This formula doesn't tell you which way it's pointing, because it's a scaler; it's not a vector equation.

这个式子没有说明方向,因为这是一个标量而非矢量方程

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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

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So the minus vector is the same vector flipped over, pointing the opposite way.

因此负矢量即为同一矢量的翻转,指向相反的方向

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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

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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

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