交流电是其方向周期地变化的电流。

Alternating current is the current whose direction changes periodically.

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周期地或者好象周期性地运动。

To move in or as if in a cycle.

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最简单的加载情况是周期地、均匀而重复地施加的载荷。

The simplest imposed conditions are those in which the loading is periodically and uniformly repeated.

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And in contrast, in the lower left hand part of the periodic table, these 2 quantities are low, so also what we're going to see is low electronegativity.

相反地，在周期表的左下部分，这两个量的值都很小，因此我们会看到这里的电负性也很低。

麻省理工公开课 - 化学原理课程节选

Depending on what kind of chemistry you go in to, you might accidentally memorize parts of the table, which is fine, but what you really want to know how to do is know how to use the periodic table.

取决于你今后从事哪个方向的化学研究，你可能会不经意地记住周期表的某一部分，这就够了，但是要注意,你真正需要知道的是如何使用元素周期表。

麻省理工公开课 - 化学原理课程节选

And so here is the Periodic Table in its glory today following on Mendeleev's scheme.

所以，这是周期表,今天非常光荣地跟在门捷列夫的图表之后。

麻省理工公开课 - 固态化学导论课程节选