Why always test charge is taken as positive?why cant it be negative because it is required to know the effect of unknown charge.
Kindly refer to the link given below for an answer to a similar query :
https://www.meritnation.com/ask-answer/question/why-is-test-charge-taken-positive/electric-charges-and-fields/2365999
https://www.meritnation.com/ask-answer/question/why-is-test-charge-taken-positive/electric-charges-and-fields/2365999