本题目来源于试卷: 美国2021年物理杯赛真题 PHYSICS BOWL 2021，类别为 美国物理杯赛(Physics Bowl)
[单选题]
Two identical metal spheres are hung from a common point by 30-cm long, nonconducting threads ofnegligible mass. The spheres each have a mass of 0.20 g and are given identical charges. It is foundthat due to the electrostatic repulsion force, the angle between the threads is 20°. How much chargewas placed on each sphere?
A. 11 nC
B. 16 nC
C. 21 nC
D. 26 nC
E. 31 nC
参考答案: C
本题详细解析:
$F_e=k \frac{q_1 q_2}{r^2}$;$F_g=mg$; $k \frac{q_1 q_2}{r^2} = mg tan \theta $
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