More Electrostatic Explorations

Corrections for Science Scope "More Electrostatic Explorations" article

In the last activity, we used very simple objects to investigate electric charge. In this activity we will build two classic devices, the electrophorus and the leaf electroscope. These are better than what we used previously, because the electrophorus allows us to make a lot of charge, and the electroscope is much more sensitive than the hanging rods, but we couldn’t understand how they worked without what we did before.

With the electrophorus, we will be able to manufacture sparks whenever we want, and begin to understand what those sparks we see when we take of a sweater or watch a lightning storm (lightning is a really big spark!) are, and what cause s them.

Sparks and the Pie Pan



In the last activity you learned why your sweater attracts your hair. (The attraction between a charged object and the charge separation induced on a neutral object.) Now you’ll find out about the sparks that your sweater makes when you take it off.

We have charged objects by rubbing them together, using friction to separate the charge. Now you'll learn how to chare and object [through the process of induction]. You'll also become familiar with a device for detecting charge, ca lled an electroscope.




Students will get further understanding of induction. They’ll experience what grounding means. They’ll also discover how to charge an object by induction and grounding. They’ll get to know that a spark is actually charge transfer be tween objects. There are two electrostatic devices used in the activity. The electrophorus is used for storing and transferring charge. The electroscope can detect whether an object is charged or not.


There is a thorough explanation of the phenomena covered at the end of this teacher's version.





The electrophorus


Take the aluminum pie pan and tape the plastic cup to the center of the pan [with the open end down]. The simple device you just built is called an electrophorus. Charge the cutting board with the plastic bag. Test if it’s tr uly charged. How do you test it?


Recharge the cutting board and put the electrophorus (pie pan) on it. Slowly bring your knuckle near the rim of the pie pan without touching the pan. What do you experience?


Now go ahead and touch the pan. Then lift it from the cutting board by the handle, without touching the metal again. Bring the edge of the pie pan near one of your knuckles. What do you experience?


Does this remind you of something? Sparks in the sweater?


Now put the pan on the cutting board again but don’t touch it before removing it from the cutting board. Bring it near your knuckle. What do you experience?


When you see or hear a spark, that means a large amount of charge is jumping from one object to another. As charges jump from the pan to your hand, the electrophorus loses its charge, discharges.


Your sweater charges up during the day as it rubs against your body. When you take it off, the charges simply jump back on you making sparks all over, [so the sweater discharges like the electrophorus].


In order to get a spark; the pan has to be charged. This only happens when you touch it before lifting it. How many times can you charge and discharge the electrophorus without recharging the cutting board?


Let’s figure out how the electrophorus works. The cutting board charges up negatively. What happens to the charges in the neutral pie pan when you put it on the board? Think of the soda can.



When you touch the rim, the charges from the rim jump on your hand. What type of charge remains on the pan? (Circle the answer)

Is the electrophorus negatively / positively charged or uncharged when you lift it?


What happens to the charges when you lift the pan in the case when you didn't touch it before lifting it?


Charges are like little balls. They can be pushed around, transferred from one place to another, selected according to sign or mixed all together. For instance a neutral object is one where positive and negative charges are mixed equall y. We can give the neutral pie pan a positive charge by removing negative charges by removing the negative charges repelled by the cutting board.





You already know a few ways you can test whether an object is charged or neutral. There is, however, a physical device called electroscope built for this. Let’s get familiar with it.


Observe the Working Model: There is a working model of a leaf electroscope at the front of the class. Charge up the golf tube with the plastic bag and observe the aluminum foil in the soda bottle when you bring it near.


Build Your Own: Stick a bare piece of wire through the lid and form a small hook in one end, or unfold a paper clip in the middle so it looks like an "S". Cut two strips of aluminum foil about 3 mm by 1.0 cm. Place the strips on the hook, or the big loop of the paper clip. If you used wire, loop the upper end down through the cap and wrap it around the other end of the hook so that no sharp end of the wire is exposed. If you used a paper clip bend the ends so that they make loops . Put the lid on the bottle. Push the top (little) loop down halfway through the hole in the lid. If the loop is not pushed down this will lead to a pointed end being loose and charges like to jump to and from points. (Think about what a lightening rod lo oks like)


Test It: Charge up a golf tube and make sure the aluminum foils deflect when the golf tube is brought near the top loop. Make sure the foil does not touch the glass when it deflects. If the leaves don't deflect, make sure they ar e free to move. If the foil touches the glass, trim the foils so that they are too short to do so.


How does an electroscope detect charge? What happens inside it?


What Do the Parts Do? The glass bottle protects the foil leaves from air currents. Aluminum foil is used because it is light so it moves even with small electrostatic forces.


Charge the Electroscope with the Electrophorus: Charge up your electrophorus. Slowly bring it up to the loop of the electroscope.[holding it by the cup] What can you observe?


Is charge transferred in the process? How do you know?


Now touch the knob of the electroscope. What happens?


The Electroscope and the Golf Tube: Charge up the golf tube and bring it near the loop without touching the loop, then move the golf tube back away. Record your observations.



