What is test charge?

Whether there is an electric field at a point or position. Or to measure exactly how far the electric field of one charge extends, we need another charge. If the other charge experiences any force of attraction or repulsion at any point, then it can be understood that there is an electric field. Now the second charge we bring to test the electric field or to measure the electric field intensity, if it has an electric field of its own, it will interfere with our first charge’s electric field and measure the electric field intensity. So to measure the electric field or electric field intensity we need a charge which has no electric field. The work of which will be only to test the electric field. This charge is called the test charge.

In reality, no charge is possible without an electric field. So it is actually just an imaginary or mathematical charge. Since there is no electric field, we can say that it has no effect of its own. It is always positive for computational convenience. But it can also be negative.