Physics-
General
Easy

Question

An ac source of angular frequency omega is fed across a resistor r and a capacitor C in series. The current registered is I. If now the frequency of source is changed to omega /3 (but maintaining the same voltage), the current in then circuit is found to be halved. Calculate the ratio of reactance to resistance at the original frequency omega

  1. square root of fraction numerator 3 over denominator 5 end fraction end root    
  2. square root of fraction numerator 2 over denominator 5 end fraction end root    
  3. square root of fraction numerator 1 over denominator 5 end fraction end root    
  4. square root of fraction numerator 4 over denominator 5 end fraction end root    

The correct answer is: square root of fraction numerator 3 over denominator 5 end fraction end root


    At angular frequency w, the current in RC circuit is given by
    i subscript r m s end subscript equals fraction numerator V subscript r m s end subscript over denominator square root of R to the power of 2 end exponent plus open parentheses fraction numerator 1 over denominator omega C end fraction close parentheses to the power of 2 end exponent end root end fraction ......(i)
    Also fraction numerator i subscript r m s end subscript over denominator 2 end fraction equals fraction numerator V subscript r m s end subscript over denominator square root of R to the power of 2 end exponent plus open parentheses fraction numerator 1 over denominator fraction numerator omega over denominator 3 end fraction C end fraction close parentheses to the power of 2 end exponent end root end fraction equals fraction numerator V subscript r m s end subscript over denominator square root of R to the power of 2 end exponent plus fraction numerator 9 over denominator omega to the power of 2 end exponent C to the power of 2 end exponent end fraction end root end fraction ......(ii)
    From equation (i) and (ii) we get
    3 R to the power of 2 end exponent equals fraction numerator 5 over denominator omega to the power of 2 end exponent C to the power of 2 end exponent end fraction rightwards double arrow fraction numerator fraction numerator 1 over denominator omega C end fraction over denominator R end fraction equals square root of fraction numerator 3 over denominator 5 end fraction end rootÞ fraction numerator X subscript C end subscript over denominator R end fraction equals square root of fraction numerator 3 over denominator 5 end fraction end root

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