A block of mass 'm' and charge ' q ' is connected to a point ' 0 ' with an inextensible string. This system is on a horizontal table. An electric field (E) is applied perpendicular to the string and in the plane of the horizontal table. The tension in the string when it becomes parallel to the electric field is

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$\frac{3 q E}{4}$

3qE

2qE

answer is D.

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Detailed Solution

As the charge moves it will gain some kinetic energy,

$\begin{aligned} ΔK \cdot E = qEI = {mv}^{2} / 2 \\ \Rightarrow {mv}^{2} / I = 2 qE \end{aligned}$

Let, T be the tension at final position

$\begin{aligned} T - qE = 2 qE \\ T = 3 qE \end{aligned}$

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