What type of test charge do we use for an electrical field ?

A positive charge with very small charge compared to the charge source of the field

Explanation:

Let's consider an electric field produced by a source charge Q.

In order to test the direction and magnitude of the electric field, we use a positive test charge q. The reason for chosing a positive charge is that a positive charge will move following the directions of the electric field lines (in fact, the positive charge will be repelled from charge Q if charge Q is positive, so it will follow the electric field lines, which point away from the charge).

Moreover, the test charge q must be small compared to charge Q. This is because we don't want to modify significantly the configuration of the electric field, so the test charge q must be negligible compared to charge Q.

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