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有一类竞赛题,带电粒子在匀强磁场、匀强电场和重力场中运动,所受的洛伦兹力大小和方向随时间而变化,运动过程复杂,常见的方法难于求解此类问题.因该类问题中带电粒子所受的重力或电场力是恒力,故可以采用构造速度的方法来巧妙地解答该类问题.构造速度法就是根据带电粒子所受重力或电场力为恒力的特点,在某一方向或某几个方向构造一对或几对大小相等、方向相反的速度,让带电粒子以其中一个速度或几个速度运动时所受的洛伦兹力与所受的重力或电场力相平衡,这样就可以将带电粒子的复杂运动看成是一个或几个匀速直线运动以及以其余分速度的合速度作匀速圆周运动的合运动.构造速度法解题思路清晰,解题步骤简便,且便于学生理解和掌握,下面举例说明.
There is a type of competition problem in which charged particles move in uniform magnetic fields, uniform electric fields, and gravitational fields. The magnitude and direction of Lorentz force vary with time, and the motion process is complicated. Common methods are difficult to solve such problems. In this kind of problem, the gravity or electric field force of the charged particle is a constant force, so the method of constructing velocity can be used to solve the problem subtly. The tectonic velocity method is based on the gravity or electric field force of a charged particle as a constant force. To construct a pair or pairs of equal and opposite speeds in one direction or in several directions. The Lorentz force and the force of gravity that the charged particles receive when they move at one or more speeds The electric field balances forces, so that the complex motion of a charged particle can be regarded as one or more uniform linear motions and the combined velocity of the remaining partial velocity is a uniform motion of a circular motion. Constructive velocity method solves problems clearly and solves problems The steps are simple and easy for students to understand and grasp. The following examples illustrate.