alleryStart learning →A charge of -4 x 10^-9 C sits 3.0 cm from a charge of +3 x 10^-9 C. What is the magnitude and direction of the force between them?
A1.2 x 10^-4 N, attractive
B1.2 x 10^-4 N, repulsive
C3.6 x 10^-4 N, attractive
D1.2 x 10^-6 N, attractive
Answer & Solution
Correct answer: A. 1.2 x 10^-4 N, attractive
1. Magnitude: F = k |q1 q2| / r^2 = (8.99 x 10^9 N m^2/C^2)(4 x 10^-9 C)(3 x 10^-9 C) / (0.030 m)^2.
2. The numerator is 1.079 x 10^-7 N m^2, and dividing by (0.030 m)^2 = 9.0 x 10^-4 m^2 gives 1.2 x 10^-4 N.
3. One charge is negative and the other is positive, so the charges are unlike and the force is attractive.
4. Option B keeps the correct size but calls it repulsive, which forgets the sign rule for unlike charges; that mix-up is the main sign trap for oppositely charged pairs.
_Source: OpenStax Physics (CC BY 4.0), Ch 18 "Static Electricity", section 18.2 Coulomb's law_
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