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Electrostatics question

2025 · 8 Apr · Shift 2 · Q66
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Electrostatics question

2025 · 8 Apr · Shift 2 · Q66

JEE MainPhysicsElectrostaticsMCQ+4 / −1
Electric charge is transferred to an irregular metallic disk as shown in the figure. If σ1\sigma_1σ1​, σ2\sigma_2σ2​, σ3\sigma_3σ3​ and σ4\sigma_4σ4​ are charge densities at given points then, choose the correct answer from the options given below: JEE Main 2025 (Online) 8th April Evening Shift Physics - Electrostatics Question 17 English A. σ1>σ3;σ2=σ4\sigma_1\gt \sigma_3 ; \sigma_2=\sigma_4σ1​>σ3​;σ2​=σ4​ B. σ1>σ2;σ3>σ4\sigma_1\gt \sigma_2 ; \sigma_3\gt \sigma_4σ1​>σ2​;σ3​>σ4​ C. σ1>σ3>σ2=σ4\sigma_1\gt \sigma_3\gt \sigma_2=\sigma_4σ1​>σ3​>σ2​=σ4​ D. σ1<σ3<σ2=σ4\sigma_1\lt \sigma_3\lt \sigma_2=\sigma_4σ1​<σ3​<σ2​=σ4​ E. σ1=σ2=σ3=σ4\sigma_1=\sigma_2=\sigma_3=\sigma_4σ1​=σ2​=σ3​=σ4​
  1. A
    B and C Only
  2. B
    A and C Only
  3. C
    D and E Only
  4. D
    A, B and C Only
View written solutionFree

Correct answer: D

  1. Principle to use

For a charged conductor in electrostatic equilibrium:

  • Charge resides on the outer surface.
  • The surface charge density is greater where the surface is more sharply curved (smaller radius of curvature).
  • At geometrically equivalent/symmetric points, charge density is equal.

So, to compare σ1,σ2,σ3,σ4\sigma_1,\sigma_2,\sigma_3,\sigma_4σ1​,σ2​,σ3​,σ4​, we use the local curvature of the irregular metallic disk.

  1. Comparing the marked points

From the figure (irregular metallic disk), point 111 is on the sharpest protruding region, point 333 is on a less sharp curved region, and points 222 and 444 are on similar broader regions.

Hence: σ1>σ3>σ2=σ4\sigma_1 > \sigma_3 > \sigma_2 = \sigma_4σ1​>σ3​>σ2​=σ4​

  1. Check each statement
  • Statement A: σ1>σ3\sigma_1 > \sigma_3σ1​>σ3​ and σ2=σ4\sigma_2 = \sigma_4σ2​=σ4​

    This is true.

  • Statement B: σ1>σ2\sigma_1 > \sigma_2σ1​>σ2​ and σ3>σ4\sigma_3 > \sigma_4σ3​>σ4​

    Since σ1>σ3>σ2=σ4\sigma_1 > \sigma_3 > \sigma_2 = \sigma_4σ1​>σ3​>σ2​=σ4​, both parts are true. So B is true.

  • Statement C: σ1>σ3>σ2=σ4\sigma_1 > \sigma_3 > \sigma_2 = \sigma_4σ1​>σ3​>σ2​=σ4​

    This matches the correct ordering exactly, so C is true.

  • Statement D: σ1<σ3<σ2=σ4\sigma_1 < \sigma_3 < \sigma_2 = \sigma_4σ1​<σ3​<σ2​=σ4​

    Opposite of the actual trend, so false.

  • Statement E: σ1=σ2=σ3=σ4\sigma_1 = \sigma_2 = \sigma_3 = \sigma_4σ1​=σ2​=σ3​=σ4​

    This would be true only for a highly symmetric shape like a sphere, not for an irregular conductor. So false.

  1. Selecting the correct option

True statements are A, B and C only.

Therefore, the correct option is: D\boxed{D}D​

  1. Comparison with stored answer

Stored correct answer: DDD

Our derived answer also is DDD, so they agree.

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