问题 实验题

有一灯泡上标有“6V   0.1A”字样,现要测绘该灯泡的伏安特性曲线,有下列器材可供选用:

A.电压表(0~5V,内阻2.0kΩ)

B.电压表(0~10V,内阻3.0kΩ)

C.电流表(0~0.3A,内阻2.0Ω)

D.电流表(0~6A,内阻1.5Ω)

E.滑动变阻器(30Ω,2A)

F.学生电源(直流9V)及开关、导线等

(1)实验中所用的电压表应选___________________,电流表应选____________________.

(2)画出实验电路图,要求电压从零开始测量

答案

(1)B , C(2)略

题目分析:(1) 小灯泡的额定电压为6V,从安全和精确度方面考虑,电压表量程选10V的;小灯泡的额定电流为0.1A,所以电流表量程选0.3A的.故选BC

(2)小灯泡在正常工作时的电阻,远小于电压表内阻,属于小电阻,“小外偏小”,电流表采用外接法.电压和电流需从零开始测起,所以滑动变阻器采用分压式接法.

点评:解决本题的关键掌握器材选取的原则,以及知道滑动变阻器分压式和限流式接法的区别,电流表内外接的区别.

问答题
单项选择题

In 1998 consumers could purchase virtually anything over the Internet. Books, compact discs, and even stocks were available from World Wide Web Sites that seemed to spring up almost daily. A few years earlier, some people had predicted that consumers accustomed to shopping in stores would be reluctant to buy things that they could not see or touch in person. For a growing number of time-starved consumers, however, shopping from their home computer was proving to be a convenient alternative to driving to the store.

A research estimated that in 1998 US consumers would purchase $ 7.3 billion of goods over the Internet, double the 1997 total. Finding a bargain was getting easier, owing to the rise of online auctions and Web sites that did comparison shopping on the Internet for the best deal.

For all the consumer interest, retailing in cyberspace was still a largely unprofitable business, however. Internet pioneer Amazon. com, which began selling books in 1995 and later branched into recorded music and videos, posted revenue of $153.7 million in the third quarter, up from $37.9 minion in the same period of 1997. Overall, however, the company’s loss widened to $45.2 million from $9.6 million, and analysts did not expect the company to turn a profit until 2001. Despite the great loss, Amazon. com had a stock market value of many billions, reflecting investors’ optimism about the future of the industry.

Internet retailing appealed to investors because it provided an efficient means for reaching millions of consumers without having the cost of operating conventional stores with their armies of salespeople. Selling online carried its own risks, however. With so many companies competing for consumers’ attention, price competition was intense and profit margins thin or nonexistent. One video retailer sold the hit movie Titanic for $9.99, undercutting the $19.99 suggested retail price and losing about $6 on each copy sold. With Internet retailing still in its initial stage; companies seemed willing to absorb such losses in an attempt to establish a dominant market position.

Finding a bargain on the Internet was getting easier partly because()

A. there were more and more online auctions

B. there were more and more Internet users

C. the consumers had more money to spend

D. there were more goods available on the Internet