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Determining the direction of torque

  1. Jul 19, 2017 #1
    pic3.png
    1. The problem statement, all variables and given/known data

    1) I need to write the equation for torque on a current carrying loop in a magnetic field, which I believe i did correctly under the relevant euations section, does this formula look correct?

    2) Use the right hand rule to determine the direction of the torque on the current carrying loop as shown in the figure

    2. Relevant equations
    T=NIABsintheta can be used to calculate torque a loop of N turns and A area, carrying I current feels in the prescence of a magnetic field B

    3. The attempt at a solution
    As for question 2, if I place my fingers in the direction of I, wouldn't I put them along the x axis pointing toward the positive x axis, then curl them toward B, which would mean by palm would have to be upwards, leaving my thumb which would be the torque pointing outwards?

    This is a little confusing to me I am not sure if I am using the correct formula or the proper use of the right hand rule

    thanks!
     
  2. jcsd
  3. Jul 19, 2017 #2

    berkeman

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    Staff: Mentor

    When the current is parallel to the B-field, there isn't any force on the charges. The force is maximum when the current and B-field are at a right angle to each other, right? So just check the forces on the ring at the top and bottom of the ring. What directions are those 2 forces pushing on the ring?
     
  4. Jul 19, 2017 #3
    would the direction be counterclockwise?
     
  5. Jul 19, 2017 #4

    berkeman

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    How do you arrive at that answer?
     
  6. Jul 19, 2017 #5
    I assume its wrong haha but I thought that the I was spinning it counterclockwise
     
  7. Jul 19, 2017 #6

    berkeman

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    Did you ever see the Tom Clancy movie "The Hunt for Red October"? There's a scene where Jack Ryan tells the American submarine Captain that the Soviet Sub's next "Crazy Ivan" will be to starboard. Turns out he was guessing, and he guessed right and got the Captain to do what he wanted.

    But you can't always get lucky when guessing. Talk through the right hand rule and how it works at the top of the ring. Tell us which way your right thumb ends up pointing. You got it right in your last thread about the right hand rule...
     
  8. Jul 19, 2017 #7
    well that was a lucky guess haha well at the top of the ring aren't your fingers pointing left, then curl upwards the b, making your thumb go into the page? but clearly that isn't counterclockwise

    nice bike btw
     
  9. Jul 19, 2017 #8

    berkeman

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    The way I interpret the drawing is that the ring mostly comes out of the page at us, and the B-field is vertical. So at the top of the ring I point my fingers in the direction of the current at the top of the ring, which means they are pointing into the page. I then curl my fingers in the direction of B...
     
  10. Jul 19, 2017 #9
    okay the picture was confusing me a little, so at the top of the wing the current is going into the page, so point your fingers towards the page, now b is vertical so your palm must be facing up, so then curling it upwards wouldn't that leave your thumb pointing right, which would be clockwise not counterclockwise?
     
  11. Jul 19, 2017 #10

    berkeman

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    Correct. And if you do the same at the bottom, you get that the force is to the left, so yes, the overall torque is clockwise seen from our vantage point. :smile:

    Good thing you weren't Jack Ryan on that submarine! :biggrin:
     
  12. Jul 19, 2017 #11
    hahaha thanks for the help, I understand what is going on, and now I have another movie to watch
     
  13. Jul 19, 2017 #12

    berkeman

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    You're welcome. Yeah, that's one of my favorites -- several unexpected turns (even though I'd read the book too).

    BTW, your puppy avatar is killing me here... :cool:
     
  14. Jul 19, 2017 #13
    hahaha not quite as cool as your bike though. One last question, if we are taking it from the top we get that it is pointing clockwise, but when taken from the bottom we get left which is counterclockwise, so the torque depends on the vantage point? so then how do you determine the actual direction if it varies from each vantage point?
     
  15. Jul 19, 2017 #14

    berkeman

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    No, the force on the top of the ring is to the right, and the force on the bottom of the ring is to the left. Both of those forces contribute to a torque in the clockwise rotation direction for the ring.
     
  16. Jul 19, 2017 #15
    oh yeah gotcha, thanks again!
     
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