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Unravelling coil and EMF

  1. Nov 22, 2006 #1
    Can i get some help with this homework problem??
    A physicist wraps a long copper wire around a toilet paper roll and places the roll in a uniform magnetic field. The axis of the roll is aligned with the field. The radius of the roll is r = 6 cm and the strength of the field is B = 0.1 T. If the physicist pulls the wire at v = 78 cm/s, what is the magnitude of the EMF induced in the wire?
    Thanks in advance.
  2. jcsd
  3. Nov 22, 2006 #2


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    Look at the equation for induced emf related to the flux through a coil of N turns. What is changing when the wire is being pulled? The wire is unwinding from the roll.
  4. Nov 23, 2006 #3
    im using dBda/dt and da/ta= .5*r*ds/dt where ds/ts is the velocity
    im taking that time the magnetic field.
    also i am watchin my units.
    so B*.5*r*v=emf??? whats wrong its not right tho?
  5. Nov 24, 2006 #4


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    I don't understand your first equation. Is d the diameter in one place and derivative in another? I think you need to be a bit creative with this one. Usually we take N to be a constant and write

    ε = -N[dΦ/dt] = -N[d(BA)/dt]

    But in this problem, B and A are constant and effectively N is changing, so

    ε = -[d(NAB)/dt)] = = -BA[dN/dt]

    How is N related to v?
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