请讲解下题目 和 解析思路，及涉及知识点---**Question Stem:** 【例题7-2】(2024·龙岗区高一期末统考)富兰克林为研究雷电现象，设计了如图甲所示的装置。帽形金属钟A上面与较长的避雷针相接，下面用等长细导线悬挂两个金属小球，形成验电器D，帽形金属钟B接地，两钟之间有一个用绝缘丝线悬挂的金属小球C。不带电时三个金属小球均静止下垂。若带负电的云层靠近避雷针时，金属小球C会左右来回撞击A、B钟发出响声，同时看到验电器D的两个金属小球张开一定角度。 **Sub-question (1):** (1) 验电器D的两个金属小球因带______电(填“正”或“负”)而张开。与金属钟B相连的接地线中电子定向移动方向是______(填“自上而下”或“自下而上”)。 **Sub-question (2):** (2) 如图乙，验电器D上的两个金属小球位置等高，质量均为m，相距为L，且可视作点电荷。悬线与竖直方向的夹角均为θ，已知重力加速度为g，静电常量为k，不考虑金属钟与小球间的作用力，求： i. 两球间的库仑力大小； ii. 假设两个金属小球带相同的电荷量，求此电荷量的大小。 **Chart/Diagram Descriptions:** **Chart 1: 图甲 富兰克林避雷装置示意图 (Figure A Schematic Diagram of Franklin's Lightning Protection Device)** * **Type:** Schematic diagram. * **Main Elements:** * **带负电的云层 (Negatively charged cloud):** Shown at the top, representing a source of charge. * **金属钟A (Metal Bell A):** A cap-shaped metal object. * **避雷针 (Lightning rod):** A long rod connected to the top of metal bell A, extending upwards towards the cloud. * **验电器D (Electroscope D):** Formed by two small metal spheres suspended by equal-length thin wires from the bottom of metal bell A. * **金属钟B (Metal Bell B):** Another cap-shaped metal object, located to the left of metal bell A. * **接地 (Ground connection):** A wire connecting metal bell B to the ground (indicated by a ground symbol). * **金属小球C (Metal Ball C):** A small metal sphere suspended by an insulating thread between metal bell A and metal bell B. * **Relative Position and Direction:** * The negatively charged cloud is positioned above metal bell A. * The lightning rod extends from the top of metal bell A towards the cloud. * Electroscope D's spheres hang vertically below metal bell A. * Metal bell B is positioned horizontally to the left of metal bell A. * Metal ball C is suspended in the space between metal bell A and metal bell B. * **Title:** 图甲 富兰克林避雷装置示意图 (Figure A Schematic Diagram of Franklin's Lightning Protection Device) **Chart 2: 图乙 验电器示意图 (Figure B Schematic Diagram of the Electroscope)** * **Type:** Schematic diagram showing a detailed view of the electroscope (part D from Figure A). * **Main Elements:** * **Upper suspension point:** Implied to be part of "金属钟A". * **Two small metal spheres:** Each labeled 'm', representing their mass. They are suspended by two threads of equal length. * **Suspension threads:** Straight lines connecting the spheres to the suspension point. * **Vertical dashed lines:** Drawn downwards from each suspension point, indicating the vertical direction. * **Angle θ:** The angle between each suspension thread and its respective vertical dashed line is labeled as 'θ'. * **Distance L:** The horizontal distance between the centers of the two spheres is labeled as 'L'. * **Labels and Annotations:** 'm' (mass), 'L' (distance), 'θ' (angle). * **Title:** 图乙 验电器示意图 (Figure B Schematic Diagram of the Electroscope)