In the case of ions, we must refine our concept of passive transport: An ion diffuses not simply down its concentration gradient but, more exactly, down its electrochemical gradient.
For example, the concentration of Na+ inside a resting nerve cell is much lower than outside it. When the cell is stimulated, gated channels open that facilitate Na+ diffusion. Sodium ions then "fall" down their electrochemical gradient, driven by the concentration gradient of Na+ and by the attraction of these cations to the negative side (inside) of the membrane.
In this example, both electrical and chemical contributions to the electrochemical gradient act in the same direction across the membrane, but this is not always so. In cases where electrical forces due to the membrane potential oppose the simple diffusion of an ion down its concentration gradient, active transport may be necessary.
In Chapter 48, you will learn about the importance of electrochemical gradients and membrane potentials in the transmission of nerve impulses.