$4.$ A commercial capacitor is to be constructed as shown in Figure. This particular capacitor is made from two strips of aluminum foil separated by a strip of paraffin$-$coated paper. Each strip of foil and paper is $7$ cm wide. The foil is $0\mathrm{.}004$ mm thick, and the paper is $0\mathrm{.}025$ mm thick and has a dielectric constant of $3\mathrm{.}7.$ What length should the strips have if a capacitance of $9\mathrm{.}5$ x $\backslash (10-8\backslash$mathrm$\{$~F$\}\backslash )$ is desired before the capacitor is rolled up$?$