Our motivation for introducing the concept of the electric field was to understand the long-range electric interaction of charges. Some charges, the source charges, create an electric field. Other charges then respond to that electric field. The first five sections of this chapter have focused on the electric field of the source charges. Now we turn our attention to the second half of the interaction.

FIGURE 23.23 shows a particle of charge q and mass m at a point where an electric field $\vec{E}$ has been produced by other charges, the source charges. The electric field exerts a force

on the charged particle. Notice that the force on a negatively charged particle is opposite in direction to the electric field vector. Signs are important!

FIGURE 23.23 The electric field exerts a force on a charged particle.

If $\vec{F}{}_{\text{on }q}$ is the only force acting on q, it causes the charged particle to accelerate with

This acceleration is the response of the charged particle to the source charges that created the electric field. The ratio $q\text{/}m$ is especially important for the dynamics of charged-particle motion. It is called the charge-to-mass ratio. Two equal charges, say a proton and a ${\text{Na}}^{+}$ ion, will experience equal forces $\vec{F}=q\vec{E}$ if placed at the same point in an electric field, but their accelerations will be different because they have different masses and thus different charge-to-mass ratios. Two particles with different charges and masses but with the same charge-to-mass ratio will undergo the same acceleration and follow the same trajectory.