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如图所示,电源电压为6V,R1为“40Ω,0.25A”滑动变阻器,R2为“10Ω,0.5A”电阻,电压表量程为0~3V。图中虚线框内接一只“20Ω,0.4A”电阻R(未画出)。已知:①闭合S1和S3,断开S2时,调节R1使其接入电路的阻值为10Ω,此时电路消耗的电功率为1.2W;②断开S3,闭合S1、S2,调节R1使其接入电路的阻值为6Ω,此时电压表的示数为1.0V。


⑴在情形①中,求电路的总阻值;

⑵根据情形①和②,在图中虚线框内将R连入电路;

⑶当闭合所有开关时,为保证电路安全,求滑动变阻器R1接入电路的阻值范围。

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解:⑴根据P=
U
2
P

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    Honey from the African forest is not only a kind of natural sugar, it is also delicious. Most people, and many animals, like eating it. However, the only way for them to get that honey is to find a wild bees' nest and take the honey from it. Often, these nests are high up in trees, and it is difficult to find them. In parts of Africa, though, people and animals looking for honey have a strange and unexpected helper - a little bird called a honey guide.

    The honey guide does not actually like honey, but it does like the wax in the beehives (蜂房). The little bird cannot reach this wax(蜂蜡), which is deep inside the bees' nest. So, when it finds a suitable nest, it looks for someone to help it. The honey guide gives a loud cry that attracts the attention of both passing animals and people. Once it has their attention, it flies through the forest, waiting from time to time for the curious animal or person as it leads them to the nest. When they finally arrive at the nest, the follower reaches in to get at the delicious honey as the bird patiently waits and watches. Some of the honey, and the wax, always falls to the ground, and this is when the honey guide takes its share.

    Scientists do not know why the honey guide likes eating the wax, but nothing can prevent the birds from making efforts to get it. The birds seem to be able to smell wax from a long distance away. They will quickly arrive whenever a beekeeper is taking honey from his beehives, and will even enter churches when beeswax candles are being lit.

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