Potential and potential energy I know when a negative charge moves in the direction of a uniform electric field its potential energy increases and its potential decreases. For example, its potential energy changes from $0.9\ \rm{mJ}$  to $1.2\ \rm{mJ}$, but its potential changes from $-90\ \rm V$ to $-120\ \rm V$. Where is the potential zero, and where is the potential energy zero?
Do they become zero at different locations? 
 A: Electric potential is just the electric potential energy per charge. In other words, electric potential just depends on the charge distribution around you, where as if you were looking at a charge in the field caused by the charge distribution you could then say it has an associated potential energy in that configuration.
In light of this, the relation between electric potential energy $U$ and electric potential $V$ for a charge $q$ is just $$U=qV$$ Therefore, these two values need to be equal to $0$ at the same point in space. Of course, this $0$ point can be chosen to be at various locations, but once you set it then you have to stay consistent, and $U$ and $V$ will both be $0$ at that location.
Therefore your example is totally fine. If the potential energy goes from $0.9\ \rm{mJ}$ to $1.2\ \rm{mJ}$ and your potential goes from $-90\ \rm{V}$ to $-120\ \rm{V}$ nothing is wrong. In either case the values are moving farther away from $0$. If you moved the charge in the other direction back to where one value was $0$, you would find the other value to be $0$ as well.
