带电粒子垂直进入磁场时,在洛伦兹力的作用下做匀速圆周运动,其轨迹是一圆。此类问题是高考的重点和热点内容,同时也是考生感到颇为棘手的难点之一。在高考中,很多考生就是因不能正确地画出轨迹圆,从而导致解题出现错误。为了解决这一难点,本文将以典型高考试题为例,运用相关几何知识,谈谈利用“两线定圆心”法构画轨迹圆的作图方法。构画轨迹圆的关键是找圆心,圆心是直径的中点或为两直径的交点,所以,可以利用通过直径的直线来确定圆心的位置。在确定圆心的同时半径也随之确定,继而可作出轨迹圆。 When the charged particles enter the magnetic field perpendicularly, a uniform circular motion is performed under the action of the Lorentz force, and the trajectory thereof is a circle. Such questions are the focus and hot topics of the college entrance examination, and they are also one of the difficulties that candidates find difficult. In the college entrance examination, many candidates are unable to correctly draw the trajectory circle, resulting in mistakes in the solution. In order to solve this difficult problem, this article will take a typical high exam topic as an example, and use related geometric knowledge to talk about the method of constructing the trajectory circle using the “two-line fixed center” method. The key to constructing the trajectory circle is to find the center of the circle, which is the midpoint of the diameter or the intersection of the two diameters. Therefore, the position of the center of the circle can be determined by using the straight line passing through the diameter. At the same time as the center of the circle is determined, the radius is also determined and then a trajectory circle can be made.