The Feynman demonstration that electromagnetic field momentum is real—even for static fields—can be made more pedagogically useful by simplifying its geometry. Instead of Feynman’s disk with charged balls on its surface, this article uses the geometry of a hollow non-conducting sphere with uniform surface charge density. With only methods available in a typical upper-division electrodynamics course, the initial field angular momentum and the final mechanical angular momentum can then be calculated in closed form and shown to be equal.

中文翻译：

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费曼静态场动量示例的球形版本


费曼证明电磁场动量是真实的——即使对于静态场也是如此——通过简化其几何形状，可以使电磁场动量在教学上更加有用。本文没有使用表面带有带电球的费曼圆盘，而是使用具有均匀表面电荷密度的空心非导电球体的几何形状。仅使用典型的高级电动力学课程中可用的方法，就可以以封闭形式计算初始场角动量和最终机械角动量，并证明它们是相等的。