题目

一矩形线圈在匀强磁场中以一边为轴匀速转动，磁场方向与转轴垂直，线圈中感应电动势e与时间t的关系如图17-1-10所示.在T/12时刻，线圈平面与磁感应强度的夹角等于___________. 图17-1-10 答案：解析：由图可知，在时，，根据电磁感应定律e=2BLvcosθ=Emcosθ，所以可以确定θ＝30°或150°.答案：30°或150°In Europe, many people make friends between their countries through town twinning. Town or city twinning means two towns in different countries agree and decide to become “twins” with a sister city relationship. The people find pen friends in the twin town. They exchange newspapers and stamps. The school teachers discuss teaching methods with the teachers in the twin town. Officials visit the twin town for celebrations. Ordinary people travel to the twin town, too, but not very often if it is far away. Sometimes, schools even exchange their classes for two or three weeks! For example, German middle school students study for a while at the school in their twin town in Britain, staying with British families. A few months later, their British friends come to study in Germany. Many British towns are so pleased with the results of the twinning that they set out to find more than one twin town! Tonbridge, a small town in Kent, for example, has twin town in both Germany and France. Richmond near London has relationships with Germany, France and even a town in a Balkan(巴尔干半岛的) country! Town twinning can help make friends. It helps students improve their language skills, and also helps people to understand the differences between nations. 70. Twin towns develop ______.    A. between European countries and China    B. in the English-speaking countries only    C. in the same country    D. between different countries 71. How many countries are mentioned in the passage? A. 4 B. 5 C. 6 D. 7 72. One of the advantages of town twinning is _____.    A. to earn more money    B. to set up schools    C. to improve students’ language skills    D. to develop industry and agriculture