Why is the test charge always positive?

why test charge is always positive ?

It isn't always positive. It generally has an arbitrary value, which can be positive or negative.

Could be your book/course uses always a positive test charge for consistency. The less changes in the system, the easier it is to compare the differences with the earlier cases, that makes teaching/learning easier.

Borek said:

Could be your book/course uses always a positive test charge for consistency.

I always use positive test charges when I teach, for simplicity. Negative test charge gives E vector in opposite direction than Coulomb force on it, which per se is not a problem, but you know, didactics When someone is learning about it for the first time it's better to avoid such things, we can come back to it later.