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Current in Seawater (Purcell 7.12)

  1. Nov 1, 2012 #1
    1. The problem statement, all variables and given/known data
    This is Purcell 7.12. Seawater moves at 1 meter/second, there is a vertical B-field of .35 gauss, and the conductivity of water is .04 (1/ohm*cm). What is the current in Amps/meter/meter? If you moved water in a bottle at this speed, would there be a current induced?


    2. Relevant equations



    3. The attempt at a solution
    I used E=(v/c)B since I know that the induced electric field comes from velocity and B-field which are perpendicular to one another... can you talk of induced electric fields like this?

    Then I get E=(35/c) esu/cm^2. J=σE, so J=42.4 esu/sec/cm^2. This converts to 1.4e-4 Amps/m^2. Is that the right number?

    I'm not sure how to answer the second part... why would it matter is the water is in a bottle or in a large body of water? All that matters it the conductivity and relative motion, so the current would flow, yes?
     
  2. jcsd
  3. Nov 1, 2012 #2

    haruspex

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    A current requires a circuit. Where's the return path in each case?
     
  4. Nov 1, 2012 #3
    Presumably the current in the ocean would return at some point on the surface of the water or maybe below the surface of the water?

    I guess for a bottle of water there would be no path for the current to return on, though.
     
  5. Nov 2, 2012 #4

    haruspex

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    Or even through the ocean floor.
     
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