在真空中，两个点电荷分别带 +2×10⁻⁶ C 和 -3×10⁻⁶ C 的电量，相距0.3 m。已知静电力常量 k = 9×10⁹ N·m²/C²，求它们之间的静电力大小。
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Correct answer
A. 0.6 N
Principle or equation
库仑定律：F = k |q1 q2| / r²。
Why this answer is correct
代入数据：F = 9×10⁹ × |2×10⁻⁶ × (-3×10⁻⁶)| / (0.3)² = 9×10⁹ × 6×10⁻¹² / 0.09 = 54×10⁻³ / 0.09 = 0.6 N。
Example
若电荷量分别为1×10⁻⁶ C和2×10⁻⁶ C，相距0.2 m，则 F = 9×10⁹×2×10⁻¹²/0.04 = 0.45 N。
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