问题 选择题

两条等长的电炉丝,均在家庭电路中使用.A的电功率为800 W,B的电功率为500W,现在800W的电炉丝断了一小截,肖强同学利用另一个500W的废电炉丝将残缺部分补接恢复原长,再接入原电路使用时,其发热能力将(  )

A.大于原800W的电炉丝

B.等于原800W的电炉丝

C.小于原800W的电炉丝

D.无法判断

答案

根据R=

U2
P
可知,500W的电炉电阻较大,又因为长度相同,故500W的电炉更细些;

因电阻丝的材料和长度相同,烧断后用一截同种材料但较细的电炉丝接入,这样造成电阻丝的横截面积减小,在材料、长度不变时,横截面积减小,电阻要比原来大;

所以根据电功率公式:P=

U2
R
,电炉的实际功率要比原来的800W小,所以A、B、D不符合题意,只有C符合题意;

故选C.

单项选择题
单项选择题

Like many of my generation, I have a weakness for hero worship. At some point, however, we all to question our heroes and our need for them. This leads us to ask: What is a hero

Despite immense differences in cultures, heroes around the world generally share a number of characteristics that instruct and inspire people.

A hero does something worth talking about. A hero has a story of adventure to tell and community who will listen. But a hero goes beyond mere fame.

Heroes serve powers or principles larger than themselves. Like high-voltage transformers, heroes take the energy of higher powers and step it down so that it can be used by ordinary people.

The hero lives a life worthy of imitation. Those who imitate a genuine hero experience life with new depth, enthusiasm, and meaning. A sure test for would-be heroes is what or whom do they serve What are they willing to live and die for The answer or evidence suggests they serve only their own fame, they may be famous persons but not heroes. Madonna and Michael Jackson are famous, but who would claim that their fans find life more abundant

Heroes are catalysts (催化剂) for change. They have a vision from the mountaintop. They have the skill and the charm to move the masses. They create new possibilities. Without Gandhi, India might still be part of the British Empire. Without Rosa Parks and Martin Luther King, Jr., we might still have segregated (隔离的) buses, restaurants, and parks. It may be possible for largescale change to occur without leaders with magnetic personalities, but the pace of change would be slow, the vision uncertain, and the committee meetings endless.

According to the passage, heroes are compared to high-voltage transformers in that ().

A. they have a vision from the mountaintop

B. they have warm feelings and emotions

C. they can serve as concrete examples of noble principles

D. they can make people feel per and more confident