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1. Electric charges and fields — Physics STD 12 Science — Question

CBSE BoardEnglish MediumSTD 12 SciencePhysics1. Electric charges and fields1 Mark
MCQ
A liquid drop having $6$ excess electrons is kept stationary under a uniform electric field of $25.5\, k\,Vm^{-1}$ . The density of liquid is $1.26\times10^3\, kg\, m^{-3}$ . The radius of the drop is (neglect buoyancy)
  • A
    $4.3\times10^{-7}\, m$
  • ✓
    $7.8\times10^{-7}\, m$
  • C
    $0.0078\times10^{-7}\, m$
  • D
    $3.4\times10^{-7}\, m$
✓

Answer

Correct option: B.
$7.8\times10^{-7}\, m$
b
$\mathrm{F}=\mathrm{qE}=\mathrm{mg}\left(\mathrm{q}=6 \mathrm{e}=6 \times 1.6 \times 10^{-19}\right)$

Density $(\mathrm{d})=\frac{\text { mass }}{\text { volume }}=\frac{\mathrm{m}}{\frac{4}{3} \pi r^{3}}$

or $r^{3}=\frac{m}{\frac{4}{3} \pi d}$

Putting the value of $\mathrm{d}$ and $\mathrm{m}\left(=\frac{\mathrm{qE}}{\mathrm{g}}\right)$ and

solving we get $r=7.8 \times 10^{-7} \,\mathrm{m}$

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