In your Mechanics physics course, you learned about the three fundamental principles (though they may have been called by different names): the momentum principle, the energy principle, and the angular momentum principle. In Electricity & Magnetism (E&M for short), these fundamental principles do not change. Instead, we focus on two new types of interactions: the electric interaction and the magnetic interaction. As we talk about these new interactions, we will be relying on the three fundamental principles to talk about how a system responds, so it is worth reviewing these principles and how they work.

The momentum principle describes how the momentum of a system will change as a result of external forces. Since momentum is a vector, this principle is really a set of three equations - one for each dimension. Therefore, it is also able to describe how an system will move in three dimensions. The momentum principle is the underlying mechanism

$$\Delta \vec{p}_{sys} = \vec{F}_{ext} \Delta t$$

If you choose the system so that there are no external forces, or if the external forces can be neglected, the momentum of the system is conserved. This means that the initial momentum of the system has to equal the final momentum of the system.

$$\Delta \vec{p}_{sys} = 0 \longrightarrow \vec{p}_{sys,i} = \vec{p}_{sys,f}$$