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Homework Help: Solenoids and Power.

  1. May 26, 2009 #1
    1. The problem statement, all variables and given/known data

    A solenoid 10.0 cm in diameter and 75.0 cm long is made from copper wire of diameter 0.100 cm, with very thin insulation. The wire is wound onto a cardboard tube in a single layer, with adjacent turns touching each other. To produce a field of 9.80 mT at the center of the solenoid, what power must be delivered to the solenoid

    I think I = 20 am I right?

    2. Relevant equations

    B=Uo * (N/l) * I

    P= IV?

    3. The attempt at a solution

    I am stuck at getting the power, I have no clue how to get potential from a magnetic field alone?
    Or if i Say P= work/time
    I don't know how to get work.
    And i'm not sure if my Current is correct. Any help is much appreciated.
  2. jcsd
  3. May 27, 2009 #2


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    The fact that they stated the dimensions of the copper wire implies that its properties will be relevant in the calculations.

    Once the magnetic field it produced, it does not change (it is static). Hence, its energy remains constant.

    This means that another source of power loss needs to be identified. With reference to the above, that source is the non-zero resistivity of Copper, which you can use to calculate the resistance of the length of Copper wire required, and hence find power with P = I^2 R

    I am getting I=8.0A.
    In this case, N/L is the number of turns per unit length. As the copper wires are adjacent to each other, this suggests that N/L = 1 / (diameter of copper wire).
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