1. Not finding help here? Sign up for a free 30min 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!

Coil Moving through a Magnetic Field

  1. Mar 14, 2009 #1
    1. The problem statement, all variables and given/known data

    At t=0, a rectangular coil of resistance R = 2 ohms and dimensions w = 3 cm and L = 8 cm enters a region of constant magnetic field B = 1.6 T directed into the screen as shown. The length of the region containing the magnetic field is LB = 15 cm. The coil is observed to move at constant velocity v = 5 cm/s. What is the force required at time t = 0.8 sec to maintain this velocity?

    F(0.8 sec) = ?? N



    2. Relevant equations



    3. The attempt at a solution

    This problem has a walkthrough to help me solve it, however I have reached the end of the walkthrough and tried using the equation but it isn't working for me.

    I find out the following in the walkthrough:

    the change in flux (dF/dt) through the loop is 1.6*0.03*0.05

    the current can be calculated using V=IR

    and finally F = IwB


    so for the answer I try: (.05/2)*.03*1.6 but it keeps saying that it is wrong.
     
  2. jcsd
  3. Mar 15, 2009 #2
    Hi there,

    Have you taken into account there are two parts of the wire perpendicular tot he B-field?
     
  4. Mar 15, 2009 #3
    As i go through the walkthrough it tells me that 2 of the wires cancel each other out and I am only left with needing to calculate the force on the wire of length w
     
  5. Mar 15, 2009 #4
    My mistake, I didn't realise it only just entered the field. Try working this through algebraicaly first to obtain a final expresion for F, then put the numbers in. I think you should have a couple of squared quantities in there somewhere.
     
  6. Mar 15, 2009 #5
    but they tell me in the walkthrough that all of those equations are correct.. at the end im told that F = IwB and that I now have enough information to solve the equation for the final answer so I don't understand why it keeps telling me that my answer is wrong.
     
  7. Mar 15, 2009 #6
    Yes, F=IwB. What does I=?
     
  8. Mar 15, 2009 #7
    According to this you have said that: current=velocity/resistance. This is not true in any circumstance.
     
  9. Mar 15, 2009 #8
    they told me to use the equation V=IR so the current should be equal to V/R.. how am i suppose to figure out what the current is then?
     
  10. Mar 15, 2009 #9
    The flux through the coil that you calculated in your first post is equal to the emf generated in the coil by the motion through the b field.

    The V in the expression of V=IR is always the voltage.

    In this situation the only voltage contribution is from the emf generated in the wire, which you have already calculated.
     
  11. Mar 15, 2009 #10
    i see now.. when i saw V=IR without thinking about it i just put the v for the velocity.. it all makes sense now, thanks for your help.
     
  12. Mar 15, 2009 #11
    Your welcome. Did it come out right in the end?
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Coil Moving through a Magnetic Field
Loading...