问题 选择题

如图所示,水平面内有一平行金属导轨(足够长),导轨光滑且电阻不计,阻值为R的导体棒垂直于导轨放置,且与导轨接触良好.导轨所在空间存在匀强磁场,匀强磁场与导轨平面垂直,时,将开关S由1掷向2,分别用表示电容器所带的电荷量、棒中的电流、棒的速度大小和加速度大小,则下图所示的图象中正确的是

答案

答案:D

题目分析:首先分析导体棒的运动情况:开关S由1掷到2,电容器放电后会在电路中产生电流.导体棒通有电流后会受到安培力的作用,会产生加速度而加速运动.导体棒切割磁感线,速度增大,感应电动势增大,则电路中电流减小,安培力减小,加速度减小.因导轨光滑,所以在有电流通过棒的过程中,棒是一直加速运动(变加速)加速度逐渐减小,速度逐渐增大.当感应电动势等于电容器的电压时,电路中无电流,达到一种平衡时,导体棒做匀速运动,加速度为零,速度达到最大值,所以C错误D正确.当棒匀速运动后,棒因切割磁感线有电动势,所以电容器两端的电压能稳定在某个不为0的数值,即电容器的电量应稳定在某个不为0的数值(不会减少到0).这时电容器的电压等于棒的电动势数值,棒中无电流,故A,B错误。

解答题
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阅读理解

     There is a growing problem of what to do with electronic waste such as old televisions, computers,

radios, cellular telephones and other electronic equipment.

     Electronic trash, or e-waste, is piling up faster than ever in American homes and businesses. People

do not know what to do with old televisions or computers so they throw them in the trash.

     National Solid Wastes Management Association(NSWMA) state programs director Chaz Miller says

the large amount of electronic waste Americans produce is not unexpected.

    The US Environmental Protection Agency (EPA) estimates more than 400 million consumer electronic

items are dumped each year, and there is a push by more states to ban the waste from landfills(垃圾填

埋场) and create recycling programs. They can be torn apart and sorted for useable parts. Mike Fannon who runs the plant in Baltimore, says, "There are a lot of valuable metals that can be recovered and

reused instead of just putting them in the landfill, and in certain components there are some materials that

should not really be in the landfill," Fannon explained.

    Fannon says nearly 20 percent of electronic waste is recycled nationwide. Thirteen years ago, it was

only about 6 percent. Recycling rates continue to rise as more communities have banned electronics from

landfills in an effort to keep e-waste poisons like lead(铅) and mercury(汞) out of garbage dumps.

    This year several states like Vermont imposed a ban on electronic waste in landfills. More than 25

other states have also adopted landfill bans, e-waste recycling programs or both. Chaz Miller says more

can be done to boost electronic waste recycling.

     "We can do much better," noted Miller. "I think clearly our goal should be to do as well as we do

recycling newspapers."

1. Which of the following does NOT belong to e-waste? 

A. Old televisions.

B. Old computers.

C. Old cell phones  

D. Old newspapers.

2. Why is e-waste banned from landfills in many states? 

A. Because it can not be recycled. 

B. Because it might damage the environment. 

C. Because it can be shipped to other countries. 

D. Because the landfills are already full.

3. According to Mike Fannon, what might be the best way of dealing with e-waste? 

A. Recycling it.

B. Selling it. 

C. Burying it.

D. Breaking it.

4. What can we learn from the passage?   

A. At present, less then 10 percent of e-waste is recycled. 

B. Chaz Miller works for EPA. 

C. All states in the US have banned e-waste from landfills. 

D. Experts are optimistic about the future of e-waste.