homework and exercises - Why do we test electric fields with positive charges and not negative ones?

Is there any difference between using a positive versus a negative charge to test an electric field?

Answer

You can use a negative charge to test an electric field. You just have to remember that the electric field points antiparallel (opposite) to the force on the charge, rather than parallel to it (in the same direction). That's just a convention, though; we could have defined the electric field to point with the force on a negative charge, and physics would work the same, except for a couple of negative signs in some formulas.