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An infinite wire placed along z-axis, has current I 1  in positive z-direction. A conducting rod placed in xy plane parallel to y-axis has current I 2  in positive y-direction. The ends of the rod subtend angles of +30o and -60o at the origin with positive x-direction. The rod is at a distance 'a' from the origin. Find net force on the rod.

PhysicsMagnetic Effects of CurrentJEE Main
Options:
  • A F = μ 0 4 π I 1 I 2 n 3 - k ^
  • B F = μ 0 4 π 2I 1 I 2 n 3 - k ^
  • C F = μ 0 4 π I 1 I 2 n 3 k ^
  • D F = μ 0 4 π 2 I 1 I 2 n 3 k ^
Solution:
1207 Upvotes Verified Answer
The correct answer is: F = μ 0 4 π I 1 I 2 n 3 - k ^


x a = tan θ
x = a tan θ 1
d x = a sec 2 θ d θ

B = μ 0 4 π 2 I 1 r · · · 2

dF = I 2 dx B sin θ - k ^ · · · 3

From (1),(2) & (3)

dF = I 2 asec 2 θ d θ . μ 0 4 π 2 I 1 a sec θ . sin θ = μ 0 4 π 2 I 1 I 2 sin θ sec θ d θ ( k ^ )

F = μ o 4π 2 I 1 I 2   30 60 tanθdθ= μ o 4π 2 I 1 I 2 [ log e cosθ ] 30 60 ( k ^ ) F = μ o 4π 2 I 1 I 2 [ log e cos 60 + log e cos( 30 ) ]= μ o 4π 2 I 1 I 2 [ log e 3 2 ×2 ] ( k ^ ) F = μ o 4π  I 1 I 2 ln3( k ^ )

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