A body having specific charge 8 μ C/g is resting on a frictionless… | JEE Main 2021 (20 Jul, Shift 1)

Q31JEE Main 2021 · 20 JUL · SHIFT 1Medium

Motion of Charges in an Electric Field

A body having specific charge 8 $\mu$ C/g is resting on a frictionless plane at a distance 10 cm from the wall (as shown in the figure). It starts moving towards the wall when a uniform electric field of 100 V/m is applied horizontally towards the wall. If the collision of the body with the wall is perfectly elastic, then the time period of the motion will be _______________ s.

Numerical answer type. Work it out, then open the answer.

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Answer: 1

Specific charge $\frac{q}{m} = 8\,\mu\text{C/g} = 8 \times 10^{-3}\text{ C/kg}$ , so $a = \frac{q}{m}E = 0.8\text{ m/s}^2$ .
Time to reach the wall from rest: $0.1 = \frac{1}{2}(0.8)t^2 \Rightarrow t = 0.5\text{ s}$ .
After the elastic bounce it decelerates back to rest at the start in another $0.5\text{ s}$ .
Time period $= 2t = 1\text{ s}$ .

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