Test charge

In physical theories, a test particle 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 behavior 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.

The easiest case for the application of a test particle arises in Newtonian gravity. The general expression for the gravitational force between two masses ${\displaystyle m_{1}}$ and ${\displaystyle m_{2}}$ is:

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

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