JEE Main 2021 · PhysicsMultiple choiceSingle correctMediumCalculation

JEE Main 26 August 2021, Shift 1, Physics Q11

Question 11 of 90 in this shift, Physics question 11 of 30, Section A.

The fractional change in the magnetic field intensity at a distance ‘r’ from centre on the axis of current carrying coil of radius ‘a’ to the magnetic field intensity at the centre of the same coil is : (Take r < a).

(1) $\frac{3}{2}\frac{a^2}{r^2}$
(2) $\frac{2}{3}\frac{a^2}{r^2}$
(3) $\frac{2}{3}\frac{r^2}{a^2}$
(4) $\frac{3}{2}\frac{r^2}{a^2}$ Official answer

Official answer

Option 4

NTA final key.

Chapter: Moving Charges and Magnetism
Topic: Biot-Savart Law
Idea tested: Ratio of Axial to Central Magnetic Field

Same idea in other shifts

Asked 5× in all

2 Apr 2025, Shift 1 · Q38Let $B_1$ be the magnitude of magnetic field at center of a circular coil of radius R carrying current I. Let $B_2$ be the magnitude of…EasySingle correct
8 Apr 2023, Shift 2 · Q59The ratio of magnetic field at the centre of a current carrying coil of radius $r$ to the magnetic field at distance $r$ from the centre…MediumNumerical value
28 Jul 2022, Shift 2 · Q13The magnetic field at the center of current carrying circular loop is $\mathrm{B_1}$ . The magnetic field at a distance of $\sqrt{3}$ times…EasySingle correct
24 Jun 2022, Shift 1 · Q49The magnetic field at the centre of a circular coil of radius r, due to current I flowing through it, is B. The magnetic field at a point…EasySingle correct

Question text from the official JEE Main paper published by NTA; answer from the final answer key. Chapter, topic and idea tags, difficulty and skill are Lumi’s. Lumi analyses 108 of the 174 JEE Main shifts held from 2017 to 2026.