问题 选择题

如图所示,竖直放置的一对平行金属板间的电势差为U1,水平放置的一对平行金属板间的电势差为U2.一电子由静止开始经U1加速后,进入水平放置的金属板间,刚好从下板边缘射出.不计电子重力,下列说法正确的是:

A.增大U1,电子一定打在金属板上

B.减小U1,电子一定打在金属板上

C.减小U2,电子一定能从水平金属板间射出

D.增大U2,电子一定能从水平金属板间射出

答案

BC

题目分析:电子在加速度电场中满足:,在偏转电场中:水平方向:L=vt;竖直方向:,联立三式可得:,由此式可知,减小U1,可增大偏转距离y,则电子一定打在金属板上,选项B正确,A错误;减小U2,可减小偏转距离y,则电子一定能从水平金属板间射出,选项C 正确,D错误。 

单项选择题

The China boom is by now a well-documented phenomenon. Who hasn’t (1) the Middle Kingdom’s astounding economic growth (8 percent annually), its mesmerizing (2) market (1.2 billion people), the investment ardor of foreign suitors ($40 billion in foreign direct investment last year (3) ) China is an economic juggernaut. (4) Nicholas Lardy of the Brookings Institution, a Washington D. C.-based think tank,

" No country has (5) its foreign trade as fast as China over the last 20 years. Japan (6) its foreign trade over a 20-year period; China’s foreign trade as quintupled. They’ve become the pre-eminent producer of labor-intensive (7) goods in the world " . But there’s been (8) from the dazzling China growth story—namely, the Chinese multinational. No major Chinese companies have (9) established themselves, or their brands, (10) the global stage. But as Haier shows, that is starting to change. (11) 100 years of poverty and chaos, of being overshadowed by foreign countries and multinationals, Chinese industrial companies are starting to (12) on the world.A new generation of large and credible firms has (13) in China in the electronics, appliance and even high-tech sectors. Some have reached critical mass on the mainland and are now (14) new outlets for their production—through exports and by building Chinese factories abroad, chiefly in Southeast Asia. One example: China’s investment in Malaysia (15) from $8 million in 2000 to $766 million in the first half of this year. (16) China’s export prowess, it will be years (17) Chinese firms achieve the managerial and operational expertise of Western and Japanese multinationals. For one thing, many of its best companies are still at least partially state-owned. (18) , China has a shortage of managerial talent and little notion of marketing and brand-building. Its companies are also (19) by the country’s long tradition of central planning, inefficient use of capital and antiquated distribution system, (20) makes building national companies a challenge.

(17)处填()

A.after

B.before

C.since

D.till

单项选择题