When the separation between the plates of an air parallel-plate capacitor is doubled (keeping plate area constant), the capacitance

1. C = ε₀ A / d. 2. Doubling d gives C' = ε₀ A / (2d) = C/2. 3. So capacitance halves; the field also halves at fixed Q. _Source: syllabus Physics Part I, Ch 2 §2.12_