问题 问答题

(14分)如图所示,在质量为M=0.99kg的小车上,固定着一个质量为m=10g、电阻R=1W的矩形单匝线圈MNPQ,其中MN边水平,NP边竖直,高度l=0.05m。小车载着线圈在光滑水平面上一起以v0=10m/s的速度做匀速运动,随后进入一水平有界匀强磁场(磁场宽度大于小车长度),完全穿出磁场时小车速度v1=2m/s。磁场方向与线圈平面垂直并指向纸内、磁感应强度大小B=1.0T。已知线圈与小车之间绝缘,小车长度与线圈MN边长度相同。求:

(1)小车刚进入磁场时线圈中感应电流I的大小和方向;

(2)小车通过磁场的过程中线圈电阻的发热量Q

(3)小车进入磁场过程中线圈克服安培力所做的功W

答案

(1)0.5A,电流方向为MQPNM

(2)48J

(3)32J

(1)线圈切割磁感线的速度v0=10m/s,感应电动势E=Blv0=1×0.05×10=0.5V(1分)

由闭合电路欧姆定律得线圈中电流 A         (2分)

由楞次定律知线圈中感应电流方向为MQPNM    (1分)

(2)线圈进入磁场和离开磁场时克服安培力做功,动能转化成电能,产生的电热

(J)(4分)

能量关系正确得 2分,结果正确再得2分。

(3)设小车完全进入磁场后速度为v

在小车进入磁场从t时刻到t+⊿t时刻(⊿t→0)过程中

(1分)

求和得 (1分)

同理得 

(1分)

又线圈进入和穿出磁场过程中磁通量的变化量相同,因而有qq(1分)

故得 v0v = vv 1 即 v = =" 6" m/s (1分)

所以,小车进入磁场过程中线圈克服安培力做功

(J)(1分)

阅读理解

Happiness, rather than working hard, is the key to success, according to research published yesterday. Cheerfu1 peop1e are more 1ikely to try new things and challenge(wk战)themselves, which strengthens their good emotion and leads to success in work,good relationships and strong health,say psychologists(心理学家).

The findings suggest that happiness is not a “ feel-good” luxury(奢侈品),but is necessary for people’s well-being. What is more, happiness can also reach an entire nation,with people in” happy” nations being more likely to help others.

The link between happiness and success was studied by a team from the University of California Riverside,led by Professor Sonja Lyubomirsky.

First, they examined questionnaires that ask peop1e about their 1ives. “ For example, they show that happy people are likely to earn higher incomes” said Lyubomirsky. Having established the link,they wanted to discover the cause.

“Almost always it has been taken for granted that things that connect closely to happiness are the causes of happiness,but it could be just the opposite that those things are likely to be caused by happiness,” said Professor Ed Diener from the University of Illinois,another author on the paper.

“ There was strong evidence that happiness leads people to be more sociable,more generous and more productive at work,to make more money,and to have stronger immune(免疫)systems,”said Lyubomirsky.

The research shows that while success can put a spring in someone’s step,peop1e need happiness in the first place to achieve success.

According to the study, around 4 out of 5 people in modern industrialized nations arc happy at any one time.

Success was not just about earning lots of money. “ We describe success as having the things that culture or society values,whether it be friends,close family, money and income,or long life,”said Diener.

However,sorrowful people arc not predetermined to lead a life of failure.

小题1:In this passage,which of the following viewpoints does Professor Lyubomirsky state?

A.People in less happy countries are more likely to help others.

B.Things connected to happiness are brought about by happiness.

C.Happiness results in richer and healthier people.

D.Success means possessing things that a cu1ture or society thinks highly of.小题2:The underlined phrase “ put a spring in someone’s step” means          .

A.put you in good spirits

B.put spring water on your feet

C.have a pleasant walk in spring

D.step into a cheerful spring小题3:According to the passage,which of the following is true?

A.People arc happy because they earn more money.

B.People earn more money because they are happy.

C.Working hard is important for being successful.

D.We can do whatever we like as 1ong as we feel happy.小题4: We can infer from the passage that          .

A.fee1ing good is a cause and not an effect of achievement

B.our life would be a failure if we were not happy

C.happiness is just a personal problem

D.being successful mainly means being wealthy

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