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Homework Help: Find Current in a Solenoid Given Magnetic Field and Velocity

  1. Mar 29, 2014 #1
    1. The problem statement, all variables and given/known data
    An electron circles at a speed of 8510 m/s in a
    radius of 2.42 cm in a solenoid. The magnetic
    field of the solenoid is perpendicular to the
    plane of the electron’s path. Find the current in the solenoid if it has
    23.3 turns/cm.
    Answer in units of mA

    2. Relevant equations
    I think you can use B=uIN
    u is the universal vacuum permittivity constant, n is the number of turns

    3. The attempt at a solution
    I already calculated force to be 2e-6.
    There are 56.386 turns total because 23.3*2.42
    so I got 2e-6/(4pi*10^-7*56.386)=0.02822
    so 28.22 mA
    I punched this in and it was wrong.
    Did I do something weird?
  2. jcsd
  3. Mar 29, 2014 #2

    rude man

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    Homework Helper
    Gold Member

    1. why calculate the force? your first task is to determine B. what relates B to q, m, v and R, all of which are given to you or known?

    2. your calculation of no. of turns does not make sense to me. Your formula for B is correct if you replace N, the number of turns, by n, the no. of turns per unit length. n is also given to you.
    Be careful to distinguish between N and n. When you have B you can calculate the current from that formula.

    Watch your units!
  4. Mar 31, 2014 #3
    What equation relates these variables? I can't find one that does
  5. Mar 31, 2014 #4

    rude man

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    How about centripetal force in circular motion = magnetic force?
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