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Published: 06 January 2020

• physics
• electromagnetism

Today we introduce another new demonstration*, this time on electrostatics. Remember that soda can from last month? It has now dried out and has returned for another adventure!  

Some materials, when rubbed together, build up an electric charge through contact; this phenomenon is known as triboelectricity. In the case of the materials we have here, after the rubber has extensive contact with the wool, the rubber is charged with an excess of electrons, giving it a net negative charge. If you touched the rod with your bare hand, you might feel a faint shock as that excess charge jumped to you. (Perfectly safe, just a little surprising!)

In this case, though, we're going to be careful not to discharge the rod too soon. Instead, we hold the rod carefully near and parallel to the soda can, being careful not to touch the rod to either the can or the table. As we do, we see an unusual phenomenon: the can will slowly start rolling towards the rod! We can even slowly pull the can across the table as it seemingly chases the rod about. How can this be?

What we are seeing is a property known as electrostatic induction. Left to its own devices, the can has no net charge - it has an equal number of positive and negative charges, distributed evenly throughout the material of the can. But because the aluminum can is an electrical conductor, some of those charges can be free to move around if acted upon by an outside force. When the negatively charged rod is brought near the can, the electrostatic force (or Coulomb force) that this negative charge exerts affects the atoms in the can. Some of those free electrons, also carrying negative charge, are repelled and move to the far side of the can. This leaves the side of the can nearest the rod with a net positive charge, which is now attracted to the rod! And so the whole can, being quite lightweight, can slowly start moving towards the rod. 

Try this out in your own classes with demonstration J1-14: Electrostatic Induction - Attracting A Can.

 (*New to us; credit for this development goes to the Physics Instructional Resource Association)