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Using thomson's model of the atom, consider an atom consisting of two electrons, each of charge $-e$, embeded in a sphere of charge $+2e$ and radius $R$. In equilibrium each electron is at a distance $d$ from the centre of the atom. What is the equilibrium separation between electrons

$R$
$\frac{R}{2}$
$\frac{R}{3}$
$\frac{R}{4}$
Solution

Considering gaussian surface
$\phi=\frac{q_{\text {in }}}{\varepsilon_{0}} \Rightarrow E 4 \pi d^{2}=\frac{2 e d^{3}}{\varepsilon_{0} R^{3}}$
$\mathrm{E}=\frac{2 \mathrm{edk}}{\mathrm{R}^{3}}$
For equilibrium of charge,
$\frac{\text { kee }}{(2 d)^{2}}=\left(\frac{\text { k } 2 \text { ed }}{R^{3}}\right) e$
$\frac{1}{4 \mathrm{d}^{2}}=\frac{2 \mathrm{d}}{\mathrm{R}^{3}} \Rightarrow \mathrm{d}^{3}=\frac{\mathrm{R}^{3}}{8} \Rightarrow \mathrm{d}=\mathrm{R} / 2$