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有关适时制的说法，不正确的是()。
电压源与电流惊的等效互换只是对() 等效，其目的是计算外电路的()和().
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The Magic of Sound
Music is one of the most beautiful forms of artistic expressions ever invented. In movies and plays， music has an added function: it not only moves people but also can shock people. Is it true that an ordinary musical instrument can be so powerful？
Our eardrums can withstand sound within 20 to 80 decibels. Once sound exceeds this limit，even beautiful music will become ear-splitting noise and harm health. A strong blast of high sound can twist and break a solid iron sheet. __________ （46）
The noise from a plane"s engine is over 140 decibels. However， the sound of a flute is at most a few decibels. __________ （47） It has been proven that people who have worked in an environment with a high sound intensity for a long time suffer varying degrees of heart disease or altered brain waves.
In movies， sometimes the hero can produce a sound that ordinary people can"t hear and only those who have the same ability can feel. In nature， there is actually sound that is beyond our hearing. In physics， the sound that exceeds 20，000 Hz is called ultrasonic. __________ （48） It does no harm to health.
Sound less than 20 Hz is called infrasonic waves. When we move， the air will vibrate. __________ （49）
As the frequency of infrasonic waves is close to that of people"s internal organs， infrasonic wave may cause resonance in human bodies. As a result， people"s vision may weaken and internal organs may rupture. However， whether an infrasonic wave can be used as a weapon depends on its intensity. If its intensity is very low， it won"t damage internal organs or a person"s health. __________ （50） When wind blows at a force of 3 or 4 over the sea， it will produce infrasonic waves of several decibels.
Only typhoons can produce infrasonic waves of over 100 decibels. At present scientist can only produce infrasonic weapons in the lab with the help of advanced scientific tools and powerful electric power.
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电路中，当电流的实际方向与电流的参考方向一致时，电流为() 值。
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