In physical theories, a test particle, or test charge, is an idealized model of an object whose physical properties (usually mass, charge, or size) are assumed to be negligible except for the property being studied, which is considered to be insufficient to alter the behaviour of the rest of the system.
The concept of a test particle often simplifies problems, and can provide a good approximation for physical phenomena.
In addition to its uses in the simplification of the dynamics of a system in particular limits, it is also used as a diagnostic in computer simulations of physical processes.
gives an electric force (Coulomb's law) exerted by the field on a test charge.
The easiest case for the application of a test particle arises in Newton's law of universal gravitation.
Newton's second law of motion of the smaller mass reduces to and thus only contains one variable, for which the solution can be calculated more easily.
This approach gives very good approximations for many practical problems, e.g. the orbits of satellites, whose mass is relatively small compared to that of the Earth.
In metric theories of gravitation, particularly general relativity, a test particle is an idealized model of a small object whose mass is so small that it does not appreciably disturb the ambient gravitational field.
According to the Einstein field equations, the gravitational field is locally coupled not only to the distribution of non-gravitational mass–energy, but also to the distribution of momentum and stress (e.g. pressure, viscous stresses in a perfect fluid).
In the case of test particles in a vacuum solution or electrovacuum solution, this turns out to imply that in addition to the tidal acceleration experienced by small clouds of test particles (with spin or not), test particles with spin may experience additional accelerations due to spin–spin forces.