Suppose you want to slide a book across the table to your friend. You give it a quick push to start it moving, but it begins slowing down because of friction as soon as you take your hand off. The book’s kinetic energy is transformed into thermal energy, leaving the book and the table slightly warmer. The only way to keep the book moving at a constant speed is to continue pushing it.

As FIGURE 27.7 shows, the sea of electrons is similar to the book. If you push the sea of electrons, you create a current of electrons moving through the conductor. But the electrons aren’t moving in a vacuum. Collisions between the electrons and the atoms of the metal transform the electrons’ kinetic energy into the thermal energy of the metal, making the metal warmer. (Recall that “makes the wire warm” is one of our indicators of a current.) Consequently, the sea of electrons will quickly slow down and stop unless you continue pushing. How do you push on electrons? With an electric field!

FIGURE 27.7 Sustaining the electron current with an electric field.

One of the important conclusions of Chapter 24 was that $\vec{E}=\vec{0}$ inside a conductor in electrostatic equilibrium. But a conductor with electrons moving through it is not in electrostatic equilibrium. An electron current is a net motion of charges sustained by an internal electric field.

Thus the quick answer to “What creates a current?” is “An electric field.” But why is there an electric field in a current-carrying wire?