It’ s said that a giant rock was located in the North Sea but once many boats and ships （strike）it and sank.
About a century years ago, a kind and imaginative minister came up with an idea to have a buoy（浮标） （fasten）to the rock. On the top of the buoy
（place）a bell, which would ring whenever any ships got closer. When hearing the bell ringing, sailors knew where the rock was and stayed safe. One day, a sea robber Rover happened
（sail） not far away from the rock.
Seeing buoy floating in the water, he decided to play a trick. Soon his crew broke the chain that held the buoy, let the bell go into the water and then sailed away. In the following weeks, they robbed many
ships around the seas and filled the ship with a great
（vary） of treasures. Unfortunately, when returning, they themselves had no idea how to get away from the terrible rock
the ring. The ship was moving swiftly in the dark
there was a great crash.
was the very rock and the ship began to sink.
将一个标有“ ”字样的灯泡，接到电压为 的电源上，此时该灯泡的实际功率为 （灯丝电阻不随温度改变）。由三节新干电池串联组成的电池组的电压为 。
如图所示，当开关 闭合后，悬挂的小磁铁处于静止状态，若滑动变阻器的滑片 向右移动，则悬挂小磁铁的弹簧将会 （选填“伸长”或“缩短” 。电动摩托是现在人们常用的交通工具，电动摩托的动力来自电动摩托上的电动机；电动机工作时是把 能转化为机械能。
如图所示，当开关 闭合时，灯 、 均不发光，电流表示数为零，但电压表指针有明显偏转，则此电路的故障是 。
如图所示，当开关 闭合和断开时，电流表的示数之比是 ，则电灯 和 的电阻之比是 。甲、乙、丙、丁四个轻质小球，靠近时的相互作用情况是：甲排斥乙，乙吸引丙，丙排斥丁，若丁带负电，则甲一定 。
两个电阻的电流随电压变化的关系图象如图甲所示，将它们连接在如图乙所示的电路中。当闭合开关 ，断开开关 、 时，电阻 、 两端的电压之比为 ；当断开开关 ，闭合开关 、 时，经过一段时间，电流通过电阻 、 产生的热量之比为 。
