Question

The plates of a parallel-plate capacitor have an area of 90 cm² each and are separated by 2.5 mm. The capacitor is charged by connecting it to a 400 V supply. How much electrostatic energy is stored by the capacitor?

(a) 3.6 J
(b) 7.2 J
(c) 14.4 J
(d) 28.8 J

Answer 1

The electrostatic energy stored by the capacitor is (c) 14.4 J

How much electrostatic energy is stored by the capacitor?

C = ε * A / d

where:

ε is the permittivity of free space (approximately 8.854 x 10^-12 F/m)

A is the area of each plate (in square meters)

d is the distance between the plates (in meters)

Convert the area and distance to meters:

A = 90 cm² * (1 m² / 10,000 cm²) = 0.009 m²

d = 2.5 mm * (1 m / 1,000 mm) = 0.0025 m

C = 8.854 x 10^-12 F/m * 0.009 m² / 0.0025 m = 3.168 x 10^-10 F

Finally, we can calculate the energy stored:

U = (1/2) * 3.168 x 10⁻¹⁰ F * (400 V)² ≈ 14.4 J

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