Why test charge should be always possitive in a an electric field?

Electric Field and the Movement of Charge
Perhaps one of the most useful yet taken-for-granted accomplishments of the recent centuries is the development of electric circuits. The flow of charge through wires allows us to cook our food, light our homes, air-condition our work and living space, entertain us with movies and music and even allows us to drive to work or school safely. In this unit of The Physics Classroom, we will explore the reasons for why charge flows through wires of electric circuits and the variables that affect the rate at which it flows. The means by which moving charge delivers electrical energy to appliances in order to operate them will be discussed in detail.

One of the fundamental principles that must be understood in order to grasp electric circuits pertains to the concept of how an electric field can influence charge within a circuit as it moves 

listen my dear,

postive and negative charge is hypothetical , we have the freedom to take any test charge,

either positive or negative its totally up to you,

if ,     Test chage +ve,       Force act on test charge will be in the direction of electric field.

         Test charge -ve       Force act on test charge will be opposite to the direction of electric field.

Thanks & Regards 

Amar Kumar, 

askIITians Faculty

M.Tech, IIT Delhi