|MadSci Network: Physics|
You are quite correct. If plastic wrap stuck by static cling, it would not stick to itself. First, suppose the entire sheet of plastic wrap was charged the same sign, say positive. That positive charge would induce a negative charge in other objects, such as the food you were trying to cover. It would tend to stick to things, especially metals. But it would strongly repel any part of itself, making it difficult to fold over. It certainly would not stick to itself at all. It would be nearly impossible to handle, too: you couldn't package it in a roll, and a stack of plastic-wrap boxes would tend to fly apart due to the net charge on each one. In general, you can assume that everything you buy in the grocery store has equal amounts of positive and negative charge. All right, suppose they made one face of the plastic wrap positive and the other negative. If the charges were not equal, we'd be back to the first case. If they were equal, I think there would be no electric fields or forces outside the plastic wrap. Just as in the case of a parallel-plate capacitor, a charge outside the plastic wrap would feel an equal attraction to the positive face and repulsion from the negative face. No forces, no sticky. The exception is that the very edges of a piece would have "fringing fields" where the cancellation wasn't perfect. The edges would be sticky in a very narrow band, comparable to the thickness of the sheet. But some simple experiments with plastic wrap show that the whole face is sticky, with the edges no more so than the middle. Next trick: suppose they made a pattern of positive and negative charges on each face, say a checkerboard. If you put the plastic wrap up against a jar, each positive square would induce a region of negative charge in the glass next to it. Each negative square in the plastic wrap would induce a positive charge in the glass next to it. So the plastic wrap would stick to other objects. If you brought two sheets of plastic wrap close to each other, though, the positive squares might be on top of the negative squares (very attractive) or they might be on top of the other positive squares (very repulsive). Although I haven't simulated it, my instinct is that the repulsive areas and attractive areas would balance out to give no stickiness. At least you would see strange patterns of sticky and non-sticky areas depending on how much you rotated the top sheet before pressing it down on the bottom sheet. So that doesn't seem to fit the facts, either. Fortunately a fellow Mad Scientist has also weighed in on the issue, who knows what plastic wrap is. (I only know what it isn't.) See this answer for more details. To summarize his explanation, the stickiness of plastic wrap is similar to the stickiness of tape: they add a small amount of a soft, mobile additive to the plastic wrap, kind of like the adhesive layer on tape. When two sheets of plastic wrap contact each other, patches of this additive move around to "glue" the sheets together. Hope this answers your question. Deron Walters Rice University
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