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NEET2026PhysicsChapterActual

A straight wire of length L carries a steady current I along the negative y-axis. When placed in a uniform magnetic field B , the wire experiences a magnetic force F = IL(3 i + 4 k ) . Which of the following could be the magnetic field B in the region? (Assume all quantities are in SI units)

Options

  1. A(4 i + 6 j - 3 k )
  2. B(-4 i + 6 j + 3 k )
  3. C(3 i + 6 j + 4 k )
  4. D(-3 i + 6 j + 4 k )

Correct answer

A. (4 i + 6 j - 3 k )

Step-by-step solution

The magnetic force on a straight current-carrying wire is given by F = I( L B ) . Since the current flows along the negative y-axis, the length vector is L = -L j . Let the magnetic field be B = B_x i + B_y j + B_z k . Substituting these into the force equation: F = I(-L j ) (B_x i + B_y j + B_z k ) Using the properties of cross products ( j i = - k , j j = 0 , j k = i ): F = -IL(B_x( j i ) + B_y( j j ) + B_z( j k )) F = -IL(-B_x k + B_z i ) = IL(-B_z i + B_x k ) We are given that the force is F = IL(3 i + 4 k ) .

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