What causes items like salt and pepper to be attracted to the balloon?​

First, the pepper polarizes much easier than the salt. That is, its electrons move mostly to one side of the pepper flake leaving one end positive and the other negatively charge. The positive end is attracted to the negative balloon. Salt does not polarize quite as much so it doesn’t feel as much of a pull. However it will jump up on the balloon if you hold it very close to the plate.

Second, the pepper flakes are much lighter than the salt crystals. So even if the salt does jump up on to the balloon, it is too heavy to stay and falls back down.

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