Suppose we have *N* source charges *N* electrostatic forces on a test charge *Q*. The net force on *Q* is (see (Reference))

We can rewrite this as

### Note:

where

or, more compactly,

### Note:

This expression is called the electric field at position *N* source charges. Here, *P* is the location of the point in space where you are calculating the field and is relative to the positions

Notice that the calculation of the electric field makes no reference to the test charge. Thus, the physically useful approach is to calculate the electric field and then use it to calculate the force on some test charge later, if needed. Different test charges experience different forces Equation 2, but it is the same electric field Equation 4. That being said, recall that there is no fundamental difference between a test charge and a source charge; these are merely convenient labels for the system of interest. Any charge produces an electric field; however, just as Earth’s orbit is not affected by Earth’s own gravity, a charge is not subject to a force due to the electric field it generates. Charges are only subject to forces from the electric fields of other charges.

In this respect, the electric field *G* is a proportionality constant, playing the same role for

To push the analogy further, notice the units of the electric field: From *E* are newtons per coulomb, N/C, that is, the electric field applies a force on each unit charge. Now notice the units of *g*: From *g* are newtons per kilogram, N/kg, that is, the gravitational field applies a force on each unit mass. We could say that the gravitational field of Earth, near Earth’s surface, has a value of 9.81 N/kg.