问题 多选题

如图所示,abcd是粗细均匀的电阻丝制成的长方形线框,长ad为宽ab的2倍,导体棒是用相同的电阻丝制成的,可在ad边与bc边上无摩擦滑动,导体棒与线框接触良好,并与ad和bc分别交于M、N两点,线框处于垂直纸面向里的匀强磁场中,导体棒与ab平行.当导体棒MN由靠近ab边处向cd边匀速移动的全过程中,以下说法正确的是(  )

A.MN中电流先增大后减小

B.MN两端电压先增大后减小

C.MN上拉力的功率先减小后增大

D.矩形线框中消耗的电功率先增大后减小

答案

导体棒MN向右运动的过程中,外电阻先增大后减小,当MN运动到线框中线时,外电路的电阻最大.导体棒匀速运动,导体棒产生的感应电动势E=BLv不变;

A、体棒MN由靠近ab边处向cd边匀速移动的全过程中,外电阻R先增大后减小,由闭合电路的欧姆定律得:

I=

E
R+r
,由于E与r不变,R先增大后减小,因此感应电流先减小后增大,则通过MN的电流先先减小后增大,故A错误;

B、MN两端的电压UMN=E-Ir,由于E与r不变,I先减小后增大,因此MN两端的电压先增大后减小,故B正确;

C、MN匀速运动,则拉力F=FB=BIL,拉力的功率P=Fv=BILv,B、L、v不变,I先减小后增大,则拉力功率先减小后增大,故C正确;

D、MN匀速运动,拉力等于安培力,安培力功率等于拉力功率,拉力功率先减小后增大,因此安培力功率先减小后增大,克服安培力做功的功率就是电路消耗的电功率,则电路消耗的电功率先减小后增大,故D错误;

故选:BC.

阅读理解

Unlike modern animal scientists, dinosaur scientists cannot sit on a hillside and use telescopes to watch dinosaurs in order to know how they lived and whether they were good parents. Instead, they have to search hard for information from dinosaurs’fossils(恐龙化石) because dinosaurs died out millions of years ago.

It’s very difficult for the scientists to reach an agreement because different results can be got from the same fossils, Many fossils of the same kind of dinosaurs have been dug out from one place. They might have formed when an entire group of dinosaurs got stuck(陷入)all at once, or they might have been the result of dinosaurs getting stuck one after another over a course of a few centuries. Thus we can say that dinosaurs might have in the first case lived in big groups and in the second lived alone.

Though there are two different results, dinosaur scientists now generally agree that at least some kinds of dinosaurs lived in big groups. “That’s pretty much settled at this point.” Says Paul Sereno, A kind of dinosaurs called Sauropods left behind tracks in the western United States that appear to run north and south, suggesting that they even moved long distances together.

As to whether dinosaurs cared for their young, dinosaur scientists have turned to the closest living relatives of dinosaurs ― birds and crocodiles ― for possible models. Birds give a lot of care to their young, while crocodiles just help their young to the water. The discovered fossils of dinosaurs sitting on their eggs and staying with their young suggest the parents were taking care of their babies, but we still cannot say that all dinosaurs did the same.

There is still a long way to go before the above questions could be answered. Dinosaur scientists will have to find more proof to reach an agreement.

60.Dinosaur scientists can get information directly by           .

A.  studying dinosaur fossils                B. examining modern animals

C. watching dinosaurs                         D. using telescopes

61.What is pretty much settled according to Paul Sereno?

A. Half of the dinosaurs lived alone.    B. Most dinosaurs moved long distances.

C. Many dinosaurs settled in the north. D.Some dinosaurs lived in big groups.

62.Dinosaur scientists can probably know whether dinosaurs were good parents by              .

A. watching many kinds of animals      B. studying dinosaurs’living relatives

C. following the tracks left behind              D. working on dug-out dinosaur eggs

63.Which of the following the according to the paragraph?

A. Birds hardly ay attention to their young.  B. Baby crocodiles can look after themselves well.

C. Some dinosaurs took care of their young.       D. Birds and crocodiles take good care of their young.

单项选择题