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Saturday, 31 October

JEE Main 2020 · PhysicsMultiple choiceSingle correctMediumMulti-step

JEE Main 8 January 2020, Shift 2, Physics Q18

Question 18 of 75 in this shift, Physics question 18 of 25, Section A.

An electron (mass m) with initial velocity v⃗=v0 i^+v0 j^\vec{v}=v_0\,\hat{i}+v_0\,\hat{j} is in an electric field E⃗=−E0 k^\vec{E}=-E_0\,\hat{k}. If λ0\lambda_0 is initial de-Broglie wavelength of electron, its de-Broglie wave length at time t is given by :
  1. (1)λ01+e2E02t2m2v02\frac{\lambda_0}{\sqrt{1+\frac{e^2E_0^2t^2}{m^2v_0^2}}}
  2. (2)λ01+e2E2t22m2v02\frac{\lambda_0}{\sqrt{1+\frac{e^2E^2t^2}{2m^2v_0^2}}}Official answer
  3. (3)λ02+e2E2t2m2v02\frac{\lambda_0}{\sqrt{2+\frac{e^2E^2t^2}{m^2v_0^2}}}
  4. (4)λ021+e2E2t2m2v02\frac{\lambda_0\sqrt{2}}{\sqrt{1+\frac{e^2E^2t^2}{m^2v_0^2}}}

Official answer

Option 2

NTA final key (Jan 2020).

Same idea in other shifts

Asked 2× in all
  1. 5 Apr 2026, Shift 1 · Q35An electron of mass mm is moving in an electric field E⃗=−2E0i^\vec{E} = -2E_0\hat{i} (E0E_0 = constant >0> 0), with an initial velocity…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.