Coulomb's law provides the formula for the electric force between two point charges. Which of the following is the correct expression?

Coulomb's law provides the formula for the electric force between two point charges. Which of the following is the correct expression?

• A. F = G m1 m2 / r^2
• B. F = k * q1 * q2 / r^2
• C. F = k / (q1 q2 r^2)
• D. F = q1 q2 / (k r)

Answer: (B)

Understanding how electric forces behave, Coulomb’s law says the force between two point charges is proportional to the product of the charges and inversely proportional to the square of the separation. The constant k sets the strength in the chosen units. This gives the magnitude as F = k q1 q2 / r^2. In vacuum, k is about 8.99×10^9 N·m^2/C^2. The force can attract or repel depending on the signs of q1 and q2, but the way it scales with distance and charges is exactly captured by this inverse-square form. The other expressions don’t match that behavior: using G and masses describes gravity, not electricity; omitting the charges or placing them in the denominator changes how the force scales with q1 and q2; and having the distance in the denominator only once or raising the distance to the first power breaks the inverse-square relationship.

Understanding how electric forces behave, Coulomb’s law says the force between two point charges is proportional to the product of the charges and inversely proportional to the square of the separation. The constant k sets the strength in the chosen units. This gives the magnitude as F = k q1 q2 / r^2. In vacuum, k is about 8.99×10^9 N·m^2/C^2. The force can attract or repel depending on the signs of q1 and q2, but the way it scales with distance and charges is exactly captured by this inverse-square form.

The other expressions don’t match that behavior: using G and masses describes gravity, not electricity; omitting the charges or placing them in the denominator changes how the force scales with q1 and q2; and having the distance in the denominator only once or raising the distance to the first power breaks the inverse-square relationship.