题目

交流发电机的发电原理是矩形线圈在匀强磁场中绕垂直于磁场的轴OO′匀速转动.一小型发电机的线圈共220匝，线圈面积S=0.05 m2，线圈转动的频率为50 Hz，线圈内阻不计，磁场的磁感应强度.为用此发电机所发出交流电带动两个标有“220 V  11 kW”的电机正常工作，需在发电机的输出端a、b与电机之间接一个理想变压器，电路如图13-36.求： 图13-36 （1）发电机的输出电压为多少？ （2）变压器原、副线圈的匝数比为多少？ （3）与变压器原线圈串联的交流电 江苏省常州市横山桥高级中学10-11学年高二上学期期中考试（英语     "Time," says the proverb, "is money". This means that every moment well spent may put some money into our pockets. If our time is usefully employed, it will either turn out some useful and important piece of work which will fetch its price in the market, or it will add to our experience and increase our abilities so as to enable us to earn money when the proper opportunity comes. Let those, who think nothing of wasting time, remember this.     Our life is nothing more than our time. To kill time is therefore a form of suicide. We are shocked when we think of death, and we spare no pains, no trouble, and no expense to preserve life.     But we often care little about the loss of an hour or of a day, forgetting that our life is the sum of the days and of the hours we live. A wasted day or an hour is therefore so much life missing. Our life is a brief span measuring some seventy or eighty years in all. But nearly one third of this has to be spent in sleep; some years have to be spent over our meals; some in making journeys on land and voyages by sea; some in merry-making; some in watching over the sick-beds of our nearest and dearest relatives. Now if all these years were to be deducted (扣除) from the term over which our life extends, we shall find about twenty or thirty years for active work. Whoever remembers this can never willingly waste a single moment of his life. All time is precious; but the time of our childhood and of our youth is more precious than any other part of our existence. For those are the periods when alone we can gain knowledge and develop our abilities.     If we allow these morning hours of life to slip away, we shall never be able to make up the loss. Just as money laid out for interest doubles or trebles (成三倍) itself in time, so the precious hours of childhood and youth, if properly used, will bring lots of advantages for us. 68. Which of the following do you think is the best title of the selection? A. Treasure your time    B. Life is short              C. Study is important     D. Save time 69. The author gives us a proverb at the beginning of the selection___________.    A. to present the topic directly               B. to inform the readers of the type of the selection C. to tell us how the selection is organized   D. to explain why he writes such a selection 70. According to the selection, if you waste several hours playing net games, __________.    A. you can make up the loss by staying up                   B. you will lose the time for ever C. you must double your work                                         D. you will be cleverer, though.