问题 选择题

在图1所示的装置中,K为一金属板,A为金属电极,都密封在真空的玻璃管中,W为由石英片封盖的窗口,单色光可通过石英片射到金属板K上,E为输出电压可调的直流电流,其负极与电极A相连,

是电流表,实验发现,当用某种频率的单色光照射K时,K会发出电子(光电效应),这时,即使A、K之间的电压等于零,回路中也有电流.当A的电势低于K时,当A比K的电势低到某一值Uc时,电流消失,Uc称为截止电压,当改变照射光的频率ν,截止电压Uc也将随之改变,其关系如图2所示,如果某次实验我们测出了画出这条图线所需的一系列数据,又知道了电子电量,则 (  )

①可求得该金属的极限频率

②可求得该金属的逸出功

③可求得普朗克常量

④可求得中子的质量.

A.①②③④

B.②③

C.①③④

D.①②③

答案

根据光电效应方程得,Ekm=hv-W0.因为最大初速度与遏止电压的关系为:

1
2
mvm2=eUc.则eUC=hv-W0,所以UC=
hv
e
-
W0
e
.知图线的斜率k=
h
e
,通过斜率可以求出普朗克常量.当UC=0时,hv=W0,可得极限频率v0=
W
h
,即横轴截距.从而可以求出逸出功.中子的质量无法求出.故D正确,A、B、C错误.

故选D.

单项选择题
单项选择题

Here in the U. S. a project of moving the government a few hundred miles to the southwest proceeds apace, under the supervision of Senator Robert Byrd of West Virginia. Apart from the usual highways and parks, Byrd has taken a special interest in transplanting pieces of federal agencies from metropolitan Washington to his home state.

Strangely, Byrd’s little experiment in de-Washingtonization has become the focus of outrage among the very people who are otherwise most critical of Washington and its ways. To these critics, it is the very symbol of congressional arrogance of power, isolation from reality, contempt for the voters, and so on, and demonstrates the need for term limits if not lynching.

Consider the good-government advantages of (let’s call it) the Byrd Migration. What better way to symbolize an end to the old ways and commitment to reform than physically moving the government What better way to break up old bureaucracies than to uproot and transplant them, files and all

Second, spreading the government around a bit ought to reduce that self-feeding and self regarding Beltway culture that Washington-phobes claim to dislike so much. Of course there is a good deal of hypocrisy in this anti-Washington chatter. Much of it comes from politicians and journalists who have spent most of their adult lives in Washington and wouldn’t care to live anywhere else. They are not rushing to West Virginia themselves, except for the occasional quaint rustic weekend. But they can take comfort that public servants at the Bureau of the Public Debt, at least, have escaped the perils of inside-the-Beltway insularity.

Third, is Senator Byrd’s raw spread-the-wealth philosophy’ completely illegitimate The Federal Government and government-related private enterprises have made metropolitan Washington one of the richest areas of the country. By contrast, West Virginia is the second poorest state, after Mississippi. The entire country’s taxes support the government. Why shouldn’t more of the country get a piece of it As private businesses are discovering, the electronic revolution is making it less and less necessary for work to be centralized at headquarters. There’s no reason the government shouldn’t take more advantage of this trend as well.

It is hardly enough, though, to expel a few thousand midlevel bureaucrats from the alleged Eden inside the Washington Beltway. Really purging the Washington Culture enough to satisfy its noisiest critics will require a mass exodus on the order of what the Khmer Rouge instituted when they took over Phnom Penh in 1975. Until the very members of the TIME Washington bureau itself are traipsing south along 1-95, their word processors strapped to their backs, the nation cannot rest easy. But America’s would-be Khmer Rouge should give Senator Byrd more credit for showing the way.

As used in the text, "the alleged Eden" (Para. 6) symbolizes()

A. "paradise."

B. "isolation."

C. "noisiness."

D. "luxuries."