First slide

Electrostatic force

Question

An isolated charge q₁ of mass m is suspended freely by a thread of length l. Another charge q₂ is brought near it (r >> l). When q₁ is in equilibrium tension in thread will be

Moderate

A

mg
B

>mg
C

<mg
D

none of these

Solution

Initial tension: $T_{1} = m g$
Final tension: $T_{2} \cos θ = m g$
or $T_{2} = \frac{m g}{\cos θ}$
Obviously, $T_{2} > m g .$
Here we have assumed $θ$ to be small so that F is almost horizontal.

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