真空中，带电量为Q的点电荷产生的电场中，在距离点电荷r处放置一试探电荷，若试探电荷电量为q，则该处的场强大小为（ ）
A．0
B．kq r2
C．kQ r2
D．与放在该点的试探电荷有关系
A、点电荷周围存在电场，电场强度不会为零，故A错误
B、由库仑定律得：F=K
，且E=Qq r2
，故E=kF q
，故B错误Q r2
C、由B分析得，C正确
D、电场强度与试探电荷无关，故D错误
故选C
Passage Four
| Global climate change, often seen as a process stretching over thousands of years, could in fact occur abruptly and unexpectedly-quickly pushing up temperatures by as much as 18 degrees Fahrenheit and wreaking havoc(大破坏，浩劫) on human society, scientists warned on Wednesday. "Climate change is not always smooth. Sometimes it is abrupt," said Richard Alley, a climate expert at Pennsylvania State University and lead author of a new National Academy of Sciences report on the threat of rapid climatic shifts. "If you have a very large, abrupt change, a lot of people and a lot of ecosystems are going to notice," he said．"The bigger and faster it is, the harder it will be to deal with." The new National Academy of Sciences report, released this week, warns that gradual global warming coupled with other human impacts on the environment could "trip the switch" for sudden climate change. At the American Geophysical Union meeting on Wednesday, Alley and other environmental scientists said the geological evidence indicated that such rapid climate shifts had occurred frequently in the past—moving temperatures drastically in the space of just a few decades. "This can happen in less than a human generation, and then it will persist for thousands of years," said David Battisti, an atmospheric scientist at the University of Washington. The most immediate dangers posed by abrupt climate change range are devastating droughts and floods which could seriously affect both water supply and agriculture across vast stretches of the planet. Longer term impacts could include changes in the basic systems which determine regional global temperatures. Scientists believe that the Gulf Stream, a current (水流) of warm Atlantic water which now keeps much of Northern Europe temperate, could theoretically reverse direction if enough cool fresh water runs into the north Atlantic from melting ice, a change that would quickly impact European weather. |
Abrupt climate change______.
A．is unprecedented in human history
B．will not happen in the near future
C．will probably cause dramatic drop in temperature
D．will last thousands of years once it happens