<s>[INST] The electron current (with energy 0.7 eV) is emitted from two identical metal blocks A and B. The metals blocks A and B are located symmetrically across the surface of a positively charged metal plate Q. At the starting time, collision between electrms and bumps electrons out from metals is very small, so that it can be negligible. Explain why only the electrons having a specific energy reach the positively charged plate Q.[/INST]