JEE Main 2020 · PhysicsMultiple choiceSingle correctEasyCalculation

JEE Main 9 January 2020, Shift 1, Physics Q12

Question 12 of 75 in this shift, Physics question 12 of 25, Section A.

A long, straight wire of radius a carries a current distributed uniformly over its cross-section. The ratio of the magnetic fields due to the wire at distance

\frac{a}{3}

and 2a, respectively from the axis of the wire is :

(1) $1/2$
(2) $2$
(3) $2/3$ Official answer
(4) $3/2$

Official answer

Option 3

NTA final key (Jan 2020).

Chapter: Moving Charges and Magnetism
Topic: Ampere's Circuital Law
Idea tested: Field Inside Thick Wire

Same idea in other shifts

Asked 4× in all

8 Apr 2024, Shift 2 · Q42A long straight wire of radius a carries a steady current I. The current is uniformly distributed across its cross section. The ratio of…MediumSingle correct
25 Jun 2022, Shift 1 · Q40A long straight wire with a circular cross-section having radius R, is carrying a steady current I. The current I is uniformly distributed…EasySingle correct
27 Aug 2021, Shift 2 · Q11A coaxial cable consists of an inner wire of radius ‘a’ surrounded by an outer shell of inner and outer radii ‘b’ and ‘c’ respectively.…MediumSingle 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.