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Find the velocity and strength of magnetic field from the given info

  1. Apr 28, 2013 #1
    1. The problem statement, all variables and given/known data

    A 7.00-cm-long wire is pulled along a U-shaped conducting rail in a perpendicular magnetic field. The total resistance of the wire and rail is 0.320Ω . Pulling the wire with a force of 1.30N causes 4.50W of power to be dissipated in the circuit.

    What is the speed of the wire when pulled with 1.30N ?

    What is the strength of the magnetic field?

    2. Relevant equations

    ε=vBL
    P=Fv
    I=ε/R

    I=sqrt(P/R)

    3. The attempt at a solution

    I=sqrt(P/R)

    I=sqrt(4.5/0.32)=3.75 amps

    I=ε/R

    3.75 amps = ε/0.32

    ε=1.2 V

    ε=vBL

    (1.2 V)=v(???)(0.07 m)

    I'm not sure how to find the magnetic field, which I obviously need for both parts. Please help! Thanks!
     
  2. jcsd
  3. Apr 28, 2013 #2

    TSny

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    You wrote P = Fv as a relevant equation. Maybe it really is relevant. :wink:
     
  4. Apr 28, 2013 #3
    Thanks!

    But for the second part, I'm not sure of what to do.

    F=qvB
    1.3=q(3.46)B
    But where does q come from?
     
  5. Apr 28, 2013 #4

    TSny

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    The equation F = qvB is not relevant here. Now that you have a value for v, why not go back to your earlier equation ε = vBL?
     
  6. Apr 28, 2013 #5
    Doh! Thanks a bunch!
     
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