What Is an Electrostatic Field

To describe how the force acting on C depends on spatial position, you compute the electrostatic field. Consider C to be a test particle, having a charge q . By test particle, we mean a charge that you are free to place at any position to probe the force there. The test particle is acted upon by the fixed charges. 

EXAMPLE 20,4 The field from a point charge. A point charge is radially symmetric so you can dispense with vector notation to compute its field, and just treat the dependence on radial distance r. The field from a fixed point charge acting on a test particle is 

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