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?
 A: 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.
A: There is no fundamental difference between positive and negative charges.  Anything that couples to positive electric charge couples to negative charge and vice versa.  Practically, the main difference is that negative charge is on electrons and positive charge is in protons in atomic nuclei, so on a day-to-day basis they play different roles.  However, these signs are reversed for antimatter, so we see there really is nothing special about the sign of a charge.
A: You can also use the negative charges. What happens is that usually the positive charges are used because, by convention, these are the charges that they decided to use to determine the direction of the electric current. Besides, you can see it from this perspective: negative charges can move on the "lattice" and they are more "fun", while the protons are "boring", there are always "stuck" in the atomic nucleus.
A: you can also take negative charges to define potential of a point.there would be electric field due to a -ve charge Q also and when you bring a point charge q(having -ve charge) near Q then you have to do work against the field.(because same charge repel each other )
A: We take positive charge as a test charge because positive charge is higher potential and negative charge is lower potential.  Therefore, influence of positive charge on other charges is greater than negative charges.  We can also take negative charge but the effect will be lower.
A: Because negative charge has attraction with positive charge which is not sure test
A: we take positive charge as a point charge because positive charge has a high potential and electrons are always move from lower potential to the higher potential.so , we use positive charge as a point charge
A: You can also take negative charge as test charge but in this case charge will move opposite to the field lines
