一、近三年“带电粒子在复合场中运动问题”解析例一:(2010年,分值18分)如下图(甲)所示,在0≤x≤a、o≤y≤a/2范围内有垂直于Oxy平面向外的匀强磁场,磁感应强度大小为B。坐标原点O处有一个粒子源,在某时刻发射大量质量为m、电荷量为q的带正电粒子,它们的速度大小相同,速度方向均在Oxy平面内,与y轴正方向的夹角分布在 First, the past three years “Moving particles in the composite field problem ” Example 1: (2010, score 18 points) As shown in Figure A below, 0 ≤ x ≤ a, o ≤ y ≤ a / 2 has a uniform magnetic field perpendicular to the Oxy plane, and the magnetic flux density is B There is a particle source at the coordinate origin O. A large number of positively charged particles with mass m and charge q are emitted at a certain time. Their velocity is the same, the velocity direction is in the Oxy plane, and the angle between the positive direction of the y-axis Distributed in