A parallel-plate capacitor has a plate separation of 3.0 mm and a potential difference of 300 V between the plates. What is the electric field strength inside the capacitor?

• A. 5×10^4 V/m
• B. 1×10^5 V/m
• C. 2×10^5 V/m
• D. 3×10^5 V/m

Answer: (B)

Between the plates, the electric field is essentially uniform and given by E = V/d. With a potential difference of 300 V and a plate separation of 3.0 mm (3.0 × 10^-3 m), the field is E = 300 / (3.0 × 10^-3) = 1.0 × 10^5 V/m. So the electric field inside is 1×10^5 V/m, directed from the positive plate toward the negative plate. The other numbers would require different separation or voltage, so they don’t match the given values.