题目

The National Gallery of Art in Washington, D.C., is one of the world's greatest art museums. Millions of people have entered its doors to see paintings by the world's fine artists. But if these priceless masterpieces are to be preserved, the Gallery must protect them carefully. The Gallery's 135-man guard force has successfully prevented them from being stolen, but protecting the paintings from nature is a greater problem. In past times, the owners of paintings did not protect them from damaging changes in humidity (湿度) and temperature. As a result, the life of these paintings wer如图所示，在y＞0的空间中存在匀强电场，场强沿y轴负方向；在y＜0的空间中，存在匀强磁场，磁场方向垂直xy平面（纸面）向外．一电量为q、质量为m的带正电的运动粒子，经过y轴上y=h处的点P1时速率为v0，方向沿x轴正方向；然后，经过x轴上x=2h处的 P2点进入磁场，并经过y轴上y=-2h处的P3点．不计重力．求（l）电场强度的大小．（2）粒子到达P2时速度的大小和方向．（3）磁感应强度的大小．