# Q & A: static charges

Q:
My friend Ray showed me a little trick with tape the other day. He took 4 pieces of scotch tape, putting 2 of them side by side on a glass surface and the other 2 on top of each other (one on the glass and one ontop so that it didn’t touch the glass). He first pulled up the 2 side by side and showed me that they repel each other. He then pulled up the other 2 and off of each other; they attracted each other. How does this work?
- Mirth (age 17)
Miami
A:
Nice question. Here's an educated guess.

The two pieces that were pulled off the glass have both been through the same experiences. If pulling tape of glass tends to leave the tape with a positive electrical charge, it will do so for both. Likewise for negative charge. Either way, they end up with like charges, and like charges repel.

When the sticky side of one piece is stuck to the smooth side of the other, whatever charge gets left in the sticky side when they're pulled apart has to be opposite to the charge left in the smooth side, if they start off with no net charge. So the pieces are oppositely charged, and attract. There's a slight complication since we know they pick up some charge when pulled off the glass, so they don't start off neutral. Apparently that's small enough (or it stays on one piece and happens to have the right sign) so that it doesn't overwhelm the opposite charges from separtating the two pieces.

Mike W.

(published on 10/22/2007)