# How Many Extra Electrons Are On This Charged Tape?

Maybe you haven’t tried this before — but it’s kind of awesome. If you put two clear sticky tapes together and pull them apart, they will become electrically charged. It produces a nice effect that even works when the air humidity is higher than normal.

Of course, electric charge is conserved, so when you pull these apart the bottom tape becomes negative and the top tape is positive. Let’s focus on the bottom tape — it’s negative because some of the electrons from the top tape where transferred. But how many electrons made this transition? Let’s find out.

# Data Collection

I’m going to vertically hang two negatively charged tapes. Just like this:

Using video analysis, (Tracker Video Analysis) the length of the two tapes are both about 15 centimeter and each hangs with a deflection of 23 degrees from the vertical. Finally, I need the mass of the tape. If you take a 4 meter long piece of tape and place it on a scale, it has a mass of 4 grams. That means the tape has a length density of 1 gram per meter. So, 15 centimeters of tape has a mass of 0.15 grams.