Two positive charges +Q are held fixed a distance d apart. A particle of negative charge -q and mass m is placed midway between them and displaced along the line joining the charges.

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Two positive charges +Q are held fixed at and from the origin of X-axis as shown in the figure. A charge q is placed at the origin which is midway between the two charges +Q and is displaced along the X-axis by a small distance x. We assume

The net Coulomb force on the charge q will be

Equation of motion of the charge q of mass m will therefore be

Scalar form of this equation is

It is of the form of equation of simple harmonic motion (SHM). The period of SHM described by this equation will be

Or