Two parallel circular plates, each with a radius of and carrying equal-magnitude surface charge densities of are separated by a distance of with only air between them. How much energy is stored in these plates? (ε0 = 8.85 × 10-12 C2/N ∙ m2)

Complete Question

Two parallel circular plates with radius 7 mm carrying equal -magnitude surface charge densities of ± 3.0μC/m2 are separated by a distance of 1 mm. How much stored energy do the plates have?  (ε0 = 8.85 × 10-12 C2/N ∙ m2)

a. 7.9 nJ     b. 25 nJ    c. 78 nJ     d. 250 nJ

The energy stored in the plate is 78 nJ

Explanation:

From the question we are given that

            The radius is  7 mm  

            The surface charge density

            The distance between the plates as d =  1mm

Generally the formula for energy stored in a capacitor is mathematically given as

Where A is the area  which is mathematically

                                                   =

 Substituting values into the equation we have

does anyone know or what?