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Question

An infinitely long wire carrying current i is along Y-axis such that its one end is at point (0 comma blank b right parenthesis while the wire extends upto infinity. The magnitude of magnetic field strength at point P blank left parenthesis a comma blank 0 right parenthesis is

  1. fraction numerator mu subscript 0 end subscript i over denominator 4 pi a end fraction open parentheses 1 plus fraction numerator b over denominator square root of a to the power of 2 end exponent plus b to the power of 2 end exponent end root end fraction close parentheses    
  2. fraction numerator mu subscript 0 end subscript i over denominator 4 pi a end fraction open parentheses 1 minus fraction numerator b over denominator square root of a to the power of 2 end exponent plus b to the power of 2 end exponent end root end fraction close parentheses    
  3. fraction numerator mu subscript 0 end subscript i over denominator 4 pi a end fraction open parentheses 1 minus fraction numerator a over denominator square root of a to the power of 2 end exponent plus b to the power of 2 end exponent end root end fraction close parentheses    
  4. fraction numerator mu subscript 0 end subscript i over denominator 4 pi a end fraction open parentheses fraction numerator b over denominator square root of a to the power of 2 end exponent plus b to the power of 2 end exponent end root end fraction close parentheses    

The correct answer is: fraction numerator mu subscript 0 end subscript i over denominator 4 pi a end fraction open parentheses 1 minus fraction numerator b over denominator square root of a to the power of 2 end exponent plus b to the power of 2 end exponent end root end fraction close parentheses


    As shown figure take an element d l blank a t blank C blank o f blank w i r e comma blank w h e r e blank O C equals l. blank L e t blank P C equals r blank a n d blank angle O P C equals blank ϕ.
    According to BiotSavarts law; magnitude of magnetic field induction at P due to current element at C is

    d B equals fraction numerator mu subscript 0 end subscript over denominator 4 pi end fraction fraction numerator i d l sin invisible function application theta over denominator r to the power of 2 end exponent end fraction (i)
    Here, theta blank equals 90 degree plus ϕ semicolon r equals a sec invisible function application ϕ
    And l blank equals a tan invisible function application ϕ semicolon d l equals a sec to the power of 2 end exponent invisible function application ϕ blank d blank ϕ
    therefore blank d B equals fraction numerator mu subscript 0 end subscript over denominator 4 pi end fraction fraction numerator i left parenthesis sec to the power of 2 end exponent invisible function application ϕ blank d blank ϕ right parenthesis sin invisible function application left parenthesis 90 degree plus ϕ right parenthesis over denominator a to the power of 2 end exponent sec to the power of 2 end exponent invisible function application ϕ end fraction
    blank equals fraction numerator mu subscript 0 end subscript over denominator 4 pi end fraction fraction numerator 1 over denominator a end fraction cos invisible function application ϕ blank d blank ϕ
    Total magnetic field induction at P is
    B blank equals blank not stretchy integral subscript ϕ subscript 1 end subscript end subscript 90 degree fraction numerator mu subscript 0 end subscript over denominator 4 pi end fraction fraction numerator 1 over denominator a end fraction cos invisible function application ϕ blank d blank ϕ equals blank fraction numerator mu subscript 0 end subscript over denominator 4 pi end fraction fraction numerator 1 over denominator a end fraction blank left parenthesis sin invisible function application ϕ right parenthesis blank subscript ϕ end subscript subscript 1 end subscript superscript 90 end superscript degree
    equals blank fraction numerator mu subscript 0 end subscript over denominator 4 pi end fraction fraction numerator 1 over denominator a end fraction left parenthesis 1 minus sin invisible function application ϕ subscript 1 end subscript right parenthesis equals fraction numerator mu subscript 0 end subscript over denominator 4 pi end fraction fraction numerator 1 over denominator a end fraction open parentheses 1 minus fraction numerator b over denominator square root of a to the power of 2 end exponent plus b to the power of 2 end exponent end root end fraction close parentheses

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