Activity 2...Sparks and the Pie Pan

Corrections for Science Scope "More Electrostatic Explorations" article


In the last activity you learned why your sweater attracts your hair. Now youíll find out about the sparks that your sweater makes when you take it off in the dark.

We have charged objects by rubbing them together. Now youíll get to know a way to charge an object in a different way. Youíll also get familiar with a device for detecting charge.



You will get to further your understanding of induction. Youíll experience what grounding means. You will also discover how to charge an object by induction and grounding. You will get to know that a spark is actually cha rge transfer between 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.




What to do?


Play with the electrophorus


Take the aluminum pie pan and tape the plastic cup to the center of the pan. The simple device you just built is called an electrophorus. Charge the cutting board with the plastic bag. Test if itís truly charged. How do you t est 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 donít 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.


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?




Letís 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?

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 type or mixed all together.



Play with the electroscope


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. Letís 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 th e 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 the y make loops. Put the lid on the bottle. Push the top (little) loop down halfway through the hole in the lid.


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.


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. 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 behav ior. It is almost the same process as in case of the electrophorus. What happens to the charges in two the different cases?