A commercial capacitor is constructed as in Figure 16.25a. This particular capac

Question

A commercial capacitor is constructed as in Figure 16.25a. This particular capacitor is made from a strip of aluminum foil separated by two strips of paraffin-coated paper. Each strip of foil and paper is 8.00 cm wide. The foil is 0.00400 mm thick, and the paper is 0.025 mm thick and has a dielectric constant of 3.7. What length should the strips be if a capacitance of9.50 ? 10-8 F is desired before the capacitor is rolled up? (Use the parallel-plate formula. Adding a second strip of paper and rolling up the capacitor doubles its capacitance by allowing both surfaces of each strip of foil to store charge.)

Explanation / Answer

You need to know the formula to calculate the capacitance of a parallel plate capacitor:

Capacitance = Permittivity x Area / thickness (Where permittivity = permittivity of free space x dielectric constant)

Rearrange the formula so that you can calculate the area of the plates.
Divide the area by the width to get the length required.

The thickness you need in the equation is the thickness of the paper, the thickness of the plates makes no difference.

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