Electrical Nature of Matter

Description

Charges are produced by rubbing plastic strips. The interaction of the charges are observed using pith balls, electroscopes, and the strips themselves. The interaction of scraps of paper with a pocket comb pulled through hair is observed.

Go to Top

1. Rub two strips of polyvinyl chloride with a dry paper towel. Bring the strips together, and note the interaction. Charge two acetate strips with a dry paper towel and note their interaction.
2. Rub a strip of cellulose acetate and a strip of polyvinyl chloride with a dry paper towel. Bring them near one another, and note the interaction.
3. Using two support stands, clamp the wooden dowel in a horizontal position. Hang the pith balls from the wooden dowel.
4. Rub a vinyl chloride strip with a paper towel. Charge pith balls by touching the balls with the charged vinyl chloride. Observe. Discharge the balls by touching them.
5. Rub the cellulose acetate with paper. Charge the pith balls by touching them with the rubbed cellulose acetate. Observe. Discharge the balls by touching them.
6. Charge the pith balls with the rubbed cellulose acetate. Charge the vinyl chloride. Bring the charged vinyl chloride close to the charged pith balls. Observe. Charge the cellulose acetate and bring it close to the pith balls. Observe.
7. Using an electroscope, charge vinyl chloride and touch it to the top of the electroscope. Observe. Charge cellulose acetate and bring it near the top of the electroscope. Observe.
8. Repeat the above procedure charging the electroscope with the cellulose acetate and then bringing the charged vinyl chloride close to the top of the electroscope. Observe. To discharge the electroscope, touch the top with your finger.
9. Tear some paper into small pieces. Comb your hair.
10. Bring the comb near the pieces of paper. Observe.

Go to Top

Name _____________________________ Class _______

Teacher______________________________

DoChem 025 Electrical Nature of Matter

1. Watch the computer movie.
2. Why is the paper attracted to the comb?
3. Construct a model to explain the charged particles. Be sure that the model reflects the evidence observed. Attractive and/or repulsive forces can be obtained by rubbing materials. These phenomena are interpreted in terms of charged particles.

Go to Top

Purpose

• To provide evidence that matter is composed of charged particles.
• To demonstrate some effects of electrical charges.

Go to Top

• 2 pith balls with nylon thread
• 2 support stands
• 2 single-buret clamps
• 30 cm length wooden dowel
• paper towels
• 15 polyvinyl chloride strips, 2.5 x 20 cm
• 15 cellulose acetate strips, 2.5 x 20 cm
• 1 electroscope
• 1 pocket comb
• 12 small pieces of paper

Go to Top

• Polyvinyl chloride strips can be obtained by cutting photograph protectors like those used in photo albums. Overhead transparency sheets are a good source of cellulose acetate. A bread wrapper on glass also produces a good opposite charge to acetate.
• In place of pith balls, styrofoam packing material may be hung from a wooden dowel using nylon thread.
• Atmospheric conditions (dampness, rain, etc.), dirt, and grease may cause charges to leak off materials. This is a dry day experiment.

Go to Top

Teacher preparation: 15-20 minutes

Presentation: 25-30 minutes

Go to Top

There are no unusual hazards in this experiment.

Go to Top

No special precautions are required in this experiment. Follow routine laboratory precautions.

Go to Top

Save the plastic strips in labeled envelopes for reuse in future classes.

Go to Top

• Demonstrate the interaction of the pith balls with like and unlike charges. See if students can deduce the existence of different types of charges. See if they can deduce that like charges repel each other and that unlike charges attract each other.
• An alternative, student-oriented exercise, is to provide each student in the class with one strip of plastic -- half with vinyl chloride, and the other half with cellulose acetate. Have them divide themselves into two groups on the basis of how the rubbed strips interact with one another.
• Have students deduce why the leaves repel and/or attract in an electroscope interacting with charged materials. Compare the reaction of the electroscope to that of the pith balls.

Go to Top

Presentation Question:

• Why is the paper attracted to the comb?
  • A charged object pulls opposite charge in adjacent material toward it and pushes like charge away from it. This "induces" a dipole which is attracted to the charge. A rubbed balloon will be attracted to nearly everything in a classroom (and stick to it) except for a similarly rubbed balloon.

Go to Top

Have the students construct a model to explain the charged particles. Be sure that the model reflects the evidence observed. Attractive and/or repulsive forces can be obtained by rubbing materials. These phenomena are interpreted in terms of charged particles.

Go to Top

• Cling free products were developed to counter the annoying effects of electrostatic charges when removing clothing from a clothes dryer.
• Some photo albums that advertise no glue involve using plastic sheets that, when rubbed, take on opposite charges. As a result they stick tightly to one another.

Go to Top

• Charles Coulomb (1736-1806) experimentally derived the magnitude of force between two charged objects. This is known as Coulomb's Law:

  F=kq₁q₂/r²

  (F = force, q₁ and q₂= charges on 2 objects, r = the distance between objects, and k = proportionality constant)

• Coulomb's Law states that the electrostatic force between two objects is directly proportional to the product of the charges and inversely proportional to the square of the distance between them.
• This is a good time to have the students solve some problems involving Coulomb's Law.

Go to Top

• charge
• positive charge
• negative charge
• Coulomb's Law
• opposite charge
• like charge
• force
• repel
• attract

Go to Top