问题 选择题

如图所示,xOy坐标系第一象限内有垂直纸面向外的匀强磁场,第三象限内有垂直纸面向里的匀强磁场,磁感强度大小均为B,第二、四象限内没有磁场。一个围成四分之一圆弧形的导体环Oab,其圆心在原点O,开始时导体环在第四象限,且导体环两边oaob恰好分别与x轴、y轴重合,从t=0时刻开始导体环绕O点在xOy坐标平面内逆时针匀速转动。规定逆时针方向为电流的正方向,在导体环转动一周的过程中,下列能正确表示环内感应电流i随时间t变化的图象是  

 

答案

答案:D

题目分析:导体环进入磁场时,磁通量发生变化,形成感应电流,根据右手定则可知,形成顺时针方向的电流,即电流为负值,故AB错误

转过90°之后开始出磁场,根据楞次定律可知,磁通量减小,形成逆时针方向的感应电流,转过180°后,进入第三象限的磁场,根据楞次定律可知形成,逆时针方向的感应电流,转过270°时,开始出磁场,会形成顺时针方向的感应电流,故D正确

感应电流大小,所以导体环匀速转动,会形成大小恒定的感应电流

故选D

点评:本题要根据感应电动势公式和欧姆定律得到感应电流的表达式,再选择图象,是经常采用的方法.

单项选择题
阅读理解

In Greek mythology(神话), the gods punished Sisyphus by forcing him to roll a rock up a steep hill for eternity(来世). But he was probably better off than if they’d forced him to sit and stare into space until the end of time, conclude the authors of a new study on keeping busy. They found that people who have something to do, even something pointless, are happier than people who sit around.

“The general phenomenon I’m interested in is why people are too busy doing what they are doing in modern society,” says Christopher K. Hsee, of the University of Chicago. “People are running around, working hard, the way beyond the basic level.” Sure, there are reasons, like making a living, earning money, and so on. But, Hsee says, “I think there’s something deeper: We have extra energy and we want to avoid idleness.”

In a study 98 students were asked to complete two surveys. After they had completed the first they were made to wait 15 minutes to receive the next one. They were given a choice of either handing in the first survey nearby or at a more distant location they had to walk to. Whichever option they chose, they received a chocolate bar. Not everyone chose to go to the faraway location. Two-thirds chose the lazy option. Yet those who chose to stay busy by going to the faraway location were found to be happier than those who had stayed put.

But if the chocolate bars offered at the  two locations were different, they were more likely to choose the far location—because they could make up a good and acceptable reason for the trip, Hsee and his colleagues say.

Hsee thinks it may be possible to use this principle—people like being busy, and they like being able to show being busy right or reasonable—to benefit society. “If we can find a way for idle people to engage in activity that is at least not harmful, I think it is better than destructive business,” he says. Hsee himself has been known to  give a research assistant a useless task when he doesn’t have anything to do, so he isn’t sitting around the office getting bored and depressed.

小题1:The author starts the passage with the Greek mythology story to ________.

A.make it easier to understand the passage

B.draw readers’ attention to Greek culture

C.show Greek people enjoyed being busy

D.bring about the subject of the passage小题2:According to Hsee, people are busy in modern society because they want to _________.

A.make others think they are not lazy

B.keep their energy at the basic level

C.earn more money to support their family

D.avoid the state of having nothing to do小题3:The underlined phrase “stayed put” in Paragraph 3 probably means __________.

A.remained there

B.kept occupied

C.got around

D.stayed awake小题4:How does Hsee’s assistant probably feel while performing a task?

A.Blue.

B.Empty.

C.Contented.

D.Trapped.