Electrostatics

Introduction

Generate electrostatic charges on balloons, then compare the negatively charged balloons with charged rods.

Concepts

• Static electricity
• Negative charge/positive charge
• Electron transfer

Prelab Preparation

1. Inflate two balloons and attach equal lengths of string to each balloon.
2. Suspend both balloons (at the same height) so they are near each other but not touching.

Procedure

1. Rub both balloons with the fur friction pad, grounding the pad after charging on the first balloon. (The balloons will obtain a net negative charge and the fur will obtain a net positive charge.)
2. Bring the two balloons close together and notice that they repel each other. Bring the fur close to one of the balloons and notice the attraction.
3. Rub the glass rod with the silk friction pad. Hold the glass rod next to one of the balloons and observe.
4. Rub the plastic rod with the flannel friction pad. Hold the plastic rod next to one of the balloons and observe.
5. Rub the rubber rod with the fur friction pad. Hold the rubber rod next to one of the balloons and observe.
6. Experiment with different combinations of rods and friction pads.

Teacher Tips

• Static electricity experiments always work best on a dry day. Lower humidity days are better than high humidity days. Airconditioned air or heated winter air tends to be drier and thus, more conducive for electrostatic demonstrations.

• If the area surrounding the balloons is highly charged, the demonstrations may not work well. Wipe down a highly charged area with a damp cloth and dry the area with a towel.
• It may be necessary to recharge the balloons with the fur friction pad if they become grounded and unresponsive during the demonstrations.

Discussion

Static electricity is a stationary electric charge. Its effects range from lightning to copy machines to the shocks you receive on a dry winter day. Static electricity is caused by an accumulation and discharge of electric charge. Lightning occurs when clouds accumulate a charge as they move through the air. If the electric charge is great enough, the electricity will be discharged to the ground, or another cloud, with an opposite charge (see Figure 1).

In copy machines, a photoconductive surface is given an electrostatic charge. A discharge occurs when the charged surface is illuminated by projecting an image upon it. White backgrounds dissipate the charge, while black backgrounds, such as lettering, shields the surface from the illuminator so the surface remains charged. An oppositely charged toner powder is dusted over the plate where it adheres to the charged areas. A sheet of paper is then placed over the plate and the toner adheres to the paper, creating the desired image.

Static electricity is also used to combat pollution in scrubbers of industrial smokestacks. Charged particles in the gaseous discharge are attracted to oppositely charged plates and removed from the “smoke.”

References

Bronowski, J. Doubleday Pictorial Library of Science, Doubleday: New York; 1960.

Heiserman, D. L. Understanding Electricity and Electronic Circuits, Howard W. Sams: Indianapolis, 1987; Chapter 1.