有些密度大的物质容易让电流通过。
Some dense substances allow electricity to flow through them freely.
没有能流通过节点（或腹点），但在其他各点，能流密度随时间作周期性变化。
There is no current flowing through any node(or antinode. ), but the current density changes periodically with time at other places.
功率谱密度是评价流通环境过程中随机振动的有效工具。
Power spectral density is an effective tool to evaluate random vibration in circulation environment.
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