JEE Main 2021 · PhysicsMultiple choiceSingle correctMediumCalculation

JEE Main 26 February 2021, Shift 1, Physics Q6

Question 6 of 90 in this shift, Physics question 6 of 30, Section A.

A large number of water drops, each of radius r, combine to have a drop of radius R. If the surface tension is T and mechanical equivalent of heat is J, the rise in heat energy per unit volume will be :

(1) $\frac{2T}{J}\left(\frac{1}{r}-\frac{1}{R}\right)$
(2) $\frac{3T}{J}\left(\frac{1}{r}-\frac{1}{R}\right)$ Official answer
(3) $\frac{3T}{rJ}$
(4) $\frac{2T}{rJ}$

Official answer

Option 2

NTA final key.

Chapter: Mechanical Properties of Fluids
Topic: Surface Tension
Idea tested: Splitting and Coalescence

Same idea in other shifts

Asked 10× in all

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2 Apr 2025, Shift 2 · Q31Two water drops each of radius $r$ coalesce to form a bigger drop. If $T$ is the surface tension, the surface energy released in this…EasySingle correct
6 Apr 2024, Shift 1 · Q54A big drop is formed by coalescing 1000 small droplets of water. The ratio of surface energy of 1000 droplets to that of energy of big…EasyNumerical value
1 Feb 2024, Shift 2 · Q37A big drop is formed by coalescing 1000 small droplets of water. The surface energy will become :EasySingle correct
30 Jan 2024, Shift 2 · Q53A big drop is formed by coalescing 1000 small identical drops of water. If $E_1$ be the total surface energy of 1000 small drops of water…EasyNumerical value
26 Jul 2022, Shift 1 · Q45A water drop of radius 1 cm is broken into 729 equal droplets. If surface tension of water is 75 dyne/cm, then the gain in surface energy…MediumSingle correct
30 Jul 2022, Shift 2 · Q27A water drop of radius 1 cm is broken into eight equal droplets. Surface tension of water is 0.075 N m $^{-1}$ . The gain in surface energy…MediumNumerical 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. Lumi analyses 108 of the 174 JEE Main shifts held from 2017 to 2026.