问题 计算题

(8分)如图所示,平行导轨ab、cd所在平面与匀强磁场垂直,ac间连接电阻R,导体棒ef横跨直导轨间,长为L,电阻值为r,所在磁场的磁感应强度为B,方向垂直纸面向里,导轨的表面光滑,其电阻可忽略,在ef棒以恒定速度v向右运动的过程中,求:

①电阻R中的电流大小和方向;

②伏特表的示数;

③所加外力的大小;

④电阻R的发热损耗的电功率。

答案

①大小:BLv/(R + r),方向:A经R到b

②示数:RBLv/(R + r)

③ B2L2v/(R + r)

④ P =   R(BLv)2/(R + r)2

阅读理解

阅读理解。

     Swimming is very popular in the world. People like swimming in summer, even in the other three

seasons. Because water makes. people feel cool and swimming makes people keep fit.

     If you swim in a wrong place, it may not be safe (安全的). A lot of people died when they were

enjoying  themselves in the water, and most of them were students. Last summer, a few students from

one school went to swim in the river near their school. As a result, three of them never came out of the

river.

     Now people are not careful enough (足够的) in swimming. They often think they can swim so well

that nothing will happen to them in water.

     Summer is here again. If you go swimming in summer,don't forget that a lot of good swimmers have

died in water. They died not because they couldn't, swim, but because of their carelessness (粗心大意).

Don't get into water when you are alone .If there is a "No Swimming" sign, don't get into water. If you

remember these, swimming will be safe and enjoyable (有趣的).

1. People like to swim in summer because ______.    

A. they feel cool in the water     

B. they are free     

C. it makes them strong

D. they are good swimmers

2. Some  swimmers died in water because ______.     

A. they think they  can swim well     

B. they are not strong enough     

C. they are students

D. they are not careful

3. The writer wants to tell us ______.     

A. swimming is good in summer     

B. that we should go swimming in summer     

C. that we must be careful in swimming

D. students should not swimming the river

4. Which of  the. statements  (叙述) is TRUE?     

A. Swimming in a wrong place may be not safe.     

B. No people died when they were enjoying swimming in the water.

C. Nothing will happen to good swimmers in winter.

D. If someone goes swimming in summer, he will die.

单项选择题