新课程提倡师生开发、整合、优化课程资源,从高考和竞赛题中总结规律,深入探究新的有用有趣的东西,是开发课程资源、提高教学教研的一个重要方面.2011年第28届全国中学生物理竞赛预赛第15题(见题2),与2009年高考浙江卷理综第25题(题1与原题相比略有简化)具有共同的特点:磁场的边界为圆,且粒子做圆周运动的轨迹半径(本文简称轨迹半径)等于磁场边界圆的半径(本文简称磁场半径).题1如图1所示,在半径为R的圆内有与平面垂直的匀强磁 The new curriculum to promote teacher-student development, integration, optimization of curriculum resources, from the college entrance examination and competition questions to summarize the law, in-depth exploration of new useful and interesting things, is to develop curriculum resources, improve teaching and research an important aspect of 2011 28th National Secondary Physics Physics Contest Series 15 (see Question 2) has the same characteristics as the 25th series of Zhejiang College Entrance Examination 2009 (Problem 1 is slightly simplified compared with the original): the boundary of the magnetic field is a circle and the particles do Circumferential trajectory radius (hereinafter referred to as the trajectory radius) is equal to the magnetic field boundary circle radius (herein referred to as the magnetic field radius.) Problem 1 As shown in Figure 1, in a radius R circle with the plane vertical uniform magnetic