题干

收音机的音量控制电路部分如图所示,调节滑动变阻器的滑片P可控制扬声器的音量,但收音机直接收到的信号有干扰,即有直流和高频信号,为此需要用电容器C1C2滤去直流和高频部分.根据电容器容抗的性质,试确定C1C2应分别用电容较大的还是较小的电容器.

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答案(点此获取答案解析)

因为容抗随交流电频率的增大而减小。由题意可知C1的作用是滤去直流成分,而且要让低频成分通过,故应选用电容较大的电容器;C2的作用是滤去高频成分且要阻碍低频成分通过,故选用电容较小的电容器.

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    I'd planned to spend my weekend in the sun, potting flowers outdoors. But that was before the pair of crows (乌鸦). Out of nowhere, they had set up their base camp in our backyard.

    We first discovered our feathered enemies on Friday morning. It was early, and the sound the birds made traveled through our neighborhood. I figured they'd go away soon. But, no. They were getting louder and probably annoying my neighbors nearly as much as us with the noise. I admit I got a little desperate.

    Then, on Sunday afternoon, I looked out the kitchen window to see our dog Quatchi staring at something on the ground. I went to explore and found a small coal-black bird in the grass. Its eyes were milky, and it stayed perfectly still as my dog inched towards it. Only when Quatchi touched his nose to its head did it walk away. I took hold of the dog and locked him inside and came back with my camera.

    It never occurred to me that the young would be hanging out on the ground, growing and gaining strength as its parents guarded it from above. The second I saw this little creature I forgave the crows.

    To be honest, I've never been much of a bird person. I also have a healthy respect for birds, and by healthy I mean slightly fear-based. They're basically living dinosaurs.

    I wish they'd chosen another backyard. There are only so many sunny days in Seattle, and I still can't do any gardening out there. But I'm also grateful that I got to see this little bit of nature unfold up close. It's pretty amazing.

    The experience has also strengthened my desire to create a beautiful backyard garden that attracts lots of bees, butterflies, and yes, birds. They are welcome here.