Attraction between the plates can be easily understood by focusing on the one-dimensional situation. Suppose that a moveable conductive plate is positioned at a short distance a from one of two widely separated plates (distance l apart). With a ≪ l, the states within the slot of width a are highly constrained so that the energy E of any one mode is widely separated from that of the next. This is not the case in the large region l where there is a large number of states (about ⁠ l / a ⁠) with energy evenly spaced between E and the next mode in the narrow slot, or in other words, all slightly larger than E. Now on shortening a by an amount da (which is negative), the mode in the narrow slot shrinks in wavelength and therefore increases in energy proportional to − ⁠ da / a ⁠, whereas all the ⁠ l / a ⁠ states that lie in the large region lengthen and correspondingly decrease their energy by an amount proportional to − ⁠ da / l ⁠ (note the different denominator). The two effects nearly cancel, but the net change is slightly negative, because the energy of all the ⁠ l / a ⁠ modes in the large region are slightly larger than the single mode in the slot. Thus the force is attractive: it tends to make a slightly smaller, the plates drawing each other closer, across the thin slot.