Numerical answer · +4 / −1
Two charges +q and -q placed 2 m apart attract with 9 N. Find q²/10⁻⁹ in C² (take k=9× 10⁹).
Answer and solution
Answer 4
Solution
Physics sir · 3 steps
The minus sign only tells you it is attraction; put the size of the force into Coulomb's law.
- Magnitudes only. Opposite charges attract, and the force is 9 N. Use the size of each charge, |q|, so the product of the two charges is q².
F = (k q²)/r² ⇒ q² = (F r²)/k
- Put in numbers. q² = (9 × 2²)/(9 × 10⁹) = 36/(9 × 10⁹) = 4 × 10⁻⁹ C².
- So. q²/10⁻⁹ = (4 × 10⁻⁹)/10⁻⁹ = 4.
The common slip: 2. Using r = 2 instead of r² = 4 in the denominator gives q² = 2 × 10⁻⁹ C², so the answer comes out as 2.
The idea to keep. In Coulomb's law the distance is squared. Use magnitudes for the size of the force, and let the signs decide only whether the charges attract or repel.
Answer from the NTA final answer key. Solution by PrepWiser.