91Ó°ÊÓ

Consider the below figure, the electric filed and electric potential on the axis of solid cylinder.

Here, the figure shows the uniformly charged solid cylinder and its axis is the along the axis at the center of the origin.

Here, Lis the Length of the cylinder, R is the radius of the cylinder and surface charge density.

Write the potential at the equatorial position due to uniform surface charge of disc is given as,

$\mathit{d}\mathit{V}\mathbf{=}\frac{\mathbf{σ}}{\mathbf{2}{\mathbf{∈}}_{\mathbf{0}}}\mathbf{(}\sqrt{{\mathbf{R}}^{\mathbf{2}}\mathbf{+}{\mathbf{Z}}^{\mathbf{2}}\mathbf{-}\mathbf{Z}\mathbf{)}}$

Here, zdistance from the center of a disc at the point P.

Consider the thickness of each disc is dz.

Consider distance of the slice from the point Pwith respect to left end is .

Consider the distance of the slice from the point Pwith respect to right end is$z-\frac{L}{2}$.Write the formula for the potential at point due to the whole cylinder is$z-\frac{L}{2}$obtained by integrating the equation with limits to$z-\frac{L}{2}toz+\frac{L}{2}$.

$$\begin{gathered} \mathrm{V}=\frac{\mathrm{p}}{2{\mathrm{Î\mu }}_0}{∫}_{\mathrm{z}-\frac{\mathrm{L}}{2}}^{\mathrm{z}+\frac{\mathrm{L}}{2}}\left(\sqrt{{\mathrm{R}}^2+{\mathrm{z}}^2-}\right)\mathrm{dz} \\ =\frac{\mathrm{p}}{2{\mathrm{Î\mu }}_0}\frac{1}{2}{\left|\left[\mathrm{z}\sqrt{{\mathrm{R}}^2+{\mathrm{z}}^2}+{\mathrm{R}}^2\mathrm{In}\left(\mathrm{z}+\sqrt{{\mathrm{R}}^2+{\mathrm{z}}^2}\right)-{\mathrm{z}}^2\right]\right|}_{\mathrm{z}-\frac{\mathrm{L}}{2}}^{\mathrm{z}+\frac{\mathrm{L}}{2}} \\ =\frac{\mathrm{p}}{4{\mathrm{Î\mu }}_0}\left\{\begin{array}{c}\left(\mathrm{z}+\frac{\mathrm{L}}{2}\right)\sqrt{{\mathrm{R}}^2+{\left(\mathrm{z}+\frac{\mathrm{L}}{2}\right)}^2}-\left(\mathrm{z}-\frac{\mathrm{L}}{2}\right)\sqrt{{\mathrm{R}}^2+{\left(\mathrm{z}-\frac{\mathrm{L}}{2}\right)}^2}\\ +\mathrm{RIn}\left[\frac{\mathrm{z}+{\displaystyle \frac{\mathrm{L}}{2}}+\sqrt{{\mathrm{R}}^2+{\left(\mathrm{z}+\frac{\mathrm{L}}{2}\right)}^2}}{\mathrm{z}-{\displaystyle \frac{\mathrm{L}}{2}}+\sqrt{{\mathrm{R}}^2+{\left(\mathrm{z}-\frac{\mathrm{L}}{2}\right)}^2}}\right]\end{array}\right\}-2\mathrm{zL} \\ \mathrm{Now}\mathrm{finding}\mathrm{the}\mathrm{electric}\mathrm{field}\mathrm{due}\mathrm{to}\mathrm{the}\mathrm{cylinder}\mathrm{at}\mathrm{the}\mathrm{point}\mathrm{p}, \\ \mathrm{E}=-∇\mathrm{V} \\ \mathrm{The}\mathrm{electric}\mathrm{field}\mathrm{along}\mathrm{the}\mathrm{z}\mathrm{axis}\mathrm{is}, \\ \mathrm{E}=-\frac{∂\mathrm{V}}{∂\mathrm{z}}\hat{\mathrm{z}} \end{gathered}$$

