问题 问答题

如图甲所示,空间有Ⅰ区和Ⅲ区两个有理想边界的匀强磁场区域,磁感应强度大小均为B,方向如图所示。两磁场区域之间有宽度为s的无磁场区域Ⅱ。abcd是由均匀电阻丝做成的边长为LLs)的正方形线框,每边的电阻为R。线框以垂直磁场边界的速度v水平向右匀速运动,从Ⅰ区经过Ⅱ区完全进入Ⅲ区,线框ab边始终与磁场边界平行。求:

(1)当ab边在Ⅱ区运动时,dc边所受安培力的大小和方向;

(2)线框从完全在Ⅰ区开始到全部进入Ⅲ区的整个运动过程中产生的焦耳热;

(3)请在图乙的坐标图中画出,从ab边刚进入Ⅱ区,到cd边刚进入Ⅲ区的过程中,

da两点间的电势差Uda随时间t变化的图线。其中E0 = BLv

 

答案

(1) ;方向水平向左   (2)

(3)

(1) dc边产生的感应电动势 E = BLv    (1分)

线框中的感应电流        (1分)

dc边所受的安培力   F = BIL   (1分)

求出           (1分)

         方向水平向左  (1分)

(2)ab边经过Ⅱ区时,设电流为I1,所用时间为t1,产生的热为Q1

有                (1分)

      (1分)

   由焦耳定律    Q1 = I12·4R t1    (1分)

ab边进入Ⅲ区到cd边进入Ⅱ区,设电流为I2,所用时间为t2,产生的热为Q,2 有             (1分)

      (1分)

由焦耳定律       Q2 = I22·4R t2   (1分)                         

整个过程中产生的热 Q = 2Q1 + Q2    (1分)

求出       (1分)                  

(3)                         

(横轴时间标对1分,

纵轴电势差标对1分,

图线每段1分)

单项选择题
单项选择题

No blueprint exists for transforming an economy from one with a great deal of government control to one based almost solely on free market principles. Yet the experience of the United Kingdom since 1979 clearly shows one approach that works: privatization, in which under-performing state-owned are sold to private companies.

By 1979, the total amount of debt, liabilities, and losses for state-controlled enterprises in the UK topped 3 billion annually. By selling off many of these companies, particularly those in the depressed industrial sector, the government decreased its debt burden and ceased pumping public funds into money losing enterprises. According to government spokesperson Alistair McBride, "Far from past practice of throwing good money after bad, the Queen’s government this year expects to take in 34 billion from the proceeds of the sale." That, say some analysts, may only be the beginning. Privatization has not only been credited with rescuing whole industries but the nation’s economy to boot.

Due to increased tax revenues from the newly privatized companies along with a rebound in the overall economy, economic forecasters predict that Britain will be able to repay nearly 12.5% of the net national debt within two years. That is good news indeed for the economy as a whole at a time when many sectors are desperate for any ray of sunshine. British Airways this week announced a 20% jump in overall ticket sales and profits over this quarter a year ago. British Gas announced its first profitable quarter in nine years. At Associated British Ports, a new labor contract was finalized, the first union contract signed at the port without a work stoppage in twelve years. Closer to home for most Britons, the nation’s phone service, British Telecom, no longer puts new subscribers on a waiting list. Prior to privatization, new customers would sometimes have to wait months before phone service could be installed in their home. Now, according to a company press release, British Telecom is promising 24-hour turnaround for all new customers.

Part of this improved productivity has to do with new efforts to allow employees to hold a stake in the company’s future. Companies now give their employees stock options that allow employees to share in the company’s success (and profits). The response has been enthusiastic to say the least. At British Aerospace; 89% of those eligible to buy company shares did so. At British Telecom nearly 92% of eligible employees took part. Finally, at Associated British Ports, long synonymous with union disagreements, walkouts, and labor strife, almost 90% of employees now can call themselves owners of the company.

"When people have a personal stake in something," said Henry Dundee of Associated British Ports, "they think about it, they care about, they work to make it prosper." At the National Freight Consortium, itself no stranger to labor problems, the new employee-owners actually voted down an employee pay-increase and, pressured union representatives to relax demands for increased wages and expanded benefits. "Privatization was only the start," says one market analyst, "what we may have here is a new industrial revolution.\

The best title for the text might be()

A. Privatization is a Double-edged Sword

B. Privatization is a Way out for UK’s Economy

C. Privatization Makes Government the Winner

D. Privatization Offers a Promising Future