Modeling Electroscope Behavior

This lab is designed to align with AAOT science outcome #1: Gather, comprehend, and communicate scientific and technical information in order to explore ideas, models, and solutions and generate further questions.

Materials

• digital device with spreadsheet program
• digital device with internet access

ObjectiveS

1. Understand the basic purpose and operation of an electroscope.
2. Apply the static charge model (single mobile charge carrier) to analyze the observed behavior of an electroscope.
3. Apply the static charge model to predict the behavior of an electroscope.
4. Experimentally test the static charge model by comparing predicted and observed behavior of an electroscope.

Methods

Experimental Methods

The electroscope is designed to indicate the presence of charged objects. The following video shows a series of electroscope charging experiments being performed. As seen in the following video, the electroscope used in for this lab consists of a flat metal plate directly connected to a vertical metal cradle that holds a metal needle that is free to rotate. These conducting components are insulated from the metal frame by two plastic washers. The conducting components can be grounded by touching the plate with a grounding rod or simply with you hand.

Modeling

The Laws of Physics aren’t different from one time to the next, or from one location to the next. Therefore, each of our explanations for the behaviors seen in the video must be consistent with one another and with our fundamental understanding of electric charge:

• In everyday conditions positively charged ____________ are bound in the nucleus of atoms, so any net charge is caused by the gain or loss of  negatively charged ____________.
• Conductors, even neutral ones, contain ______________ that are free to move throughout the conductor.
• When conductors acquire additional ______________, they are also free to move throughout the conductor.
• Insulators may lose or acquire _____________, but they are not free to move around.
• Like charges repel and opposite charges attract.

First Experiment

1) In the first experiment a balloon was rubbed on a shirt and becomes charged according to a process known as tr____electric charging.  Examining the relative electron affinity of rubber and cotton (you may need to look this up), we can assume the balloon acquires a __________ charge.

2) When the charged balloon is brought near the electroscope we see the needle move away from the cradle on both ends.  Draw a basic diagram of the electroscope and balloon that indicates any charge on the balloon and any induced charge on the plate, cradle and needle. Use your diagram to support a written explanation for why the needle moves away from the cradle when the balloon is brought nearby. Be sure to cite any and all sources used. [Hint: no net charge was added to the electroscope, but individual parts of the electroscope did become temporarily charged due to movement of free electrons in the metal parts.]

3) The temporary charging of the electroscope parts you described above is called charging by ____________. (Provide citation below.)

Second Experiment

4) The second experiment in the video shows the balloon rubbed on the plate of the electroscope. When the balloon is removed the we see that the needle remains separated from the cradle. What type of charge is left on the balloon?  What type of charge is left on the plate? [Hint: You will need to look up where a balloons and aluminum fall in a triboelectric series]. Be sure to cite any and all sources used.

5) Draw a basic diagram of the electroscope and balloon that indicates any charge on the balloon and any induced charge on the plate, cradle and needle. Use your diagram to support a written explanation for why the needle stays away from the cradle when the balloon is brought nearby. Be sure to cite any and all sources used.

Third Experiment

6) The third experiment in the video shows a neutral metal pie pan brought near the electroscope after it has been charged as described above. We see that the separation of the needle and cradle decreases, as the pie pan nears and increases again as the pan is removed.  Draw a basic diagram of the electroscope and balloon that indicates any charge on the balloon and any induced charge on the plate, cradle and needle. Use your diagram to support a written explanation for why the needle moved back toward the cradle when the neutral pie pan is brought nearby. Be sure to cite any and all sources used.

Fourth Experiment

7) The fourth experiment in the video shows a charged balloon brought near the electroscope while it has been grounded (by touching the plate). We see that needle does not initially move as it did in the first experiment when the electroscope was not grounded. Explain why. Be sure to cite any and all sources used.

8) Continuing in the fourth experiment, the electroscope is ungrounded while the charged balloon is still nearby and then the balloon is removed. We see that the electroscope needle remains separated from the cradle. Draw a basic diagram of the electroscope and balloon that indicates any charge on the balloon and any induced charge on the plate, cradle and needle. Use your diagram to support a written explanation for how the electroscope acquired a charge in this case. Be sure to cite any and all sources used.

9) The charging of the electroscope as you described above is called charging by ____________. (Provide citation below.)

FURTHER QUESTIONS

Fifth Experiment

Our modeling of the electroscope behavior was based on very basic concepts, so we should be able to apply those same concepts to model the electrostatic behavior of other systems, such as the Van de Graff Generator and pie plates seen in the fifth experiment. The Van de Graff Generator builds up a large net charge on the dome. You can watch the video below to see how:

10) Draw a basic diagram of the Van de Graff Generator dome and pie plates that indicates any charge on the dome and plates. Use your diagram to support a written explanation for the behavior of the pie plates. Be sure to cite any and all sources used.