And his eyes have all the seeming of a Demon that is dreaming, And the lamplight o'er him streaming throws his shadow on the floor;

VOA: special.2010.10.31

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

er is a vector at each point of length one pointing radially away from the center.

r 是一个模长恒为 1 的矢量,方向沿半径向外

耶鲁公开课 - 基础物理课程节选

So sometimes what we do is introduce a little vector here called er.

通常我们的做法就是引入一个矢量 er

耶鲁公开课 - 基础物理课程节选

So er is a new entity I've introduced for convenience.

因此,er 是为了方便而引入的

耶鲁公开课 - 基础物理课程节选

er is not a fixed vector.

r 不是一个固定矢量

耶鲁公开课 - 基础物理课程节选