JEE Advanced 2015 · Paper 1 · Physics Q13

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PhysicsMultiple correct+4 / -2Question 13 of 60

The figures below depict two situations in which two infinitely long static line charges of constant positive line charge density $\lambda$ are kept parallel to each other. In their resulting electric field, point charges $q$ and $-q$ are kept in equilibrium between them. The point charges are confined to move in the $x$ direction only. If they are given a small displacement about their equilibrium positions, then the correct statement(s) is(are)

Two panels, each with two parallel vertical line charges λ and a dashed x-axis between them. Left: a point charge +q midway between the lines. Right: a point charge −q midway between the lines.

Both charges execute simple harmonic motion.

Both charges will continue moving in the direction of their displacement.

Charge

+q

executes simple harmonic motion while charge

-q

continues moving in the direction of its displacement.

Charge

-q

executes simple harmonic motion while charge

+q

continues moving in the direction of its displacement.

Answer and worked solution

Official answer: C

Field between the lines. Two equal positive line charges give zero field at the midpoint. Displaced towards one line, the field there points back towards the midpoint, and it grows linearly for small displacements.

$+q$ : the force is restoring, so it executes SHM.
$-q$ : the force points away from the midpoint, so it keeps moving in the direction of its displacement.

(C)

Topics:Electric field Simple harmonic motion

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JEE Advanced 2015 · Paper 1, Physics question 13