It's good to have a vector pointing in the radial direction of length one.

所以引入这么一个模长为 1,方向沿圆心向外辐射的矢量作用很大

耶鲁公开课 - 基础物理课程节选

At each point, er is a different vector pointing in the radial direction of length one.

矢量 er 在每一点处都不同,方向都从圆心指向该点,模长为1

耶鲁公开课 - 基础物理课程节选

It could form in any direction because the Coulombic field is radial in all directions.

能够在任何方向形成,因为库仑产在全方位发生分散。

麻省理工公开课 - 固态化学导论课程节选