题干

某太阳能热水器公司研制出一种新的太阳能、电能两用热水器.天气晴朗时使用太阳能;在阳光不足时,用电热丝辅助加热.实物如图所示,热水器铭牌如表:

**牌热水器

型号﹣18A

集热管数28支

额定频率50Hz

水箱体积200L

集热管管长180cm

额定电压220V

防水等级A

系统水压3.0MPa

额定功率4400W

(1)电热水器正常工作时,电热丝的电阻和电流各是多少?

(2)太阳能热水器贮满水,水温从20℃加热到50℃时吸收了多热量?

(3)若通过完全燃烧天然气加热一满箱水,使其温度从20℃升高到50℃时,不考虑热量的损失,需要多少立方天然气?(天然气的热值为4.0×109J/m3

(4)由于天气不好,太阳能热水器只将水加热到35℃,现要通过电热丝将水加到50℃使用,需电加热器正常工作多少秒?(设电能转化为内能的效率为70%)

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答案(点此获取答案解析)

解:
(1)因为电热水器正常工作,
所以U=U=220V,P=P=4400W;
根据P=IU
可得电热水器正常工作时的电流:
I=
P
U
=

同类题2

阅读下面的短文,从短文后各题所给的A、B、C和D四个选项中,选出可以填入空白处的最佳选项。

    “Daily Star, sir,” called Jason, carrying some news-papers under his arm. The little boy had been running up and down the street, but there were still twenty 1left. His voice was almost gone and his heart was 2. The shops would soon close, and all the people would go home. He would have to go home too, carrying the papers 3 money. He had hoped to sell more papers tonight to make more money to buy a 4for his mother and some seeds for his bird. That was why he had bought the papers with all his money. He 5 as he thought of his failure to sell all his papers.

    “You don't know the 6 of selling papers. You must shout, 'Hot news! Bomb bursting!'” another newsboy Chad told Jason. “7 it's not in the paper at all,” replied Jason. “Just run away quickly 8 they have time to see, and you'll 9out and get your money,” Chad said.

    It was a new 10 to Jason. He thought of his bird with no11and the cake he wanted to buy for his mother, but was12 that he would not tell a lie. Though he was 13 a poor newsboy, he had been 14 some good things.

    The next afternoon Jason went to the office for his papers15. Several boys were crowding around Chad, who declared with a 16 smile that he sold six dozen the day before. He added that Jason 17 money because he would not tell a lie. The boys 18 at Jason. “You wouldn't tell a lie yesterday, my boy?” A gentleman at the office came up and patted Jason's shoulder 19, “You're just the boy I am looking for.” A week later Jason started his new 20. He lost the sale of twenty papers because he would not tell a lie, but got a well-paid job because he told the truth.