问题 选择题

如图所示,边长为的L的正方形区域abcd中存在匀强磁场,磁场方向垂直纸面向里。一带电粒子从ad边的中点M点以一定速度垂直于ad边射入磁场,仅在洛伦兹力的作用下,正好从ab边中点N点射出磁场。忽略粒子受到的重力,下列说法中正确的是

A.该粒子带负电

B.洛伦兹力对粒子做正功

C.粒子在磁场中做圆周运动的半径为L/4

D.如果仅使该粒子射入磁场的速度增大,粒子做圆周运动的半径也将变大

答案

答案:D

题目分析:粒子进入磁场后向上偏转,说明M点受到洛伦兹力竖直向上,根据左手定则判断粒子带正电,选项A错。洛伦兹力始终与速度垂直,洛伦兹力不做功,选项B错。根据洛伦兹力与速度垂直,M点受洛伦兹力沿Ma方向,在磁场中做匀速圆周运动,从N点出磁场,MN即为所对应的一条弦,那么MN的垂直平分线与Ma的交点即a点就是圆周运动的圆心,根据几何关系可得半径,选项C错。根据洛伦兹力提供向心力,可得圆周运动半径,粒子速度越大,圆周运动半径越大,选项D对。考点:

单项选择题
单项选择题

When Newsweek recently asked 1,000 U. S. citizens to take America’s official citizenship test, 29 percent couldn’t name the vice president. Seventy-three percent couldn’t correctly say why we fought the Cold War. Forty-four percent were unable to define the Bill of Rights. And 6 percent couldn’t even circle Independence Day on a calendar.

Don’t get us wrong: civic ignorance is nothing new. For as long as they’ve existed, Americans have been misunderstanding checks and balances and misidentifying their senators. And they’ve been lamenting the ignorance of their peers ever since pollsters started publishing these dispiriting surveys back in Harry Truman’s day. According to a study by Michael X. Delli Carpini, dean of the Annenberg School for Communication, the yearly shifts in civic knowledge since World War II have averaged out to "slightly under 1 percent. "

But the world has changed. And unfortunately, it’s becoming more and more inhospitable to incurious know-nothings—like us. To appreciate the risks involved, it’s important to understand where American ignorance comes from. In March 2009, the European Journal of Communication asked citizens of Britain, Denmark, Finland, and the U.S. to answer questions on international affairs. The Europeans outdid us. It was only the latest in a series of polls that have shown us lagging behind our First World peers.

Most experts agree that the relative complexity of the U. S. political system makes it hard for Americans to keep up. In many European countries, parliaments have proportional representation, and the majority party rules without having to "share power with a lot of subnational governments," notes Yale political scientist Jacob Hacker. In contrast, we’re saddled with a nonproportional Senate; a tangle of state, local, and federal bureaucracies; and near-constant elections for every imaginable office (judge, sheriff, school-board member, and so on). "Nobody is competent to understand it all, which you realize every time you vote," says Michael Schudson, author of The Good Citizen. "You know you’re going to come up short, and that discourages you from learning more. "

It doesn’t help that the United States has one of the highest levels of income inequality in the developed world, with the top 400 households raking in more money than the bottom 60 percent combined. As Dalton Conley, an NYU sociologist, explains, "it’s like comparing apples and oranges. Unlike Denmark, we have a lot of very poor people without access to good education, and a huge immigrant population that doesn’t even speak English. " When surveys focus on well-off, native-born respondents, the U. S. actually holds its own against Europe.

For more than two centuries, Americans have gotten away with not knowing much about the world around them. But times have changed—and they’ve changed in ways that make civic ignorance a big problem going forward. We suffer from a lack of information rather than a lack of ability. Whether that’s a treatable affliction or a terminal illness remains to be seen. But now’s the time to start searching for a cure.

America’s civic ignorance()

A. is largely attributable to its unwillingness to learn

B. can not be corrected by any kind of method

C. has become an affliction in the fast-changing world

D. can be traced back to its rising dominance in the world’s affairs