Two charged particles, placed at a distance d apart in vacuum, exert a force F on each other. Now, each of the charges is doubled. To keep the force unchanged, the distance between the charges should be changed to ________ .
4d
2d
d
d/2
2d
According to Coulomb′s law, the force between two charges is given by F = k q1q2/d2 . When each charge is doubled, to keep the force constant, the distance d must be increased by a factor of √4 =2, so the new distance is 2d