Why test charge is positive?
1 Answer
A test charge can be positive or negative.
Explanation:
The direction of the electric field (force per unit charge) is (arbitrarily) chosen to be the direction of the force it would exert on a positive test charge, so if you want to find the direction of the electric field you place a positive test charge in the field and see in which direction it flows and this is why it seems like only a positive test charge can be used . If you placed a negative test charge in the field then it would flow in the direction opposite to the direction of the electric field.
This kind of arbitrary definition happens all over physics e.g. defining where a zero of potential is. You need to choose one option, so you choose the best one you can e.g. for potential you usually choose a reference points such that there are the fewest arbitrary constants.
