Electrophorus charge/discharge, rinse, and repeat
In this experiment we’ll create an object called an electrophorus. Using the materials listed above, we’ll charge the object and then discharge it creating a snap, a little electrical shock, and a bright spark.
- Use pliers to remove the pen cartridge from the insides of a writing pen. This will be our ‘handle’.
- Place the pie pan upside down on the table.
- Push a thumbtack down through the center of the pie pan.
- Turn the pan back over so you are looking at the inside of the pan. The point of the thumbtack should be sticking up through the middle of the pan.
- Coat the thumbtack point with hot glue.
- Push the bottom of pen body down onto the extending thumbtack point. You could also use a pencil for this step and press the eraser end of the pencil down onto the thumbtack.
- Let the glue dry for a little while.
- Rub the Styrofoam plate with the wool rag for about 45 seconds.
- Place a Styrofoam plate upside down on the table.
- Using the pen ‘handle’ that we just created, place the pie pan on top of the upside-down Styrofoam plate (the pen should be sticking up).
- Quickly touch the pie pan with your finger. It may produce a small shock.
- Remove the pie pan off the Styrofoam plate using the pen ‘handle’.
- Discharge the ‘charged’ pie pan by touching it with your finger.
You can recharge the pan by starting at step 8. Rubbing the Styrofoam plate with the wool rag creates a negative charge on the plate (that is, it attracts electrons from the wool). When you place the pie pan on top of the Styrofoam, the electrons on the Styrofoam repel the electrons on the pan. The pan at this point has a neutral charge. But when you touch the pan (while it is on the Styrofoam plate) the electrons travel off of the pan and onto your finger (possibly creating a spark). Now the pan has a positive charge (it was charged by induction.
Now, by carrying this contraption with the insulated handle (the pen), you can carry a positive charge all around the room. When you bring this positive charge near your finger, or any other object that is a source of electrons, the positively charged pan will attract electrons, creating a spark.
Notes about the Making sparks with Styrofoam and a metal pie pan experiment
The electrophorus device consists of a dielectric plate (the Styrofoam plate) and a metal plate (the pie pan) with an insulating handle (the plastic pen). The dielectric plate is first charged through the triboelectric effect by rubbing it with something like fur or cloth. The metal plate is then placed onto the dielectric plate to produce the effect.
The dielectric plate does not transfer a significant fraction of its charge to the metal because the contact at a microscopic level is poor. Instead, the electrostatic field of the charged dielectric causes the charges in the metal plate to separate. It develops two regions of charge – the positive charges in the plate are attracted to the side facing down toward the dielectric, charging it positively, while the negative charges are repelled to the side facing up, charging it negatively, with the plate remaining electrically neutral as a whole. Then, the side facing can be grounded (which can be done by touching it with a finger), draining off the negative charge and creating a spark.
Since the charge on the dielectric is not depleted in this process, the uncharged metal plate can be placed back on the dielectric and the process repeated to get another charge. This can be repeated as often as desired, so in principle an unlimited amount of induced charge can be obtained from a single charge on the dielectric.
Charge in the universe is conserved so no physics principles are violated. The electrophorus simply separates positive and negative charges. A positive or negative charge ends up on the metal plate (or other storage conductor), and the opposite charge is stored in another object after grounding (in the earth or the person touching the metal plate).
Making sparks with Styrofoam and a metal pie pan experiment supplies
Supplies: BIC pen, Pie pan, Thumbtack, Glue (hot), Styrofoam plate
Image CreditsDrawing of an electrophorus showing electric charges via Wikimedia Commons with usage type - Creative Commons License. November 9, 2010
Drawing of electrophorus electrostatic generator, invented by Johan Carl Wilcke in 1764, from an 1840 chemistry text via Wikimedia Commons by Robert hare with usage type - Public Domain. 1840
Diagram showing how an electrophorus is used, from a 1909 magazine via Wikimedia Commons by Edwin J. Houston with usage type - Public Domain. August 1909