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Homework Help: Full-scale deflection?

  1. Feb 3, 2007 #1
    1. The problem statement, all variables and given/known data
    The needle of a moving coil meter gives a full-scale deflection for a current of 50uA. What current would give a full-scale deflection if the magnetic field weakened to 80% of its original value?

    2. Relevant equations

    3. The attempt at a solution
    Decrease magnetic field => needle not deflected as much for a given current such as 50uA. A full-scale deflection now would result in 20% more current than before because of the 20% decrease in magnetic field. I assume the powers of magnetic strength and current are equal and unity. So now 50uA*1.2=60uA would result. However the answers suggested 63uA. Why?
  2. jcsd
  3. Feb 3, 2007 #2


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    Science Advisor
    Homework Helper

    Because "a 20% decrease" and a "20% increase" are not the inverse of each other.

    0.8 times 1.2 = 0.96, not 1.0

    Your basic idea about how to solve the problem is OK.
  4. Feb 4, 2007 #3
    That is good. It's liitle things like this that tests your mathematical intuition. For me, it's obviously not very good but I'm working on it.
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