题目

用波长λ=5×10－7  m的光照射一金属表面.已知其逸出功W=2 eV，需加怎样的电压才能把从阴极逸出的全部电子挡住而不致飞到阳极? 答案：0.5 V 解析:照射光的能量 E =hν=h =6.63×10－34×J=2.5 eV                                                                   (2分) 由爱因斯坦光电方程 E k=hν－W                                                                                                     (2分) 得  E k=h－W=2.5 eV－2 eV=0.5 eV                                                            (2分) 要使光电子被挡住，电场力应做功W电 W电＝E k＝0.5 eV                                                                                             (1分) 所以应在阳极和阴极间加0.5 V的反向电压.                                                     (1分)

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