SAMPLE 10
[物理学]
The use of heat pumps has been held back largely by skepticism about advertisers claims that heat pumps can provide as many as two units of thermal energy for each unit of electrical energy used, thus apparently contradicting the principle of energy conservation.
Heat pumps circulate a fluid refrigerant that cycles alternatively from its liquid phase to its vapor phase in a closed loop. The refrigerant, starting as a low-temperature, low-pressure vapor, enters a compressor driven by an electric motor. The refrigerant leaves the compressor as a hot, dense vapor and flows through a heat exchanger called the condenser, which transfers heat from the refrigerant to a body of air. Now the refrigerant, as a high-pressure, cooled liquid, confronts a flow restriction which causes the pressure to drop. As the pressure falls, the refrigerant expands and partially vaporizes, becoming chilled. It then passes through a second heat exchanger, the evaporator, which transfers heat from the air to the refrigerant, reducing the temperature of this second body of air. Of the two heat exchangers, one is located inside, and the other one outside the house, so each is in contact with a different body of air: room air and outside air, respectively.
The flow direction of refrigerant through a heat pump is controlled by valves. When the refrigerant flow is reversed, the heat exchangers switch function. This flow-reversal capability allows heat pumps either to heat or cool room air.
Now, if under certain conditions a heat pump puts out more thermal energy than it consumes in electrical energy, has the law of energy conservation been challenged? No, not even remotely: the additional input of thermal energy into the circulating refrigerant via the evaporator accounts for the difference in the energy equation.
Unfortunately there is one real problem. The heating capacity of a heat pump decreases as the outdoor temperature falls. The drop in capacity is caused by the lessening amount of refrigerant mass moved through the compressor at one time. The heating capacity is proportional to this mass flow rate: the less the mass of refrigerant being compressed, the less the thermal load it can transfer through the heat-pump cycle. The volume flow rate of refrigerant vapor through the single-speed rotary compressor used in heat pumps is approximately constant. But cold refrigerant vapor entering a compressor is at lower pressure than warmer vapor. Therefore, the mass of cold refrigerant and thus the thermal energy it carries is less than if the refrigerant vapor were warmer before compression.
Here, then, lies a genuine drawback of heat pumps: in extremely cold climates where the most heat is needed heat pumps are least able to supply enough heat.
1. The primary purpose of the text is to
[A] explain the differences in the working of a heat pump when the outdoor temperature changes.
[B] contrast the heating and the cooling modes of heat pumps.
[C] describe heat pumps, their use, and factors affecting their use.
[D] advocate the more widespread use of heat pumps.
2. The author resolves the question of whether heat pumps run counter to the principle of energy conservation by
[A] carefully qualifying the meaning of that principle.
[B] pointing out a factual effort in the statement that gives rise to this question.
[C] supplying additional relevant facts.
[D] denying the relevance of that principle to heat pumps.
3. It can be inferred from the text that, in the course of a heating season, the heating capacity of a heat pump is greatest when
[A] heating is least essential.
[B] electricity rates are lowest.
[C] its compressor runs the fastest.
[D] outdoor temperatures hold steady.
4. If the authors assessment of the use of heat pumps is correct, which of the following best expresses the lesson that advertisers should learn from this case?
[A] Do not make exaggerated claims about the products you are trying to promote.
[B] Focus your advertising campaign on vague analogies and veiled implications instead of on facts.
[C] Do not use facts in your advertising that will strain the prospective clients ability to believe.
[D] Do not assume in your advertising that the prospective clients know even the most elementary scientific principles.
5. The text suggests that heat pumps would be used more widely if
[A] they could also be used as air conditioners.
[B] they could be moved around to supply heat where it is most needed.
[C] their heat output could be thermostatically controlled.
[D] people appreciated the role of the evaporator in the energy equation.
国内英语资讯:Xinjiang reports robust foreign trade growth in Q1
国内英语资讯:China-made large amphibious aircraft finishes 1st glide test
4月资讯热词汇总
国际英语资讯:Five killed, dozens injured by tornadoes in U.S. state of Texas
世界最美的国家是哪?
“一带一路”国际合作高峰论坛亮点前瞻
国内英语资讯:China, Britain urge peaceful solution to DPRK nuclear issue
体坛英语资讯:Chinas Ding levels at 12-12 with Selby at snooker worlds semifinals
国际英语资讯:EU to discuss crucial issues in complex Brexit talks
国内英语资讯:Ministry of Public Security stresses traffic safety during holiday
国内英语资讯:China, Cambodia pledge to cooperate under Belt and Road Initiative
《沉默的羔羊》导演乔纳森 戴米去世
国内英语资讯:Chinese FM calls for non-proliferation efforts, peace talks on Korean Peninsula
体坛英语资讯:Bayern crush Wolfsburg 6-0 to defend Bundesliga title
国内英语资讯:Feature: Polish dairy farmer turns east as China pushes Belt and Road Initiative
国内英语资讯:China Focus: CPC creates four-tier system to measure political discipline
国内英语资讯:Italy, China hold forum on poverty reduction with partners
体坛英语资讯:Brazilian striker on Sevilla radar
国际英语资讯:Thousands of Americans join May Day protests across U.S. cities for workers, immigrantsrig
国内英语资讯:Chinese submersible Jiaolong completes fourth dive in South China Sea
体坛英语资讯:Schalke upset Leverkusen 4-1 in German Bundesliga
国际英语资讯:High winds knock out power to thousands of homes in south California
体坛英语资讯:Selby beats Higgins to win third world snooker title
国内英语资讯:Chinese naval fleet starts friendly visit to Philippines
国际英语资讯:Brazils president dismisses general strike as democracy in action
国内英语资讯:Chinese FM stresses two directions in dealing with nuclear issue on Korean Peninsula
国内英语资讯:Taiwan cracks 54,873 illegal drug cases in 2016
国际英语资讯:Conservation found not associated with reduced levels of infectious disease
国内英语资讯:Philippines open to joint war drills with China: Duterte
国内英语资讯:World Expo Museum opens to public in Shanghai
不限 |
英语教案 |
英语课件 |
英语试题 |
不限 |
不限 |
上册 |
下册 |
不限 |