题目

As we know birds do not have teeth. However, according to a new study, this was not always the case. About 116 million years ago, the ancient birds had teeth and used them for eating, just like what they do today. The new study was conducted by a team of researchers that analyzed the remains of the tooth gene in modern birds and tried to find out why the teeth suddenly disappeared in birds during the process of evolution.Because the fossils of ancient birds, buried deeply in the earth, are either broken or not preserve(保护)well, their use in determining this evolutionary(进化)change has n一实验小组同学在“测量小灯泡电功率”的实验中，已知电源电压是6V，小灯泡的额定电压是3.8 V，电阻约为8Ω. （1）请你用笔画线代替导线帮他将图甲中的实物图连接完整；正确连接电路后，闭合开关前，滑动变阻器的滑片应该置于         端（选填“A”或“B”）； （2）实验小组的同学调节滑动变阻器，进行了几次实验，得到部分U和I的数据如下表所示： 实验次数 1 2 3 4 5 6 7 8 U/V 1.00 1.50 2.00 2.50 3.00 3.50 3.80 4.20 I/A 0.20 0.30 0.40 0.44 0.47 0.49 0.51 灯泡亮度 不亮 微亮 很暗 较暗 偏暗 较亮 正常 很亮 ①实验小组同学在对数据进行分析时发现：小灯泡灯丝电阻随电压的增大而         （填“增大”、“减小”或“不变”）。在第1次实验中,小灯泡不发光的原因是_____________________________； ②灯泡正常发光时，电流表的示数如图乙所示，则灯泡的额定功率为        W； ③当小组内的同学测量灯泡地电功率在继续调节滑动变阻器时，不小心把灯丝烧断了（其余元件完好），则此时电压表示数约为         V； （3）一同学在实验室借了甲：“10Ω  1.5 A”、乙：“20Ω  1.2 A”和丙：“30Ω  1 A”三种滑动变阻器各一个，你认为为个实验小组在实验中使用的滑动变阻器是：                ， 理由是：______________________________________________________________。