In Raymond Serway's physics textbook, the definition of the electric field vector, was that it's force vector acts on a positive test charge, given as force divided by the test charge:
$$\overrightarrow{E}=\frac{\overrightarrow{F}}{q_0} \tag{1}$$
Then he provided another equation which is:
$$q\overrightarrow{E}=\overrightarrow{F} \tag{2}$$
where $q$ is any charge, no matter if it is positive or negative.
My question is: the book defined the electric field as the force acting on a unit positive test charge, so where has equation $(2)$ come from? We should say that:
$$\overrightarrow{E}=\frac{\overrightarrow{F}}{q}$$
not
$$\overrightarrow{E}=\frac{\overrightarrow{F}}{q_{0}} \ \ ?$$