$$\begin{gathered} \mathrm{Substitute}\frac{\mathrm{p}}{4{\mathrm{Î\mu }}_0}\left\{\begin{array}{c}\left(\mathrm{z}+\frac{\mathrm{L}}{2}\right)\sqrt{{\mathrm{R}}^2+{\left(\mathrm{z}+\frac{\mathrm{L}}{2}\right)}^2}-\left(\mathrm{z}-\frac{\mathrm{L}}{2}\right)\sqrt{{\mathrm{R}}^2+{\left(\mathrm{z}-\frac{\mathrm{L}}{2}\right)}^2}\\ +\mathrm{RIn}\left[\frac{\mathrm{z}+{\displaystyle \frac{\mathrm{L}}{2}}+\sqrt{{\mathrm{R}}^2+{\left(\mathrm{z}+\frac{\mathrm{L}}{2}\right)}^2}}{\mathrm{z}-{\displaystyle \frac{\mathrm{L}}{2}}+\sqrt{{\mathrm{R}}^2+{\left(\mathrm{z}-\frac{\mathrm{L}}{2}\right)}^2}}\right]-2\mathrm{zL}\end{array}\right\}\mathrm{for}\mathrm{V}\mathrm{in}\mathrm{above} \\ \mathrm{equation}. \\ \mathrm{E}=-\frac{\hat{\mathrm{z}}\mathrm{p}}{4{\mathrm{Î\mu }}_0∂\mathrm{z}}\left\{\begin{array}{c}\left(\mathrm{z}+\frac{\mathrm{L}}{2}\right)\sqrt{{\mathrm{R}}^2+{\left(\mathrm{z}+\frac{\mathrm{L}}{2}\right)}^2}-\left(\mathrm{z}-\frac{\mathrm{L}}{2}\right)\sqrt{{\mathrm{R}}^2+{\left(\mathrm{z}-\frac{\mathrm{L}}{2}\right)}^2}\\ +\mathrm{RIn}\left[\frac{\mathrm{z}+{\displaystyle \frac{\mathrm{L}}{2}}+\sqrt{{\mathrm{R}}^2+{\left(\mathrm{z}+\frac{\mathrm{L}}{2}\right)}^2}}{\mathrm{z}-{\displaystyle \frac{\mathrm{L}}{2}}+\sqrt{{\mathrm{R}}^2+{\left(\mathrm{z}-\frac{\mathrm{L}}{2}\right)}^2}}\right]-2\mathrm{zL}\end{array}\right\} \\ \mathrm{Now}\mathrm{partially}\mathrm{differentiating}\mathrm{the}\mathrm{above}\mathrm{equation}\mathrm{with}\mathrm{respect}\mathrm{to}\mathrm{z}. \end{gathered}$$

$E=-\frac{\hat{z}p}{4{\mathrm{Î\mu }}_0}\left\{\begin{array}{c}\left\{\sqrt{R^2+{\left(z+\frac{L}{2}\right)}^2}+\frac{{\left(z+\frac{L}{2}\right)}^2}{\sqrt{R^2+{\left(z+\frac{L}{2}\right)}^2}}-\sqrt{R^2+{\left(z-\frac{L}{2}\right)}^2}-\frac{{\left(z-\frac{L}{2}\right)}^2}{\sqrt{R^2+{\left(z-\frac{L}{2}\right)}^2}}\right\}\\ +R^2\left[\frac{1+{\displaystyle \frac{z+{\displaystyle \frac{L}{2}}}{\sqrt{R^2+{\left(z+\frac{L}{2}\right)}^2}}}}{\left(z+\frac{L}{2}\right)+\sqrt{R^2+{\left(z+\frac{L}{2}\right)}^2}}-\frac{1+{\displaystyle \frac{z-{\displaystyle \frac{L}{2}}}{\sqrt{R^2+{\left(z-\frac{L}{2}\right)}^2}}}}{\left(z-\frac{L}{2}\right)+\sqrt{R^2+{\left(z-\frac{L}{2}\right)}^2}}\right]-2L\\ \end{array}\right\}$

Simplify the above equation,

$$\begin{gathered} \mathrm{E}=\frac{-\hat{\mathrm{z}}\mathrm{p}}{4{\mathrm{Î\mu }}_0}\left\{2\sqrt{{\mathrm{R}}^2+{\left(\mathrm{z}+\frac{\mathrm{L}}{2}\right)}^2}-2\sqrt{{\mathrm{R}}^2+{\left(\mathrm{z}-\frac{\mathrm{L}}{2}\right)}^2}-2\mathrm{L}\right\} \\ =\frac{\mathrm{p}}{2{\mathrm{Î\mu }}_0}\left[\mathrm{L}-\sqrt{{\mathrm{R}}^2+{\left(\mathrm{z}+\frac{\mathrm{L}}{2}\right)}^2}+\sqrt{{\mathrm{R}}^2+{\left(\mathrm{z}-\frac{\mathrm{L}}{2}\right)}^2}\right]\hat{\mathrm{z}} \\ \mathrm{Therefore},\mathrm{the}\mathrm{electric}\mathrm{field}\mathrm{is}\frac{\mathrm{p}}{2{\mathrm{Î\mu }}_0}\left[\mathrm{L}-\sqrt{{\mathrm{R}}^2+{\left(\mathrm{z}+\frac{\mathrm{L}}{2}\right)}^2}+\sqrt{{\mathrm{R}}^2+{\left(\mathrm{z}-\frac{\mathrm{L}}{2}\right)}^2}\right]\hat{\mathrm{z}} \end{gathered}$$