Read the following passage and answer the questions.
A student studies the magnetic field produced by a steady current. She uses the Biot-Savart law. First she takes a short straight piece of wire, 1 cm long, placed at the origin. It carries a current of 5 A in the +x direction. She marks some points, each 10 cm from this piece. Some of these points lie on the x-, y- and z-axes. One point, Q, lies in the x-y plane, on a line that makes an angle of 30° with the +x-axis. She also moves a point P along the +y-axis to see how the field changes with distance. Next, she bends a long wire into a flat circular loop of radius \(R\) and passes a steady current \(I\) through it. She treats the loop as a large number of small current elements. She then studies the field at the centre of the loop and at points on its axis.
The loop lies flat on a table, and the student looks down on it. The current in the loop flows anticlockwise as she sees it. The magnetic field at the centre of the loop is
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → perpendicular to the table, pointing up towards her
The field at the centre of a circular loop is along its axis, normal to the plane of the loop. Its direction comes from the right-hand thumb rule.
Option (1) perpendicular to the table, pointing up towards her — Correct. Curl the fingers of the right hand along the anticlockwise current. The thumb then points up, out of the table.
Option (2) perpendicular to the table, pointing down away from her — Incorrect. This is the field for a clockwise current. The direction is reversed when the current is taken the wrong way round.
Option (3) zero, because the fields of opposite elements cancel — Incorrect. Opposite elements carry current in opposite directions, but they also lie on opposite sides of the centre. Both give fields on the same side of the loop, so the fields add up.
Option (4) along the table, pointing outward from the centre — Incorrect. An outward field would be like the electric field of a charged ring. The magnetic field of each element is at right angles to the plane holding dl and r, so it is normal to the table.
NCERT (2026-27):
Magnetic Field on the Axis of a Circular Current Loop – “Curl the palm of your right hand around the circular wire with the fingers pointing in the direction of the current. The right-hand thumb gives the direction of the magnetic field.”