But when using just the electromagnetic fields, the effect depends on the field values in a region from which the test particle is excluded.

The concept of a test particle often simplifies problems, and can provide a good approximation for physical phenomena.

The easiest case for the application of a test particle arises in Newtonian gravity.

The acceleration of a free falling test particle is:

To go with the section on test particles above, here are the equations for the fields in which they move.

In the absence of gravity and other external forces, a test particle moves along a straight line at a constant speed.

Where such objects are concerned, the laws governing the behavior of test particles are sufficient to describe what happens.

The test particles were lost exponentially with a half life of 553 Myr.

Calculations showed that when test particles were inserted into it, they flew away from each other.

The area enclosed by the test particles does not change and there is no motion along the direction of propagation.