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Saturday, 31 October

JEE Main 2021 · PhysicsNumerical answerNumerical valueMediumMulti-step

JEE Main 4 August 2021, Shift 1, Physics Q21

Question 21 of 90 in this shift, Physics question 21 of 30, Section B.

A wire of length L is coated by another conducting material of double resistivity and the thickness of the layer is equal to the radius of the wire. The resistance of the wire between its ends after coating will decrease by ________%.

The figure, in words

Cylindrical wire with its outer coating hatched; the left end is labelled A and the right end B; a double-headed arrow below labelled L spans the length.

Official answer

60

NTA final key.

Chapter
Current Electricity
Topic
Ohm's Law and Resistance
Idea tested
Resistivity and Geometry

Same idea in other shifts

Asked 10× in all
  1. 6 Apr 2023, Shift 1 · Q55The length of a metallic wire is increased by 20% and its area of cross section is reduced by 4%. The percentage change in resistance of…EasyNumerical value
  2. 10 Apr 2023, Shift 2 · Q60A rectangular parallelopiped is measured as 1 cm ×\times 1 cm ×\times 100 cm. If its specific resistance is 3×10−7 Ω3\times10^{-7}\ \Omega m,…EasyNumerical value
  3. 25 Jul 2022, Shift 1 · Q58Eight copper wire of length ll and diameter dd are joined in parallel to form a single composite conductor of resistance RR. If a…EasyNumerical value
  4. 29 Jul 2022, Shift 1 · Q42Two metallic wires of identical dimensions are connected in series. If σ1\sigma_1 and σ2\sigma_2 are the conductivities of the these wires…MediumSingle correct
  5. 30 Jun 2022, Shift 1 · Q45An electric cable of copper has just one wire of radius 9 mm. Its resistance is 14 Ω\Omega. If this single copper wire of the cable is…EasySingle correct
  6. 26 Aug 2021, Shift 1 · Q17What equal length of an iron wire and a copper-nickel alloy wire, each of 2 mm diameter connected parallel to give an equivalent…MediumSingle correct
  7. 22 Jul 2021, Shift 2 · Q7A Copper (Cu) rod of length 25 cm and cross-sectional area 3 mm2^2 is joined with a similar Aluminium (Al) rod as shown in figure. Find…EasySingle correctHas a figure
  8. 18 Mar 2021, Shift 2 · Q23Two wires of same length and thickness having specific resistances 6 Ω\Omega cm and 3 Ω\Omega cm respectively are connected in parallel.…EasyNumerical value

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.