问题 问答题

电暖气是常见的家用电器之一,某电暖气的电路示意图如图甲所示.L1和L2是两个指示灯,R1和R2为电热丝,通过调节旋转开关可以改变电暖气的档位.为避免引起火灾,使电暖气被碰倒时能够自动断开电路,可以在电路中接入一个“防倾倒开关”.“防倾倒开关”及其原理如图乙所示,在电暖气的底部有一个伸出外壳的塑料杆,当电暖气放置在水平地面时,塑料杆受到地面竖直向上的力而被顶起,接通顶部的触点.如果电暖气发生倾倒,塑料杆底部不受力,在弹簧的作用下顶部触点断开,电暖气便不再工作,该装置起到了“自动开关”的作用.在某次使用中,电暖气的旋钮开关接到档位“1”时,2min消耗的电能是48000J.已知R1:R2=4:7,指示灯L1和L2的电阻很小可忽略不计.

(1)“防倾倒开关”最好应安装在图甲的电路示意图中A、B、C、D、E哪一点处;

(2)求电热丝R2的阻值;

(3)当旋钮开关分别置于档位1和档位2时,求此电暖气的最大和最小功率的比值.

答案

(1)为了防止短路现象,在电热器被碰倒时,切断火线,“倾倒开关”要接在C点;

(2)旋钮开关接触点“1”时的等效电路如图1所示:

∵指示灯L2的电阻很小可忽略,

∴旋钮开关接静触点“1”时,2min消耗的电能:

W=

U2
R1+R2
t,

R1+R2=

U2
W
t=
(220V)2
48000J
×120s=121Ω,

∵R1:R2=4:7,

∴R1=44Ω,R2=77Ω;

(3)旋钮开关接触点“2”时的等效电路如图2所示

∵指示灯L1的电阻很小可忽略,

∴图2中,电热器消耗功率最大,P1=

U2
R1

图1中,电热器消耗功率最小,P2=

U2
R1+R2

P1
P2
=
U2
R1
U2
R1+R2
=
R1+R2
R1
=
121Ω
44Ω
=
11
4

答:(1)“防倾倒开关”最好应安装在图甲的C处;

(2)电热丝R2的阻值为77Ω;

(3)当旋钮开关分别置于档位1和档位2时,电暖气的最大和最小功率的比值为11:4.

阅读理解

.

Beijing — Premier Wen Jiabao has urged the leadership of all countries to deepen their political will in tackling climate change woes (灾难) as uncertainties mount in the months before the United Nation’s conference in Cancun, Mexico, at the end of this year.

Wen made the remarks on Friday when he met with nearly 20 politicians, climate change and environmental ministers worldwide, who are in Beijing this weekend to attend an international forum on green economy and climate change.

Based on the achievements made at the Copenhagen Summit last December, Wen urged the rich countries to further clarify their compulsory carbon reduction targets. “At the same time, we developing countries should make clear our voluntary goal of slowing down climate change,” Wen said.

Wen also urged the developed countries to transfer climate change friendly technologies to the developing countries, especially the poorest countries and island nations.

During the meeting, Wen said China will redouble its efforts in reducing carbon emissions and improving energy efficiency during the coming years after it finishes its 2006-2010 goal of cutting energy consumption per unit of economic output by 20 percent.

While praising China for its green efforts, former Australian Prime Minister Robert J. Lee Hawke asked China to share its experiences and lessons on energy savings and pollution reduction to the rest of the world to tackle global environmental woes and climate change.

Wen responded: “China, together with the rest of the world, is willing to make its due contribution in coping with global warming and climate change.”

At a banquet for the foreign guests on Friday night, former vice-premier Zeng Peiyan said mounting uncertainties are ahead for the climate change negotiations. The world needs more dialogue and cooperation, Zeng said.

“Climate change woes are our shared challenges but the root lies in the historical emissions by the developed countries,” said Zeng, as president of the China Center for International Economic Exchanges, which organized the weekend climate change summit.

Zeng expected the forum to work as a platform for different countries to communicate their positions freely before the Bonn climate change negotiation in June and the Cancun conference at the end of this year.

51. Which title suits this passage best?

A. We must work on climate change.

B. We should share our ‘green’ technology.

C. We need to clarify our action targets.

D. We ought to cooperate to control the climate.

52. Why did Wen Jiabao urge the developed countries to clarify their carbon reduction targets?

A. Because the developed countries are not willing to deal with global environmental woes.

B. Because the developing countries have not set any goal of slowing down climate change.

C. Because there are mounting uncertainties ahead for the climate change negotiations.

D. Because agreements were already made at the Copenhagen Summit last December.

53. What kind of goal should a developing country have to slow down climate change?

A. A less important goal.                                          B. A self-assessed goal.

C. A negotiation-based goal                               D. A more compulsory goal.

54. The underlined part in the last paragraph but one implies ____________.

A. Air pollution just existed in the developed countries in the past.

B. In the past, the developed countries slowed down the development of the world.

C. Climate change woes were originally caused by the developed countries.

D. The rich countries should be more responsible since they polluted most.

55. It can be inferred from the passage that _____________.

A. The Chinese government intends to be regarded as a developed country.

B. The whole world has got the same opinion about the measures to take.

C. The developed countries have not found any effective ways to cut their emissions.

D. There is still a long way to go before an agreement can be reached.

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