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NEET 2025 · Dielectrics

Dielectrics question from NEET 2025

Q34NEET 2025Medium

Dielectrics

The plates of a parallel plate capacitor are separated by dd. Two slabs of different dielectric constant K1K_1 and K2K_2 with thickness 38d\frac{3}{8}d and d2\frac{d}{2}, respectively are inserted in the capacitor. Due to this, the capacitance becomes two times larger than when there is nothing between the plates. If K1=1.25K2K_1 = 1.25K_2, the value of K1K_1 is:

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Answer: (c)

  1. Air gap left =d−3d8−d2=d8=d-\frac{3d}{8}-\frac{d}{2}=\frac{d}{8}. All in series: d8+3d8K1+d2K2=d2\frac{d}{8}+\frac{3d}{8K_1}+\frac{d}{2K_2}=\frac{d}{2}
  2. K2=K11.25⇒12K2=0.625K1K_2=\frac{K_1}{1.25} \Rightarrow \frac{1}{2K_2}=\frac{0.625}{K_1}
  3. 0.375+0.625K1=12−18=38⇒K1=83\frac{0.375+0.625}{K_1}=\frac{1}{2}-\frac{1}{8}=\frac{3}{8} \Rightarrow K_1=\frac{8}{3}
  4. So the answer is (c) 2.66.
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