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  4. The electric field strength in N C -1 that is required to just prevent a water drop carrying a charge 1.6 × 10-19 C from falling under gravity is (g = 9.8 ms-2, mass of water drop = 0.0016 g)

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Q. The electric field strength in $N \,C\,{}^{-1}$ that is required to just prevent a water drop carrying a charge $1.6 \times 10^{-19} \, C$ from falling under gravity is $(g\, = \,9.8\, ms^{-2}$, mass of water drop = $0.0016\,g$)

KEAMKEAM 2016Electric Charges and Fields

Solution:

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Charge on drop $=1.6 \times 10^{-19} C$
Mass of the drop $=0.0016\, g$
$=16 \times 10^{-4}\, g $
$=16 \times 10^{-7}\, kg $
$g=9.8\, m / s ^{-2}$
In balance position,
$m g=q E$
where, $E$ is electric field strength.
$E =\frac{m g}{q}=\frac{16 \times 10^{-7} \times 9.8}{1.6 \times 10^{-19}}$
$=9.8 \times 10^{13} N / C$