题目

天然气灶烧水，燃烧0.5m3的天然气，使100kg的水从20℃升高到70℃．已知水的比热容为C=4.2×103J/（kg•℃），天然气的热值为q=7.0×107J/m3．求： （1）0.5m3天然气完全燃烧放出的热量Q放． （2）水吸收的热量Q吸． （3）燃气灶的效率η． 答案：解：（1）天然气完全燃烧放出的热量： Q放=Vq=0.5m3×7.0×107J/m3=3.5×107J． （2）水吸收的热量：Q吸=cm（t﹣t0）=4.2×103J/（kg•℃）×100kg×（70℃﹣20℃）=2.1×107J． （3）燃气灶的效率：η=×100%=×100%=60%． 答：（1）0.5m3天然气完全燃烧放出的热量是3.5×107J； （2）水吸收的热量是2.1×107J； （3）燃气灶的效率是6They leap from helicopters or speeding boats, bringing aid to swimmers who get into trouble off Italy’s popular beaches. Hundreds of specially trained dogs from Italy’s corps of canine(犬类的) lifeguards set out each summer to help swimmers in need of rescue. These "life dogs" wear a harness that victims can grab to be dragged back to shore, and unlike human lifeguards, they can easily jump from helicopters and speeding boats to reach swimmers in trouble. With millions flocking to Italy’s crowded beaches each summer, the Italian Coast Guard says it rescues about 3,000 people every year —and their canine helpers have saved several lives. It takes three years for the canines to reach expert rescue status, and currently 300 dogs are fully trained for duty, said Roberto Gasbarri, who directs the Italian School of Canine Lifeguards program. "Dogs are of good physical strength. They can increase the speed at which victims are rescued," Gasbarri said. "The dog becomes a sort of intelligent lifebuoy(救生圈). It is a buoy that goes by itself to a person in need of help, and comes back to the shore also by himself, choosing the best landing point and swimming through the safest currents," he said. The school will train any breed, as long as they weigh at least 30 kilograms, but New found lands and golden retrievers are most commonly used because they are good at swimming. Each dog works together with a human lifeguard, who also acts as the animal’s trainer. "Being retrievers, they set out to pick up anything we tell them, be it a human being, an object, or a fish, and they bring it back to the shore," said lifeguard Monia Luciani. "They do not associate it with a physical activity, but it is rather a game for them." 1.The dogs wear a harness so that __________. 2.The dogs are helpful to _________. 3.Why does the school usually choose to train the New found lands and golden retrievers? ___________________________________. 4.How do the dogs regard the pickup training? ____________________________________.