Two large parallel conducting plates carry equal and opposite surface charge density σ. The electric field BETWEEN the plates is:

1. Each conducting plate, by itself, would give E = σ / (2 ε₀) on EACH side (treat as sheet). 2. Between the plates, the fields ADD (both point from + to −). 3. Total E = 2 · σ/(2ε₀) = σ / ε₀ between the plates. 4. Outside the pair, the two plate fields CANCEL → E = 0. _Source: syllabus Physics Ch 1 §1.10 Parallel-Plate Field_