在真空中，两个点电荷分别带电量+2×10⁻⁶ C和-4×10⁻⁶ C，相距0.3 m。已知静电力常量k=9×10⁹ N·m²/C²，求它们之间的库仑力大小。
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Correct answer
A. 0.8 N
Principle or equation
库仑定律：F = k|q₁q₂|/r²。
Why this answer is correct
F = 9×10⁹ × |2×10⁻⁶ × (-4×10⁻⁶)| / (0.3)² = 9×10⁹ × 8×10⁻¹² / 0.09 = 72×10⁻³ / 0.09 = 0.8 N。
Example
若电荷量分别为1 μC和2 μC，距离0.1 m，则F = 9×10⁹×2×10⁻¹²/0.01 = 1.8 N。
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