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Homework Help: Force on a wire, by a current carrying wire.

  1. Nov 24, 2012 #1
    1. The problem statement, all variables and given/known data
    What is the magnitude of the force per unit length on a long wire of charge density λ from a wire that carries a current of 3 A and is a distance of 3 m away?
    A)μλ/2 B)μλ/6 C)3μλ/2 D)μλ/3 E)0

    3. The attempt at a solution
    F=BIL→ (F/L)=BI →(F/L)=μI1I2L/2∏R

    I dont know how to apply that λ. i would think that λ=Q/L, and i also dont know which current is which in that final equation i put (that may be a slightly tangential question, but i also need help understanding which is which if one could aid with that).
  2. jcsd
  3. Nov 24, 2012 #2
    You have one wire with a constant charge density λ, and another with a constant current 3A. They are 3m apart. What is the force between them.
    What kind of field does a charge density create?
    What kind of field does a current create?
    What are the forces between one wire's field and another wire's source?
  4. Nov 24, 2012 #3
    -a charge density doesn't create a field, the charge needs to be moving to generate a magnetic field.
    -a current creates a field that circles the wire radially, either clockwise of counterclockwise depending on the direction of the current. But there is no indication of the direction the wires sit with respect to each other, neither parallel nor perpendicular.
    -considering what this question is giving, there is no reference to a wire's source (which im guessing you mean voltage?)
  5. Nov 24, 2012 #4
    Charges create electric fields, moving or not.
    Charges are the sources of electric fields. Currents are the sources of magnetic fields.
    My question could be better stated as:
    What is the force of a Magnetic field on a charge?
    What is the force of an Electric field on a current?
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