Two long, straight, parallel wires a and b lie side by side on a horizontal table. Both carry currents in the same direction. At the position of wire b, the magnetic field due to wire a is
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → vertical, at right angles to the table
The field lines of a long straight wire are circles around the wire. At any point, the field acts along the tangent to the circle through that point.
Option (1) horizontal, along the length of wire b — Incorrect. The field of a straight wire is always at right angles to the wire. So it has no part along the length of a wire parallel to it.
Option (2) vertical, at right angles to the table — Correct. Wire b lies on one of the field circles of wire a. The tangent at that point is at right angles to the radius, which is the line joining the wires. It is also at right angles to the wires. On a horizontal table, this direction is vertical.
Option (3) horizontal, along the line joining the two wires — Incorrect. The line joining the wires is a radius of the field circle, and the field is at right angles to it. It is the force on wire b that acts along this line, towards wire a.
Option (4) zero, because the two currents are in the same direction — Incorrect. Wire a sets up a field all around it, whatever the current in wire b. Its value at wire b is \( \mu_0I_a/2\pi d \). This field is what pulls wire b towards wire a.
NCERT (2026-27):
Ampere’s Circuital Law – “The field direction at any point on this circle is tangential to it.”
Force Between Two Parallel Currents, the Ampere – “… The conductor ‘a’ produces, the same magnetic field Ba at all points along the conductor ‘b’. The right-hand rule tells us that the direction of this field is downwards (when the conductors are placed horizontally).”