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1. Electric Charges and Fields
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A charge $Q$ is distributed over a line of length $L.$ Another point charge $q$ is placed at a distance $r$ from the centre of the line distribution. Then the force expericed by $q$ is

A
$\frac{{qQ}}{{4\pi { \in _0}({r^2} - {L^2})}}$
B
$\frac{{qQ}}{{4\pi { \in _0}({r^2} - {{(L/2)}^2})}}$
C
$\frac{{qQ}}{{4\pi { \in _0}{r^2}}}$
D
$\frac{{qQL}}{{4\pi { \in _0}{r^3}}}$
Solution

${\rm{E}} = \int_{{\rm{r – }}\frac{{\rm{L}}}{2}}^{{\rm{r}} + \frac{{\rm{L}}}{2}} {\rm{k}} \lambda {\rm{dx}}/{{\rm{x}}^2}$
$ = \frac{{{\rm{KQ}}}}{{\rm{L}}}\left( { – \frac{1}{{\rm{x}}}} \right)_{{\rm{r – }}\frac{{\rm{L}}}{2}}^{{\rm{r}} + \frac{{\rm{L}}}{2}}$
$=\frac{\mathrm{KQ}}{\left(\mathrm{r}^{2}-(\mathrm{L} / 2)^{2}\right)}$
$\mathrm{F}=\mathrm{q} \mathrm{E}$
Standard 12
Physics
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