Objective:

Develop a basic understanding of electric fields by making and using a simple electroscope.

Materials per group of four students:

• Aluminum foil, ±10 cm square
• Scissors (not needed if teacher does prep work)
• Paper clip, metal, regular size
• Clay
• Clear plastic cup (must be taller than 4 cm)
• Balloon, small
• Wool fabric

Optional:

• Plastic - 10 cm x 10 cm
• Flannel cloth
• Paper - 10 cm x 10 cm
• Plexiglas rod
• Silk cloth
• Paper clip, plastic

Time:

40-60 minutes should be enough time to complete the
initial activity and discuss the results.

Preparation:

• To save class time and to facilitate the construction and use of the electroscope, you might want to cut out the foil strips and put the holes near the very top of each strip.
• Make extra strips because a few will be damaged.
• Drill a small hole into the center of the bottom of the cup before beginning the activity when working with elementary students.

Procedure:

• Cut two strips of aluminum foil 4 cm long and 0.5 cm wide for the “leaves” of the electroscope.
• Holding the strips horizontally, make the hole along the horizontal midline about 0.5 cm from the left-hand edge using the tip of a straightened paper clip.
• Partially straighten a paper clip into the shape of a “J.” Hang the two aluminum strips on the curve of the clip.
• Working from the inside of the cup, push the straight end of the clip through the hole in the cup, so that it hangs as shown in the illustration.
• Use a small piece of clay to secure the clip at the hole opening.
• Crumple a small piece of aluminum foil into a ball. Add this ball to the top of the clip that sticks up through the cup.
• Rub a scrap of wool fabric across an inflated balloon. Now pass the balloon near the crumpled aluminum. Observe the behavior of the aluminum strips.

Charges on the electroscope can accumulate. Before starting any new experiments, always discharge the electroscope first by touching its metal ball with your finger.

Questions:

• What happens when the balloon approaches the electroscope? The foil strips of the electroscope spread apart.

• What happens to the foil strips when the charged balloon is taken away? The foil strips of the electroscope move back together. This will occur only if the electroscope has not been touched or if no spark has jumped between the balloon and electroscope.

• When the foil strips are apart, are the kinds of charges on the strips the same or different? Charges of the same kind repel one another. Therefore, when the strips are spread apart, the kinds of charges on each strip are the same.

• How does the distance to the balloon affect the behavior of the aluminum strips?

The closer the balloon, the more active the strip.

• What is the relationship between the distance the “leaves” spread apart and the electrical charge?

The greater the charge, the farther apart the spread will be.

• What happens if you do not rub the balloon?

Nothing.

• Why did you have to rub the balloon?

So it would become electrically charged.

• Will the detection of the presence of electrostatic charges be possible without the plastic cup?

The plastic cup is needed to assure that air currents do not disturb the sensitive foils.

• Do you think that the balloon’s electric field can penetrate paper, aluminum foil or plastic? Why? Test your hypothesis by placing a piece of paper between the charged balloon and the aluminum ball.

What is happening?

In this activity, students bring charged objects near the aluminum ball and observe that the foil strips move apart. The strips must have the same type of charge on them (both + or both -) because like charges repel one another.

When the negatively charged balloon is brought near the ball, the electrons (negative charges) on the ball are repelled by the negative charges on the balloon. Some electrons move from the ball down onto the foil strips. With more negative than positive charges on the strips, the net charge on each strip is negative. Because like charges repel, the negatively charged strips repel one another.

In a similar fashion, when a positively charged Plexiglas rod is brought near the aluminum ball, the electrons on the ball and the strips are attracted to the positive charges on the Plexiglas rod. This results in more positive charges than negative charges on the strips. The strips each have a net positive charge. Because like charges repel, the positively charged strips repel one another.

Further investigations:

• Does rubbing the balloon against silk or flannel change the results. Chart the results.
• What would happen if you used a Plexiglas rod instead of a balloon? Chart your results.
• Try hanging the metal paper clip from different objects (paper or plastic). What happens? Do these objects help or hinder the detection of charges?
• What happens when you use a plastic paper clip?
• Redesign the electroscope to make it easier to detect and see the effects of electrical charges.
• Can you design a portable electroscope, which can be used when held in different positions?
• Find out more about how the electric phenomena is measured in living things (EKG, EEG, for example.)

Interesting Data:

Sharks use their senses to help them navigate and detect prey. The shark’s electromagnetic sense is an interesting adaptation; sensory pores detect weak electric fields equivalent to one AA battery. Sensing the bioelectric activity produced by muscle contractions in their prey helps sharks find their dinner. The sense is well developed in hammerhead sharks, which are said to use their heads like metal detectors.

Supplemental materials from the PUD:

Resources:

• Graf, Rudolf. 1964. Safe and Simple Electrical Experiments. Dover Publications, pp. 11-15, 18-19.
• National Science Teachers Association (1993) Taking Charge: An Introduction to Electricity. Pp., 49-61.
• Scientific American Frontiers.1996. Teachers’ guide.

Contact:

Jenni Lamarca
education@snopud.com
425-783-8292
(M-F, 8am to 5pm)