University of Virginia
Physics Department

## Balloon Electroscope

A Physical Science Activity

2003 Virginia SOLs

• PS.1
• PS.11

Objectives

Students will

• show and understand that like charges repel each other and opposite charges attract;
• apply information about static electricity to identify missing charges in static electricity problems.

Motivation for Learning

Demonstration of an Electroscope

The teacher should have a basic electroscope that is assembled and ready when the students come in.

Materials
 Glue Flexible Straw Thin aluminum foil Styrofoam cup Thread Fake fur

Procedure

1. Apply glue to a small strip of aluminum foil and wrap a single layer around a straw.
2. Cut the foiled straw into one centimeter long pieces and place a piece of thread (about 15 cm long) between the straw and the foil before the glue dries. You now have a pith-ball! (The thread can also be added later).
3. Tape the straw onto an inverted styrofoam cup with the flexible end of the straw facing up. Bend the straw so that the short end is horizontal and about 7 cm above the cup.
4. Cut several slits into the straw and hang a pith-ball by slipping the string into the slit of the straw. You now have an electroscope!

• What do you think will happen when I rub another Styrofoam cup with the fur and bring it near the pith-ball?
• What do you think will happen when I touch the cup to the pith ball?

You may want to hang several pith-balls from the straw and repeat part 3.

• What do you think will happen if I touch one pith ball and not the other?

Background Information

### Student Activity

Materials

• 2 identical balloons per group of students
• Fur or Wool cloth for each group
• Water sprayer per 2 groups

Procedure

1. Blow-up the balloons and tie thread to the ends of each balloon.
2. Tie the balloons together.
3. Have the students hold the uncharged balloons by the thread and move balloons together. Record their observation and make a sketch. (Figure A).
4. Rub each balloon with the wool. Move one balloon near the other but do not allow them to touch. (Balloons will not touch like Figure B). Hold by thread.
5. While the balloons are repelling each other, have the students gently mist the balloons with water. Record their observations and make a sketch. (They will fall back against together like Figure A).

Data Sheet

 Why did the balloons repel each other after they were rubbed with the fur/wool?   Why did the balloons fall back towards each other after they were sprayed with water?   Are there other ways besides water that we could use to have the balloons come back together?   During which time of the year would it be best to do experiments using static electricity?   What effect does damp weather have on electrical charges?

Explanation: After rubbing the balloons with the fur/wool, the balloons have the same charge and will repel each other. Using water droplets on the balloons carries the charge away from the balloons which leaves them neutral. As the balloons become neutral, they will fall back together but will not be attracted to each other. Another way to neutralize the balloons is to touch them with a moist or damp hand. One needs to remember that only where the balloons are wet is where the charges are carried away. The part of the balloon that is not wet will still attract to the other balloon where it is not wet.

Extensions

Let the students work in groups and have the students make a new design on paper or actually build an electroscope. They can use different charged objects to get their electroscopes to work. If they are designing on paper or building one, I would have them write down the purpose of the different parts (bottle, stopper, wire, foil, and charged object). If building, let the students try out different things to see what works and what doesn't.

Students with Special Needs

Adapt as needed for children who have physical disabilities. These children may need help blowing up the balloons, tying the thread to the balloon, and rubbing the balloon with the fur or wool. I would ask your resource teacher for additional help if you need it.

Click here for further information on laboratories with students with special needs.

Assessment

Data sheet to be completed during the laboratory.