ABSTRACT
For an electron coming toward the spacecraft, the effect of the negative potential is to slow down the electron. However, in nature, a spacecraft would not easily charge to a highly negative voltage even though adding electrons to a spacecraft surface would increase the negative potential of the surface. The resultant potential depends very much on the energy distribution of the incoming electrons. Since an electron has a negative charge, the potential profile accelerates the electron as it travels from the high negative potential. As an analogy, the acceleration from left to right is like a ball rolling downhill. As a second exercise, write down the total energy for a positive ion coming in from right to left.
