So, instead of having the periodic variation of water, or the periodic variation of air density here we're talking about an electric field.

不像水或者空气密度的,周期变化，我们这里,讨论的是电场。

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The calorie density has changed because food is processed, so the calories in the fat and the sugar that can get squeezed into things made possible by technological changes has shifted things a lot.

食品经过加工,热量密度发生了变化,添加到食品中的脂肪和糖类,其卡路里含量可通过技术手段进行调整,进而造成很大不同

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So, for example, we have this average level, and then it can go high where we have the peak, or it can go very low. We can also discuss sound waves, so again it's just the periodic variation of some property -- in this case we're talking about density, so we have high density areas and low density areas.

例如这是平均位置，在峰的位置水位高，在谷水位低，我们也可以,讨论声波它也是某种,量的周期变化，在这里我们,讨论的是密度,我们有高密度区和低密度区。

麻省理工公开课 - 化学原理课程节选