Charge Q is a source of an electric field and charge q is the test charge placed in that field.
Electric charges change the electrical properties of the space around them, causing an electric field. It is this field that will interact with another electrical charge, producing a force of attraction or repulsion. To measure the electric field at a location in space, we place a positive charge, called a test charge q 0 , and measure the electric force acting on that charge.
The expression above allows us to calculate the electric field strength, whatever the charges that create this field. Let’s apply it to a particular case, where the charge that creates the field is a point charge. Let’s assume that an electrified particle with charge Q is the source of an electric field, as in the figure above.
We easily get
This expression allows us to calculate the electric field strength at a certain point, when we know the value of the point charge Q that created it and the distance from the point considered to the charge. Note, however, that this expression can only be used for a field created by a charge that can be considered a point charge.
Remember: in the above equation, the value of the electrostatic constant is:
k = 9 x 10 9 Nm 2 .C -2 .