Video narration
Inside a metallic conductor, there are many free electrons. With no battery connected, these electrons move randomly in all directions due to thermal motion, so the average net motion is zero—and no current flows. Now connect a battery. An electric field E is set up in the wire from left to right. Electrons are negatively charged, so they feel a force opposite to E. They still keep their fast random zig-zag motion, but now there is a tiny average drift from right to left. That slow average net velocity is the drift velocity, v_d. Because electrons drift right to left, the conventional current I is defined in the opposite direction, left to right. Current is related to drift by: v_d = I divided by n A e, where n is the number of free electrons per unit volume, A is cross‑sectional area, and e is the charge of one electron. Larger I means larger v_d.