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General
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Question

Three charges Q,(+q) and (+q) are placed at the vertices of an equilateral triangle of side l as shown in the figure. If the net electrostatic energy of the system is zero, then Q is equal to

  1. open parentheses negative fraction numerator q over denominator 2 end fraction close parentheses    
  2. (-q)    
  3. (+q)    
  4. Zero    

The correct answer is: open parentheses negative fraction numerator q over denominator 2 end fraction close parentheses


    Potential energy of the system
    U equals k fraction numerator Q q over denominator l end fraction plus fraction numerator k q to the power of 2 end exponent over denominator l end fraction plus fraction numerator k q Q over denominator l end fraction equals 0
    Þ fraction numerator k q over denominator l end fraction left parenthesis Q plus q plus Q right parenthesis equals 0 Þ Q equals negative fraction numerator q over denominator 2 end fraction

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