Two charged particles repel each other with a force F. If the charge of both particles is tripled and the distance between them is also tripled, then the force will be
A)F.
B)2 F.
C)F/2.
D)F/4.
E)none of the above

Question

Quizplus · Accepted Answer

The force between two charges is given by Coulomb's law, $F = k \frac{q_1 q_2}{r^2}$. Tripling both charges makes the numerator 9 times larger ($3q \times 3q = 9q^2$), and tripling the distance makes the denominator 9 times larger ($3r \times 3r = 9r^2$). These changes cancel out, leaving the force unchanged.

Two Charged Particles Repel Each Other with a Force F