# Source field

In theoretical physics, a source field is a field $J$ whose multiple
$S_{source} = J\Phi$
appears in the action, multiplied by the original field $\Phi$. Consequently, the source field appears on the right-hand side of the equations of motion (usually second-order partial differential equations) for $\Phi$. When the field $\Phi$ is the electromagnetic potential or the metric tensor, the source field is the electric current or the stress–energy tensor, respectively.