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.

In simulations with electric fields the most important characteristics of a test particle is its electric charge and its mass. In this situation it is often referred to as a test charge.

The electric field created by a point charge q is

where ε₀ is the vacuum electric permittivity.

Multiplying this field by a test charge ${\displaystyle q_{\textrm {test}}}$ gives an electric force (Coulomb's law) exerted by the field on a test charge. Note that both the force and the electric field are vector quantities, so a positive test charge will experience a force in the direction of the electric field.

The easiest case for the application of a test particle arises in Newton's law of universal gravitation. The general expression for the gravitational force between any two point masses ${\displaystyle m_{1}}$ and ${\displaystyle m_{2}}$ is:

where ${\displaystyle \mathbf {r} _{1}}$ and ${\displaystyle \mathbf {r} _{2}}$ represent the position of each particle in space. In the general solution for this equation, both masses rotate around their center of mass R, in this specific case:

In the case where one of the masses is much larger than the other (${\displaystyle m_{1}\gg m_{2}}$), one can assume that the smaller mass moves as a test particle in a gravitational field generated by the larger mass, which does not accelerate. We can define the gravitational field as