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问题 计算题

图甲是某工厂的水平传送带上产品运输及计数器的示意图。S为激光光源，能连续发射粗细忽略不计的激光束；R为光敏电阻，当有激光照射时，其阻值较小，无激光照射时，其阻值较大。每当水平传送带上的产品和传送带一起匀速通过S与R之间时，射向光敏电阻的激光被产品挡住，通过电压表测出光敏电阻R两端电压随时间变化的规律如图乙所示。已知计数器的电源电压为3V，保护电阻R0=200Ω。若水平传送带上的产品均是边长a=0.lm、质量m=2.5kg的实心正方体，求：

(l）产品的密度；

(2）产品对传送带的压强；

(3）传送带传输产品的速度；

(4）计数器工作1min，光敏电阻R在有激光照射时消耗的电能。

答案

(l）2.5×103kg/m3 (2）2.5×103Pa(3）0.5m/s(4）0.4J

解：(l）产品的体积：V=a3=（0.1m）3=10-3m3，

产品的密度：ρ=m/V=2.5kg/10-3m3=2.5×103kg/m3，

(2）产品的重力：G=mg=2.5kg×10N/kg=25N，

因为产品放在水平传送带上，所以F=G=25N．

产品对传送带受力面积：S=a3=（0.1m）2=10-2m3

产品对传送带的压强P=F/S=25N/10-2m3=2.5×103Pa

(3）当无光照射时（即产品挡住了射向光敏电阻的激光），光敏电阻的阻值较大，电流较小，R0两端的电压较小，而R两端的电压较大

由图乙可知，R两端的较大电压是2V，

此时对应的时间是t=0.6s-0.4s=0.2s（或1.2s-1.0s=0.2s），

传送带传输产品的速:V=a/t=0.1m/0.2s=0.5m/s

(4)当有光照射时，光敏电阻的阻值较小，则此时电路中的电流较大，R0两端的电压较大，而R两端的电压较小，由图乙可知，R两端的较小电压是1V，则R0两端的较大电压是：U0=U-UR=3V-1V=2V，电路电流I= U0/ R0=2V/200Ω=0.01A

由图乙所示图象可知，1min内有激光照射的时间t=(60S/0.6S)×0.4=40S

则1min内有激光照射时，光敏电阻消耗的电能W=URIt=1V×0.01A×40s=0.4J；

答：（1）产品的密度为2.5×103kg/m3；

（2）产品对传送带的压强是2500Pa；

（3）传送带传输产品的速度是0.5m/s；

（4）计数器工作1min，光敏电阻R在有激光照射时消耗的电能为0.4J．

（1）已知正方体的边长，可以求出它的体积，由密度公式可以求出产品的密度．

（2）求出产品对水平传送的压力，然后由压强公式求出产品对传送带的压强．

（3）由图乙所示图象求出，产品的挡光时间，然后由速度公式求出传送带的速度．

（4）由串联电路特点求出由激光照射时保护电阻两端电压，由欧姆定律求出电路电流，

根据图象求出1min激光照射时间，然后由W=UIt求出光敏电阻在有激光照射时消耗的电能．

【考点】密度的计算；速度的计算；压强的大小及其计算；电功的计算。

单项选择题

Friday marks the 40th anniversary of one of the biggest, most expensive, most destructive social policy experiments in American history: The war on drugs. On the morning of June 17, 1971, President Richard Nixon, speaking from the Briefing Room of the White House, declared: "America’s public enemy No. 1 in the United States is drug abuse. In order to fight and defeat this enemy, it is necessary to wage a new, all-out offensive. I have asked the Congress to provide the legislative authority and the funds to fuel this kind of an offensive. This will be a worldwide offensive dealing with the problems of sources of supply, as well as Americans who may be stationed abroad, wherever they are in the world. "

So began a war that has waxed and waned, ,sputtered and sprinted, until it became an unmitigated disaster, an abomination of justice and a self-perpetuating, trillion-dollar economy of wasted human capital, ruined lives and decimated communities. Since 1971, more than 40 million arrests have been conducted for drug-related offenses. And no group has been more targeted and suffered more damage than the black community. As the American Civil Liberties Union pointed out last week, "The racial disparities are staggering: despite the fact that whites engage in drug offenses at a higher rate than African-Americans, African-Americans are incarcerated for drug offenses at a rate that is: 10 times greater than that of whites. "

An effort meant to save us from a form of moral decay became its own insidious brand of moral perversion-turning people who should have been patients into prisoners, criminalizing victimless behavior, targeting those whose first offense was entering the world wrapped in the wrong skin. It feeds our overwhelming thirst for punishment.

Last week, the Report of the Global Commission on Drug Policy, a 19-member commission that included Kofi Annan, a former U. N. secretary general, declared that: "The global war on drugs has failed, with devastating consequences for individuals and societies around the world. Fifty years after the initiation of the U.N. Single Convention on Narcotic Drugs, and 40 years after President Nixon launched the U. S. government’s war on drugs, fundamental reforms in national and global drug control policies are urgently needed. "

The White House immediately shot back: no dice. The Obama administration presented a collection of statistics that compared current drug use and demand with the peak of the late 1970s, although a direct correlation between those declines and the drug war are highly debatable. In doing so, it completely sidestepped the human, economic and societal toll of the mass imprisonment of millions of Americans, many for simple possession.

No need to put a human face on 40 years of folly when you can wrap up its inefficacy in a patchwork quilt of self-serving statistics.

It is obvious that the Obama administration（）

A. is sympathetic to the wrongly imprisoned victims

B. is less tolerant of drug abuse than the Nixon administration

C. is dissatisfied with Annan’s assessment of the situation

D. has done better than the Nixon administration in the war on drugs
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单项选择题

称为增强因子，表示（）的量度。

A.液体溶解在超临界流体相中溶解度增大

B.固体溶解在超临界流体相中溶解度增大

C.流体溶解在超临界流体相中溶解度减小

D.固体溶解在超临界流体相中溶解度减小
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