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Question 31591
JEE Physics · Hard

Capacitance: Capacitor and Dielectric Slab Force Calc MCQ

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Capacitor and Dielectric Slab Force CalcHardQuestion 31591

Question

A dielectric slab (K=3, width w, area A) is pulled out of a capacitor (plate separation d) connected to voltage V. Force required is:
  1. A
    ε₀V²w(K−1)/(2d)
    Correct
  2. B
    ε₀V²w(K+1)/(2d)
  3. C
    ε₀KV²w/(2d)
  4. D
    ε₀V²w/(2d)

Correct answer

ε₀V²w(K−1)/(2d)

Explanation

F = dU/dx = (V²/2)·dC/dx. As slab is withdrawn, C decreases. |dC/dx| = ε₀w(K−1)/d. So F = ε₀V²w(K−1)/(2d).

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