Two parallel identical conductors, A and B of equal lengths, carry currents I and 10 I, respectively, in opposite directions. Then
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → A and B will repel each other with same force
Two parallel conductors carrying currents in opposite directions repel each other. The field of each wire at the other, together with the force \( I\vec{L}\times\vec{B} \), gives a force pushing them apart, of magnitude
\( \displaystyle F = \frac{\mu_0 I_1 I_2 L}{2\pi d} = \frac{\mu_0 (I)(10I)L}{2\pi d} \)
This same expression gives the force on A and the force on B: the forces form an action-reaction pair, equal and opposite by Newton’s third law, even though the currents are different.
Why the other options are incorrect
Option (2) A and B will attract each other with same force — Incorrect. Parallel conductors attract only when their currents are in the same direction.
Option (3) A will attract B but B will repel A — Incorrect. Mutual forces between two conductors cannot be of different kinds; that would violate Newton’s third law.
Option (4) A and B will attract each other with different force — Incorrect. The conductors repel, and the forces on them are equal in magnitude.
NCERT (2026-27):
Force Between Two Parallel Currents, the Ampere – “… From considerations similar to above we can find the force \( \mathbf{F}_{ab} \), on a segment of length \( L \) of ‘a’ due to the current in ‘b’. It is equal in magnitude to \( F_{ba} \), and directed towards ‘b’.”
Force Between Two Parallel Currents, the Ampere – “… One can show that oppositely directed currents repel each other. Thus, Parallel currents attract, and antiparallel currents repel.”