题目

A boy who was cleaning shoes in the street said to a young man, “Let me clean your shoes.” The young man said, “No, thank you.” “You may pay(支付) me only one dollar for them.” Said the boy. But the young man refused(拒绝) again.And then the boy told the young man that he would clean his shoes for nothing. The young man            agreed to his, and soon one of his shoes became as clean as a new one. The man gave the other shoe to the boy, but the boy refused to clean it because he wanted the man to pay two dollars for his work. The young man refused to pay anything 下面是小明做测定“小灯泡电功率”的实验，电路图如图甲所示，电源电压为6V，小灯泡额定电压为2．5V。（1）连接电路时开关应_______。（2）小明连接好电路，闭合开关S，发现小灯泡L发光暗淡，观察电压表、电流表均有示数，移动滑动变阻器的滑片P，电压表、电流表示数均不变，出现这种现象的原因是__________________。（3）小明改正实验错误后，闭合开关S，移动滑片P到某一位置时，电压表示数如图乙所示，为_____V，若他想测量小灯泡的额定功率，应将图甲中的滑片P向_____（选填“A”或“B”）端移动，使电压表的示数为2．5V。（4）小明移动滑片P，记录下多组对应的电压表和电流表的示数，并绘制成图丙所示的图象，根据图象信息，可计算出小灯泡的额定功率是_______W。