How would you explain this? What happens to the charges in the electroscope?


Is charge transferred from the golf tube to the electroscope? Yes / No (circle one).


Bring a charged golf tube near the loop. This time touch the loop with your finger while the golf tube is near the loop. Remove your finger and then the tube. (Pay special attention to the sequence of performing events!) Record your observations.


Discharge your electroscope (touch it with your hand). Charge the golf tube and bring it near the loop. Touch the loop with your finger then remove the golf tube. Finally, remove your finger. Record your observations.


Just like in case of the electrophorus, touching is a crucial point. Whether you touch the electroscope or not, whether you remove your finger before removing the golf tube or not makes a big difference. Try to explain this behavior. It is the same process as in case of the electrophorus. What happens to the charges in two the different cases?



Hints and explanations


- Do not touch the metal part of the charged electrophorus when you lift it up.

- Discharge the electroscopes (by touching) before each experiment.

- To get the charging by induction to work right you need to make sure the bottle itself is not charged. (Some charge gets deposited on the glass.) Do this like you did with the charged rods, carefully rubbing your hands over the sides. If you are stil l having problems, have someone hold the jar during the experiment with one hand, while holding on to something grounded (like a water faucet) with the other. The amounts of charge we are moving here are tiny, but the device is very sensitive.

If it still doesn't work right, take off the lid, and "pant" into the bottle then let it dry. The moisture in your breath will absorb the charge on the inner surface of the bottle and remove it as it evaporates

Remember when we used a charged rod to attract the soda can? Well we could have actually ended up with two charged cans instead of just separating the charge on one object if we had done this:



The + charge on can 1 is there because the - charge on the tube attracts it. The - charge on can 2 is there because charge is conserved and nothing is holding it, and the char ge on the tube is repelling it. Now, suppose you ground the two cans by touching them, then the charge which is not attracted to the tube escapes into your body.

Now, if you take away your finger BEFORE you remove the golf tube the spheres will have a net + charge. This is also the way you put a charge opposite to that on the tube on a single sphere.

Charging an Electrophorus


To get a lot of charge, we use the same technique on a different "geometry" of conductors. Geometry here means that the conductors are of a different size and shape. To get a lot of charge, we want the insulator with the charge o n it as close to the metal as we can get, so we use flat plastic and a flat conductor like:



I also put an insulating handle on the metal, otherwise as soon as I picked it up I would have grounded it. The way I finally put it together was:

To charge the electrophorus (which is what the do-hickey is called) I rub the cutting board with the oven bag. I found that the oven bag which I used to charge the cutting board repels the oven bag I used to charge the golf tube, so the cutting board and the golf tube must be charged the same. I put the pie pan on the charged plastic and grounded it with my finger. I felt a spark when I grounded it. I picked up the plate (by the handle) and tried to ground it again, and again I felt a s park.


Here's a picture of what happened:




Diagram of an electroscope:



The aluminum foil is loose on the loop of copper wire, so when you fill the electroscope with charge by touching the bolt with a charged rod, the charge goes equally onto each of the two aluminum foil strips and forces them apart (reason: like charges repel). The purpose of the jar is to keep the leaves from blowing around and to help insulate the leaves from electric fields. I cheated when I made mine because the physics department had a few broken electroscopes, so I got the ja r and bolt for free. When I had to make more, I found a paper clip worked well instead of the bolt. I punch a small hole in the top of a plastic lid for a glass jar, unbend the clip in the middle and poke it through so one loop is inside the jar holding t he leaves and the other loop is outside to touch. Be careful to have the ends of the loops come back and touch the center, because charge really likes to bleed off of sharp ends.


Make sure you have a working electroscope built for the students to observe before they try to build there own. Just before class, make sure the leaves are moving freely and that there is no charge built up on the glass.


After I built the electroscope, I charged up a golf tube and observed the following:




Charging the Electroscope by Induction


Why do the leaves of the electroscope spread apart before the tube touches the bolt? It is the same process as we saw in the electrophorus. The positive charges are attracted to the golf tube, and negative charges are pushed into the leaves. This gives them both a negative charge so they repel, even though the device as a whole is neutral. However, if I touch it while the golf tube is present, I can put a charge on it, just like the electrophorus:


1) Remove the finger first, then the tube and you have a charged electroscope.

2) Touch the charged electroscope with the finger and the charge escapes into the body and the electroscope is grounded.



- Humidity can, once again, cause problems.

- The notion of electric field is not mentioned, and should only be if you are discussing it in class. However, students get a pretty good understanding of its properties. Sparks are being explained as charges jumping from one object to another. What actually happens is the dielectric breakdown of air. This means that if the strength of an electric field reaches a certain limit value (E=3* 106N/C) then the air gets ionized and becomes a conductor. An enormous amount of charge moves very quickly along a narrow ionized path from one object to another (usually to the ground). The spark is the same physical phenomenon as the lightning. The light is due to the air ions colliding with each other, getting excited and getting rid of their energy in the form of light. The sound, just like the thunder, is due to the rapid warming and extension of air.