1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: What force does the current carrying coil feel?

  1. Aug 4, 2017 #1
    1. The problem statement, all variables and given/known data

    What direction does the force that the current carrying coil feel point to when a magnetic field is turned on?

    A magnetic field is turned on and points from bottom to top and passes through a current carrying coil that has current passing through it CCW. Here is a diagram https://stuff.mit.edu/afs/athena/course/8/8.02t/www/materials/PRS/Raw/PRS_W11D2.pdf
    (bottom of page 2)

    2. Relevant equations

    F = ILBsintheta

    Lenz's law

    3. The attempt at a solution

    From Lenz's law I know that once the magnetic field is turned on there is an increase in magnetic flux upwards. Which means there is an induced magnetic field downwards, which means there is a current passing through the wire that should be clockwise. But this question is weird because there is already current passing though the wire CCW, so I'm not sure exactly sure what happens to the current or the force when the magnetic field is turned on. I would really appreciate some help!
  2. jcsd
  3. Aug 4, 2017 #2


    User Avatar
    Homework Helper
    Gold Member
    2017 Award

    This question is about the force on the loop when there is a steady current in the loop. So, you don't need to worry about what happens when you first turn on the current or when you first turn on the magnetic field (when things are changing with time). So, Lenz's law is not relevant to this question.

    Did you learn how to get the direction of the magnetic force on a steady current placed in a magnetic field using the "right hand rule"?
  4. Aug 4, 2017 #3
    So the right hand rule would be fingers point in direction of B field, thumb in direction of conventional current and palm in direction of force, but in this case it's a weird more difficult to use since it's a loop rather than a straight wire.
  5. Aug 4, 2017 #4


    User Avatar
    Homework Helper
    Gold Member
    2017 Award

    You can look at small sections of the loop. A very short section is approximately straight. For example, consider a small section at the far right side of the loop where you see dF2 indicated. The picture below shows the cross-section of the wire at this point. Can you see how the right hand rule works there?


    Note that the current in this section is essentially into the page
  6. Aug 4, 2017 #5
    Ohh I see it now. Your diagram really helped visualize it better. Thank you!
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted