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Saturday, 31 October

JEE Main 2021 · PhysicsMultiple choiceSingle correctEasyCalculation

JEE Main 22 July 2021, Shift 2, Physics Q7

Question 7 of 90 in this shift, Physics question 7 of 30, Section A.

A 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 the resistance of the combination between the ends A and B. (Take Resistivity of Copper =1.7×10−8 Ω= 1.7\times10^{-8}\ \Omegam Resistivity of Aluminium =2.6×10−8 Ω= 2.6\times10^{-8}\ \Omegam)

The figure, in words

Two cylindrical rods, Cu (upper) and Al (lower), drawn one above the other; their left ends are joined by a wire to terminal A and their right ends are joined by a wire to terminal B (the two rods are connected side by side between A and B).
  1. (1)1.420 mΩ1.420\ \mathrm{m\Omega}
  2. (2)0.858 mΩ0.858\ \mathrm{m\Omega}Official answer
  3. (3)0.0858 mΩ0.0858\ \mathrm{m\Omega}
  4. (4)2.170 mΩ2.170\ \mathrm{m\Omega}

Official answer

Option 2

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. 4 Aug 2021, Shift 1 · Q21A 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…MediumNumerical valueHas a figure
  7. 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
  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.