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Electric field due to uniformly charged disc is considered one the most difficult concept.
8 Questions around this concept.
A thin disc of radius b=2a has a concentric hole of radius 'a' in it (see figure). It carries uniform surface charge $\sigma$ on it. If the electric field on its axis at height' h ' (h<<a) from its centre is given as 'Ch' then the value of 'Ch' is :
A uniformly charged disc of radius R having surface charge density $\sigma$ is placed in the $x y$ plane with its center at the origin. Find the electric field intensity along the $z$-axis at a distance $Z$ from origin :
The value of electric field due to a charged disc of surface charged density $\sigma$ and radius R , for points situated near the disc is:
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If a charged spherical non - conductor of radius 15 cm has a potential V at a point on its surface then the potential at a point distance 20 cm from the center is:
Electric field due to uniformly charged disk
Let us take a disk of radius R with a uniform positive surface charge density (charge per unit area) . Our aim is to find an electric field at a point on the axis of the disk at a distance x from its centre.
From the figure, we can see that the we have taken a typical ring has charge $d Q$, inner radius $r$ and outer radius $\mathrm{r}+\mathrm{d} \mathrm{r}$. Its area $\mathrm{d} A$
$
d E_x=\frac{1}{4 \pi \varepsilon_0} \frac{(2 \pi \sigma r d r) x}{\left(x^2+r^2\right)^{3 / 2}}
$
If we integrate from 0 to $R$, we will get the total field -
$
E_x=\int d E_x=\int_0^R d E_x=\int_0^R \frac{1}{4 \pi \varepsilon_0} \frac{(2 \pi \sigma r d r) x}{\left(x^2+r^2\right)^{3 / 2}}
$
Here, ' $x$ ' is constant and 'r' is the variable. After integration we get -
$
E_x=\frac{\sigma x}{2 \varepsilon_0}\left[-\frac{1}{\sqrt{x^2+R^2}}+\frac{1}{x}\right]=\frac{\sigma}{2 \varepsilon_0}\left[1-\frac{x}{\sqrt{x^2+R^2}}\right]
$
As this disc is symmetric to $x$-axis, so the field in the rest of the component is zero i.e., $E_y=E_z=0$
Special case -
1) When $R \gg>x$, then $\quad E_x=\frac{\sigma}{2 \varepsilon_0} \quad$ Note that this equation is independent of ' x '
2) When $x \rightarrow 0$ (i.e very near to disc), then $E_x=\frac{\sigma}{2 \varepsilon_0}$
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