First slide

Electric field

Question

A pendulum of bob of Mass $80 m g$ and carrying a charge of $2 \times 10^{- 8} C$ is at rest in a uniform horizontal electric field of $20 k V m^{- 1}$ . Find the tension in the string

NA

Solution

At equilibrium $T \cos θ = m g a n d T \sin θ = E q$
$∴ T^{2} = {(m g)}^{2} + {(E q)}^{2} T = \sqrt{{(80 \times 10^{- 6} \times 10)}^{2} + {(20 \times 10^{3} \times 2 \times 10^{- 8})}^{2}} = \sqrt{80 \times 10^{- 8}} = 8.8 \times 10^{- 4} N$

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