题目：
如图所示，边长为L的等边三角形ABC为两有界匀强磁场的理想边界，三角形内的磁场方向垂直纸面向外，磁感应强度大小为B，三角形外的磁场（足够大）方向垂直纸面向里，磁感应强度大小也为B．把粒子源放在顶点A处，它将沿∠A的角平分线发射质量为m、电荷量为q、初速度为v0的带电粒子（粒子重力不计）．若从A射出的粒子
①带负电，v0=
，第一次到达C点所用的时间为t1qBL m
②带负电，v0=
，第一次到达C点所用的时间为t2qBL 2m
③带正电，v0=
，第一次到达C点所用的时间为t3qBL m
④带正电，v0=
，第一次到达C点所用的时间为t4（ ）qBL 2m
A．t1=t3＜t2=t4
B．t1＜t2＜t4＜t3
C．t1＜t2＜t3＜t4
D．t1＜t3＜t2＜t4
答案：
粒子在磁场中做匀速圆周运动，洛伦兹力提供圆周运动的向心力，如图所示，
当v0=qBL m
所以有：qvB=mv2 r
⇒r=
=Lmv qB
根据几何关系可知作出运动轨迹，
根据轨迹可知，当电荷带正电，粒子经过一个周期到达C点，即为t3；
当粒子带负电，粒子经过
T第一次到达C点，即为t1．1 6
当v0=qBL 2m
所以有：qvB=mv2 r
⇒r=
=mv qB L 2 
根据轨迹可知，当电荷带正电，粒子经过
T到达C点，即为t4；5 6
当粒子带负电，粒子经过
T第一次到达C点，即为t2．1 3
所以B正确，ACD错误．
故选：B．
er what personality characteristics go along with success in intercultural experiences. Their findings have often been unclear or incomplete. But three characteristics stand out in their reports: patience, a sense of humor, and the awareness of being unclear.
humor. Foreigners often find themselves in situations that are unclear to the newcomers. That is, they do not know what is happening in the situation. Perhaps they do not understand the local language well enough, or they do not know how some system or organization works, or they can’t be sure of different people’s roles in what is going on. “It’s like I just got here from the moon,” a Chinese graduate student who newly arrived in the United States said, “things are just so different here.”
seful for communicating with foreigners小题2:According to the passage, what characteristic should be most focused on?