题目

在纸面上有一个等边三角形ABC，顶点处都通有相同电流的三跟长直导线垂直于纸面放置，电流方向如图15-2-9所示，每根通电导线在三角形的中心O产生的磁感应强度大小为B0.中心O处磁感应强度的大小为______. 图15-2-9 答案：解析：直线电流的磁场是以直线电流为中心的一组同心圆，中心O点处三个直线电流的磁场方向如图15-2-10所示.由于对称性，它们互成120°角，由于它们的大小相等，均为B0，根据矢量合成的特点，可知它们的合矢量为零. 图15-2-10答案：零After a busy day of work and play, the body needs to rest. Sleep is necessary for good health. During this time, the body recovers from the activities of the previous day. The rest that you get while sleeping enables your body to prepare itself for the next day.　　There are four levels of sleep, each being a little deeper than the one before. As you sleep, your muscles relax little by little. Your heart beats more slowly, and your brain slows down. After you reach the fourth level, your body shifts back and forth from one level of sleep to the other.　　Although your mind slows down, from time to time you will dream. Scientists who study sleep state that when dreaming occurs, your eyeballs begin to move more quickly (Although your eyelids are closed). This stage of sleep is called REM, which stands for rapid eye movement.　　If you have trouble falling asleep, some people recommend breathing very deeply. Other people believe that drinking warm milk will help make you drowsy. There is also an old suggestion the counting sheep will put you to sleep!1.A good title for this passage is ____A.SleepB.DreamsC.Good HealthD.Work and Rest2.The word drowsy in the last paragraph means ____A.sickB.asleepC.stand upD.a little sleepy3.This passage suggests that not getting enough sleep might make you _____A.dream more oftenB.have poor healthC.nervousD.breathe quickly4.During REM, ____A.your eyes move quicklyB.you dreamC.you are restlessD.both A and B5.The average number of hours of sleep that an adult needs is ______A.approximately six hoursB.around ten hoursC.about eight hoursD.not described here