Asmall ball of charge q and mass m has a velocity v at infinity. it collides head-on with a ball of the same charge and mass which is initially at rest. what is the minimal distance between the charges during the collision?

Explanation:

Kinetic energy of ball in motion = 1/2 m v² . Potential energy = 0

Let the minimum distance between the balls be d  on collision.

Electric potential energy at that time= k Q²/d , Here kinetic energy is converted into potential energy . So

1/2 m v² = kQ²/d

d =2 k Q² / mv²,= 18 x 10⁹ x Q²/ m v².

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