Elecric Forces

Straws and Pens

Seat Experiment

Using such everyday items as a straw, tape, plastic pen and various types of cloth, we'll explore static electricity and how items can be charged, discharged, and recharged.

You'll need a straw that still has the paper covering on it. Carefully tear one end of the paper and slide about one inch of the straw out of the paper.  Partially push and pull the straw in and out of the paper five times. Remove the paper and lay it on the table while holding one end of the straw with your fingertips.

Like a wand, lower the straw and hold it next to the paper, then raise the straw again.  What happens? 

Now take the straw and "wipe" it through your bare palm a couple of times. Once again hold the straw to the paper. What happened? 

Q1: When you first pull the straw out of the paper there is a static electric charge present. Where do you think this charge originated from?

Q2: The Coulomb force (attractive or repulsive force between charged objects) between the paper and the straw allows you to initially attract the paper and "lift" it off of the table. What must be true of the relative sizes of Coulomb force  and force due to Gravity for the paper to be lifted by static electricity?

Why won't the straw pick up the paper after you wipe the straw through your bare hand a few times? 

Take the straw and rub it with some cloth, like your shirt or coat. Locate the point on the straw that you think is about half its length. Use a long narrow piece of tape to hang the straw from the edge of your desk. Make sure the straw can move freely. 

Hold one end of a plastic pen near one end of the straw. Does anything happen? Now take the pen and rub it over a piece of fabric, ten or twenty times. Again hold the end of the pen near the end of the straw. What happens? 

Q4: What does rubbing the plastic pen or the straw over fabric do to the pen and straw?

Q5: Why does the straw "spin" when you hold the end of the pen near it? What would happen if you rubbed the pen over a different material, like rubber or leather and then brought the pen near the straw? See the table of triboelectronegativities.

Q6: Draw a free-body diagram of the paper while it is being lifted by the straw. Label the Coulomb force and force due to gravity and indicate their relative sizes.

Advanced Question: What must be the minimum Coulomb force present for you to lift the paper if you know its mass is 0.15 grams? 



Seat Activities