Why a test charge is always positive. WHy cant we assume the test charge to be negative? Just that if we assume it to be negative test charge, will the electric field turns to be negative? Additional Details reply: does it mean that if we put a negative charge as a test charge near a positive charge source, it shows that the positive charge actually exerts a field pointing inward? before, if we put a positive charge near a positive source, it means field radially outward, how if we change the test charge to a negative charge?