This simulation shows the electric field vectors; the colors indicate the relative strength of the field at that point. You can drag the charges around to see the change in the electric field due to the changes in charge position. If you overlap two charges, their charge values will add and you will see the field due to the combined charges.
With two positive charges describe the electric field around each charge. What happens if you lay a positive charge exactly on top of a positive charge of the same magnitude?
With a positive and a negative charge showing describe the electric field around each charge. What happens if you lay a positive charge exactly on top of a negative charge of the same magnitude? Both charges are still there so why you don't you see a field?
In this simulation there is a way to tell from the picture which charge is negative and which is positive. Expain how.