what is test charge (檢驗電荷) ?

"A test charge is an object (usually a point particle) that has negligible charge; one can ignore the electrical field generated by the object itself. This concept is very useful when one wants to understand the properties of a given electrical field. The test charge is the electrodynamic counter-concept to the test particle in gravitation." (wikipedia)

easier to understand is: a point where you calculate electrostatic field using Coulomb's Law (electric force per unit charge, vector quantity)