In this demonstration, science teacher John Goering explains how you can build and use a classic device to demonstrate basics of voltage, positive and negative charges, and resistance. To reproduce this experiment, you will need:
- Foam plate
- Aluminum pie plate
- Aluminum foil
- Drinking straw
- Needle or pin
- Neon bulb (NE2-)
- Wool sock or rag
This device is called an electrophorus. That name was coined by Mr. Volta many years ago. It consists of some very simple equipment. You have a styrofoam plate, and you rub the styrofoam plate with something such as a wool sock; that works great. Then it also consists of an aluminum plate with a foam cup. You set the aluminum plate on the foam cup, bring your finger near the aluminum plate, and listen real carefully. You may have heard a spark. Then you lift the aluminum plate up by holding on to the foam cup, and then you bring your finger close again. You can repeat the process multiple times.
Notice how the aluminum plate pulled up the foam plate when I did not hold the foam plate down. That shows us that the electrostatic force is greater than the force of gravity. I need to hold the foam plate, and I can just keep repeating this multiple times.
So, actually, I have students do this.
Now to see a little better what’s going on it is useful to take a drinking straw that has a thread attached. At the end of the thread is a tiny piece of a straw that has been covered with aluminum foil. It can be attached using glue or using two sided tape. We stick this in the styrofoam cup. The cup has two holes in it. We can stick this right in here. Then this little piece of straw with aluminum foil will serve as a conductor. We will want the string to be hanging straight down with the metal of the conductor against the edge of the aluminum plate. Then as I lift up the plate the conductor will swing away from the plate. As I set it back down it will go back to the plate again. If I lift the plate and then touch the plate, the conductor comes back to the plate. If I set it down, the conductor swings out now. If I touch the plate, it will hit the plate again. I can keep repeating that process.
Now if I have the prop for this conductor on my side, now I can lift the plate, touch it, conductor goes against the plate. Set it back down and it swings out. Now if I very carefully bring my finger close to this conductor. It will be attracted to my finger. Look at it being attracted to my finger there if I bring it real close. When it hits my finger it loses its charge and it falls back against the plate, gets more charge, and then comes back against my finger. And so as the conductor swings back and forth between the plate and my finger it is conducting electrons from the plate to my finger, until it eventually conducts all of the electrons away that want to leave the plate. So I can lift it up and I can repeat the process up here, except this conductor is acting in a very rambunctious manner at the moment. I will touch the plate, put it back against the plate, and lower it, so it’s swinging out. I can again repeat that process many times.
This can be used to teach about the concept of voltage and current, and resistance.
As the charge leaves the plate, the voltage on the plate decreases and the electrons are no longer able to transfer across the gap to my finger because the resistance is too high. I can decrease the resistance by bringing my finger closer to this thing and then the current will flow again as electrons flow to my finger. Then I can bring it all the way in until all the electrons are off.
Now I’m going to set this up again. This time I will bring a needle next to this conductor. I will aim it at the conductor. If you watch very carefully as I bring it close to the conductor it falls away from the needle before the needle ever gets to it. The electrical charge gets induced on the needle since the plate has a negative charge. A positive charge gets induced on the needle. Nearby air molecules lose their electrons to the point of the needle, and those positively charged air molecules now head towards this little conductor. They actually conduct the electricity through the air without the need of the needle actually touching the conductor. So the conductor hits against the plate. Then also it vibrates enough to vibrate against the needle as well.
And so this experiment can be done with very simple equipment. Now if you have access to an electronic store, you can get a neon lamp, just a small neon bulb. This is called an NE2H. This neon bulb can be brought against this conductor and you may observe the neon bulb flashing. As the conductor vibrates between the neon bulb and the plate, there is a flash as electrons are transferred through the neon bulb. I’ll do that on the other side here. I will go ahead and raise the plate. This time I will bring the neon bulb against the plate and the neon bulb will flash. Then I can lower the plate. The neon bulb as it’s brought against here will now flash again.
Now this neon bulb has two tiny electrodes in it. The negative electrode is the one that flashes if you watch the neon bulb very carefully. When the plate is raised, the electrode next to my hand is the one that flashes. When the plate is lowered, the electrode next to the plate is the one that flashes. That shows us that when the plate is lowered the electrons are leaving the plate and going to my hand, but when the plate is raised the electrons leave my hand and go to the plate. This process can continue on indefinitely because the styrofoam plate does not lose very many electrons at all to the aluminum plate. Instead, the negative charge in the styrofoam plate repels the electrons from the negative plate so that they go into my hand. The plate now has lost electrons because of the influence of the styrofoam plate. When I lift this plate above the styrofoam plate so it’s no longer under its influence, the electrons on this aluminum plate reposition themselves and the electrons now jump from my hand to the plate because the plate has developed an overall positive charge.
See John’s Electrophorus Lab lesson plan:
CONTRIBUTOR: John Goering