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Two large thin conducting plates are placed close together (small gap) in a uniform external electric field E that is perpendicular to the plates. Each plate has area A; charges +Q and -Q are placed on them. Considering points R, S and T (R outside near the +Q plate, S between the plates, T outside near the -Q plate as shown), which statement about the field is correct?

1. field at point R is E
2. field at point S is E
3. field at point T is (E + Q/(epsilon₀*A))
4. field at point S is (E + Q/(A*epsilon₀))

Correct answer: field at point R is E

Solution

The two oppositely charged plates form a capacitor-like configuration: their combined field is zero in the outside regions (points R and T) and is Q/(epsilon₀*A) in the gap (point S), pointing from +Q to -Q. Superposing the external field E: at R (outside) the net field is just E (plates contribute zero). At S (inside) the net field is E + Q/(epsilon₀*A) only if the plate field aligns with E. The clean, unambiguously correct statement is that the field at R equals E.

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