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问题 填空题

如图所示电路．电源电压恒定不变，滑动变阻器的最大阻值R1为20Ω，定值电阻R2、R3的阻值分别是10Ω、20Ω，两个电流表的电阻不计．闭合开关S，当滑动变阻器的滑片P置于R1中间位置时，电流表A1和A2的示数之比为______；当滑动变阻器的滑片P置于R1最右端时，电流表A1和A2的示数之比为______．

答案

当滑片P置于R1中间位置时，R1=10Ω；

并联部分的电阻为R并=

R1R2
R1+R2
=
10Ω×10Ω
10Ω+10Ω
=5Ω，

电路中的总电阻为R=R并+R3=5Ω+20Ω=25Ω，

电路的总电流为I=

U
R
=
U
25
，

并联部分的电压为U并=IR并=

U
25
×5Ω=
U
5
，

R1支路的电流为I1=

U并
R1
=
U
5
10Ω
=
U
50
，

所以两电流表的示数之比为I1：I=

U
50
：
U
25
=1：2；

当滑片P置于R1最右端时，R1，R2都短路，电路为R3的简单电路，两电流表测电路中的电流；

根据串联电路各处的电流相等可知，两电流表的示数相等，即1：1．

故答案为：1：2，1：1．

单项选择题

Every silver lining has its cloud. At the moment, the world’s oceans absorb a million tonnes of carbon dioxide an hour. Admittedly that is only a third of the rate at which humanity dumps the stuff into the atmosphere by burning fossil fuels, but it certainly helps to slow down global warming. However, what is a blessing for the atmosphere turns out to be a curse for the oceans. When carbon dioxide dissolves in water it forms carbonic acid. At the moment, seawater is naturally alkaline -but it is becoming less so all the time.
The biological significance of this acidification was a topic of debate among scientists. Many species of invertebrate have shells or skeletons made of calcium carbonate. It is these, fossilized, that form rocks such as chalk and limestone. And, as anyone who has studied chemistry at school knows, if you drop chalk into acid it fizzes away to nothing. Many marine biologists therefore worry that some species will soon be unable to make their protective homes. Many of the species most at risk are corals.
The end of the Permian period, 252m years ago, was marked by the biggest extinction of life known to have happened on Earth. At least part of the cause of this extinction seems to have been huge volcanic eruptions that poured carbon dioxide into the atmosphere. But some groups of animals became more extinct than others. Sponges, corals and brachiopods were particularly badly hit.
Rather than counting individual species of fossils, which vary over time, paleontologists who study extinction usually count entire groups of related species, called genera. More than 90% of Permian genera of sponges, corals and brachiopods vanished in the extinction. By contrast, only half of the genera of mollusks and arthropods disappeared.
This is because mollusks and arthropods are able to buffer the chemistry of the internal fluids from which they create their shells. This keeps the acidity of those fluids constant. Sponges, corals and brachiopods, however, cannot do this.
The situation at the moment is not as bad as it was at the end of the Permian. Nevertheless, calculations suggest that if today’s trends continue, the alkalinity of the ocean will have fallen by half a pH unit by 2100. That would make some places, such as the Southern Ocean, uninhabitable for corals. Since corals provide habitat and food sources for many other denizens of the deep, this could have a profound effect on the marine food web.
No corals, no sea urchins and no who-knows-what-else would be bad news indeed for the sea. Those who blithely factor oceanic uptake into the equations of what people can get away with when it comes to greenhouse-gas pollution should, perhaps, have second thoughts.

Which category of writing does the passage belong to

A. Narration.
B. Description.
C. Persuasion.
D. Exposition.
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单项选择题

病人症状好转，尿化验正常，为防止复发采取低剂量抑菌疗法，正确的方法应该是

A．用常规一次量，睡前一次口服

B．用常规一次量，清晨一次口服

C．用常规一次量，中餐后一次口服

D．用全天剂量的1/2，睡前一次服

E．减少用药剂量，分次口服
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