A parallel-plate capacitor with air has capacitance C. When a dielectric slab (k = 4) fills half the gap in series, the new capacitance is
Correct: C
Two capacitors in series: air gap C₁ = ε₀A/(d/2) = 2C, dielectric gap C₂ = 4·2C = 8C. C_eq = (C₁C₂)/(C₁+C₂) = 16C²/10C = 8C/5. But original C = ε₀A/d, so new C = 4C/5.