[填空题]
In a photocell, ultraviolet (UV) light provides enough ene0m2ncsgmi h5) (7ym fnrgy to some elects0k aij 4*c**thksn *lrons in barium metal to eject them from a surface at high speed. See Fig. 17–33. To measure the maximum energy of the electrons, another plate above the barium surface is kept at a negative enough potential that the emitted electrons are s*khst 4**ns i*ckl a0jlowed down and stopped, and return to the barium surface. If the plate voltage is $-3.02V$ (compared to the barium) when the fastest electrons are stopped, what was the speed of these electrons when they were emitted? $ \times10^{6 }\,m/s$
