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Saturday, 31 October

JEE Main 2021 · PhysicsMultiple choiceDiagram basedMediumConceptual

JEE Main 3 August 2021, Shift 1, Physics Q13

Question 13 of 90 in this shift, Physics question 13 of 30, Section A.

A current I enters a circular coil of radius R, branches into two parts at A and then recombines at B (as shown in figure). The resultant magnetic field at the centre of the coil is -

The figure, in words

A circle with centre O (filled dot). A straight wire carrying current I enters from the left (horizontal arrow, labelled I) at point A on the left of the circle; a dashed horizontal line joins A to O. A second dashed line joins O to point B on the upper right of the circle, labelled R, with an angle of 120° marked at O between OA and OB (on the upper side). The upper arc from A to B carries I1 (arrow along the arc, labelled I1) and the lower arc from A round the bottom to B carries I2 (double arrowhead, labelled I2). At B the wire continues straight up and to the right with an arrow labelled I. The label 'I = I1+I2' is printed to the right of the circle.
  1. (1)23(μ0I2R)\frac{2}{3}\left(\frac{\mu_0 \mathrm{I}}{2\mathrm{R}}\right)
  2. (2)12(μ0I2R)\frac{1}{2}\left(\frac{\mu_0 \mathrm{I}}{2\mathrm{R}}\right)
  3. (3)μ0I2R\frac{\mu_0 \mathrm{I}}{2\mathrm{R}}
  4. (4)ZeroOfficial answer

Official answer

Option 4

NTA final key.

Chapter
Moving Charges and Magnetism
Topic
Biot-Savart Law
Idea tested
Field at center of circular arc

Same idea in other shifts

Asked 8× in all
  1. 3 Apr 2025, Shift 1 · Q48A loop ABCDA, carrying current I=12\mathrm{I} = 12 A, is placed in a plane, consists of two semi-circular segments of radius…MediumNumerical valueHas a figure
  2. 30 Jan 2024, Shift 1 · Q42Two insulated circular loop A and B of radius 'a' carrying a current of 'I' in the anti clockwise direction as shown in the figure. The…MediumDiagram basedHas a figure
  3. 31 Jan 2024, Shift 2 · Q57Two circular coils PP and QQ of 100 turns each have same radius of π\pi cm. The currents in PP and RR are 1A1A and 2A2A respectively.…MediumNumerical value
  4. 10 Apr 2023, Shift 2 · Q59A straight wire carrying a current of 14 A is bent into a semicircular arc of radius 2.2 cm as shown in the figure. The magnetic field…MediumNumerical valueHas a figure
  5. 26 Jul 2022, Shift 1 · Q39BXB_X and BYB_Y are the magnetic fields at the centre of two coils X and Y respectively each carrying equal current. If coil X has 200…EasySingle correct
  6. 30 Jul 2022, Shift 2 · Q16In the following loop, magnitude of the magnetic field produced at the centre (O) of the loop is : (Assume AB and CD are infinitely…MediumSingle correctHas a figure
  7. 4 Sep 2020, Shift 1 · Q13A wire A, bent in the shape of an arc of a circle, carrying a current of 2 A and having radius 2 cm and another wire B, also bent in the…MediumDiagram basedHas a figure

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. This question needs a figure we do not reproduce; it is described in words above. Lumi analyses 108 of the 174 JEE Main shifts held from 2017 to 2026.