# cylindrical capacitor

 $D (r) = \frac{Q}{2 \pi r l} = \frac{\tau}{2 \pi r}$ $E (r) = \frac{\tau}{2 \pi \epsilon_0 \epsilon_r} \frac{1}{r}$ $C = \frac{2 \pi \epsilon_0 \epsilon_r}{\ln (\frac{b}{a})}$