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Magnetic field caused by long wire

  1. Oct 24, 2011 #1
    1. The problem statement, all variables and given/known data

    A long, thin wire carrying constant current I = 2 A into the page is surrounded by a concentric cylindrical hollow wire of inner radius a = 0.12 m, and outer radius b = 0.26 m, carrying total current I = 4 A directed out of the page, as shown. Assume the current in the cylindrical hollow wire is distributed uniformly over its cross-sectional area.

    Which one of the following statements is valid?
    (a) For radii r > b, the magnetic field is the same as the field a distance r from the center of a long wire carrying total current of 6 A directed out of the page.

    (b) Ampere's law is valid only in highly symmetric situations, such as the situation depicted in the diagram
    (c) The magnetic field at the point labeled C is directed out of the page.

    (d) The magnitude of the magnetic field at r = a is less than the magnitude of the magnetic field at point C (assume that the distance from the center to point C is 3a).

    (e) None of the statements above is valid.

    Answer is E



    2. Relevant equations

    int[B dot dl]= miu I

    3. The attempt at a solution

    I can understand first A,D is wrong, but what does option B even means? Is that means Biot-Savart Law is always true? What is relationship between Biot-Savart Law and Ampere's Law? C is wrong because the out of screen current is larger which is the enclosed current direction at C?

  2. jcsd
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