack of washers.

    pss, a piece of thick paper is a way to make the air gaps repeatable if you don't want them to approach zero distance. The paper drops out when the gap starts to increase, making a strong visual indicator of when the weight exceeds the magnetism.
     
    Last edited: Jul 20, 2015
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  6. BR-549

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    Sep 22, 2013
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    Wery interesting.

    If we measure the un-energized difference......we will have the inertia and
    mass figures if needed.

    It's true that the strength of the field being cut is the same.

    The first(small coil) has to cut a certain area of the flux.

    If the second(large coil) is twice the turns, and therefore twice the
    area......it should take twice the force to cut.

    I have never experienced a circuit where the number of turns did not influence force.

    I don't know your test setup and dimensions, but you should be able to feel the difference with your fingers.

    Let us know what you find.
     
  7. Kermit2

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    The TOTAL number of turns is the same, if I understood the question correctly. In the formula for solenoids, I recall LENGTH of winding being a multiplier along with current, so the total force induced in the iron would be the same in the air gaps, which translates to no difference in attractions between the two examples.
    In actual application there IS a short length at the ends of the segments which is not overwound by the coil. If this short length becomes a large fraction of the total length of the moveable segment, I would suspect less weight would be required to dislodge it.
     
  8. cmartinez

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    Jan 17, 2007
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    I'm under the impression that the coil's density in relation to the iron it involves (that is, the number of turns per kg of iron) would be important too. And in this case, it would remain constant when contemplated as a whole. So it is my theory that the attractive force would remain constant too.
     
  9. Kermit2

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    The formulas I recall have a factor for square area, such as 20 Sq mm, so this would contain the kilogram, or Mass, you mention. The other factor is the permittivity or permability? Shit, it's been too long ago. I'd have to dig out the dusty tomes of knowledge to get it straight. Kilograms are most certainly involved in solenoid operation calculations, but not in magnetic force formulas involving the simple air gaps shown.
     
  10. nsaspook

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    Aug 27, 2009
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    What you have is a series magnetic circuit so the flux in the air gaps is the same (like series resistance moving the actual resistor to a different position in the circuit loop). Having coils over the cut-out piece reduces leakage from the air-gaps.

    http://www.consult-g2.com/course/chapter1/chapter.html
     
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