Re: Why does microwave-cooked popcorn have an electric charge?

Hi Dan. Weird! I don't know if I understand this entirely. As the popcorn pops, it collides with the plastic. The surfaces are dissimilar, so charging by contact takes place. The plastic surface will aquire a strong charge of one polarity, while the popcorn has an opposite charge. The oppositely-charged fragments should cling to the plastic surface.

When you bring your hand near the outside surface, a capacitor forms and it concentrates the lines of field in the space between your hand and the plastic. Another way to say it: the charged plastic attracts opposite charges from within your conductive body, so the surface of your hand develops an opposite charge. Your hand has an opposite charge compared to the plastic, but so do the popcorn fragments, and so your hand will repel the popcorn. (Or, your hand makes the e-field stronger between the plastic and your hand, and this makes the field weaker between the plastic and the popcorn fragments, and so the fragments are attracted to other regions of the plastic which are not being "shielded" by being so close to your oppositely-charged hand.)

Now why should things be different when you put your hand inside? FOr one thing, the force on the popcorn fragments is reversed: before it was pushing them up off of the plastic, but now it is pushing them against it. Also, the shape of the field is reversed: the fragments before were in a field which spread outwards from the plastic, and this would tend to drive them sideways across it. Since the plastic is curved, the field now spreads inwards, towards your hand, and would tend to be perpendicular to the inner plastic surface. The force would not tend to drive the fragments sideways. The popcorn would tend to move far less than before, but I can't explain why the forces now seem to be totally zero.

If you had some charged popcorn fragments clinging to a FLAT piece of oppositely charged plastic, I bet that they would flee from your fingers no matter which side you approached.