题目

如图所示，电源电压U保持不变，定值电阻R1的阻值是30Ω，滑动变阻器R2的最大阻值为50Ω，当S、S1闭合，S2断开时电流表示数为0.2A，当S、S2闭合，S1断开，且滑片P在b端时，电流表示数为0.1A。（不考虑温度对灯泡电阻的影响），求：(1)电源电压U；(2)灯泡的阻值；(3)电路中消耗的最小总功率。 答案：【答案】(1)6V；(2)10Ω；(3)0.6W【解析】(1)当S、S1闭合，S2断开时,只有R1工作，R1两端电压就是电源电压(2)当S、S2闭合，S1断开时，R1断路，灯泡与滑动变阻器R2串联，串联的总电阻为由于这时滑动变阻器滑片P在b端，所以滑动变阻器接入电路的阻值为50Ω,灯泡电阻为(3)根据由于电路总电压保持不变，所以电路A recent survey in the United States showed that the average family spent more money on its pets than on its children. Although rather shocking, it should not surprise anyone who has seen the doggy parlors(客厅) where loved pets rest. Are Americans unique in treating their little friends in this way? No, the English, too, pay more attention to their pets.This can clearly be seen when we look at pet foods, which often contain more vitamins than human food. They certainly cost much. Last year the British public spent two hundred million pounds on pet food alone, to say nothing of veterinary bills or animal furniture. It is difficult not to feel angry about this when considering what the same amount could do for victims of starvation and poverty, so it is not unusual for me___________when I read an old man left all his money to his dog instead of his children.There are a variety of reasons why I find pets-raising alarming. They cause physical problems. An example of this is New York where they have great difficulty getting rid of the mess that dogs leave on the streets. Many people find this funny, but in a number of large cities it is a major problem. Animals can cause disease, too. It is the threat of rabies — a disease with no known cure.Another problem is the carelessness of pet owners. Most little children want a dog or a cat, and they continually push their mothers and fathers until they get one. It is only when the "sweet little thing" has been brought home that the parents realize how much time and money must be spent on "Rover" or "Bonzo". Then they just abandon it. As a result, they are allowed to run free. English farmers lose hundreds of sheep a year, killed by someone's pet and you must have read of children being hurt by some pets of their own.Lastly, I would only suggest that we have got our priorities wrong and that something should be done about it. In my view, it's time we stopped being sentimental about pets. I can see no reason why we should get upset when animals are cut up for medical experiments. This will lead us to discovering cures for serious human diseases, then I say, “keep cutting!”【1】The doggy parlors are mentioned in the 1st paragraph to show ________.A. where Americans keep their petsB. what costly lives the pets are livingC. why children love their pets so muchD. how much pets depend on their masters【2】In the second paragraph "to get hot under the collar" probably means ________.A. getting quite hot B. feeling overwhelmingly angryC. becoming excited D. receiving a fashionable shirt【3】According to the author, in which field can animals be most useful?A. Pet. B. Food. C. Medicine. D. Sports.【4】What is author’s attitude towards pets-raising?A. Pitiful. B. Neutral. C. Opposed. D. Supportive.