V1-1 WILL THAT BE CASH OR "CHARGE"?

Exploration

Problem

How many types of electrical charge exist and how do they interact with each other?

Sheet of colored paper, small sheet of scrap paper, scraps of cloth, aluminum foil and styrofoam, comb, salt and pepper, transparent tape, and a ring stand with a non-metal crossbar.

Part A

1. Make at least ten very, very small pieces of white paper from the scrap paper. Place these on the colored paper. Then sprinkle a small amount of salt and pepper on the colored paper. Align the teeth of the comb parallel to the desk top and slowly move close to the paper, the salt, and the pepper. Observe what happens and record your observations.

2. Now vigorously rub the comb back and forth several times with a piece of cloth. Again, slowly move the comb towards the paper, salt and pepper. Once more observe what happens and record your observations.

   Repeat with a different type of cloth or other material. Record your observations.

   Suggest and try other variables beside cloth that might be changed. See your teacher for additional materials. Clean up the materials.

3. Predict what will happen if you substitute tiny bits of aluminum foil and styrofoam for the salt and pepper. Now test your prediction. Keep the colored paper and the small pieces of white scrap paper. You will need these for Part B.

Part A

1. What happened when you brought the rubbed comb close to the paper, salt and pepper? What explanations can you offer for why this happened? Were all three substances affected equally?

2. What combinations of cloth and combs or other objects or materials seemed to produce the greatest effects?

3. You actually created an electrical charge on the combs that were rubbed. In light of what you know (?) about atomic structure, explain whether electrons or protons were being transferred when the comb was rubbed with cloth or fur.

Part B

1. Stick a 20 cm piece of transparent tape to your desk top with abaut two centimeters hanging loose over the edge. Use your thumb or a finger to rub the tape against the desk top so it is stuck very well. Pull up on the loose end and carefully lift the tape away from the top of your desk. Touching only the ends of the tape with your fingers, align the tape parallel to the table surface. Slowly bring the non-sticky side of the tape close to the tiny scraps of paper. Record your observations.

2. Charge the tape as you did before. Suspend the tape from a plastic, glass or wood rod. (You could hang it over the edge of your desk.) Next, charge a similar length of tape in the same way and then bring the non-sticky side close to the non-sticky side of the suspended length of tape. Record your observations and why you think this happened. Suspend the second length of tape a few centimeters away from the first. Charge a third piece of tape in the same manner as the others and observe the reactions between this tape and the two suspended tapes. Again record your observations and possible explanation of the reactions. Charge a comb from part 1 and bring it near the suspended strips and observe and record what now happens.

3. Discard the strips of tape. Charge a new length and suspend it. Stick another strip down on your desk top and stick another piece of tape on top of it. Rub them well against the desk top. Peel this pair, still stuck together, away from your desk. Run the non-sticky side of the duo over your lips or a water pipe. Bring the pair close to the suspended strip. What happens?

4. Carefully and slowly pull the two tapes apart. What happens as you separate these? Hold one tape in each hand and slowly bring the two non-sticky sides close to each other. Now bring the tapes, one at a time, near the suspended strip and observe what happens in each case.

5. Suspend each of these strips and bring a charged comb near to each and once more, and record your observations.

6. Tear paper into a strip approximately the same size and shape as the hanging tapes. Bring the uncharged paper strip close to each of the hanging strips and record your observations.

Part B

1. Why do you think the charged comb affected the original suspended strip as it did?

2. What happened when you brought the two separated tapes close to each other? What explanations can you offer for this?

3. How many types of charge were you working with in this activity? How do you know?

4. If a third type of charge existed, how would it affect the two oppositely charged strips in this activity?

5. Why do you think the charged comb affected the two suspended tapes as it did?

6. How would you explain the attraction or repulsion between each of the suspended tapes and the uncharged paper strip?

7. How would you explain the fact that a charged comb can attract an uncharged object like the paper bits, salt and pepper?

Teacher Notes

VI-1 WILL THAT BE CASH OR "CHARGE"?

Exploration

            Lab setup        easy           moderate     difficult
            Calculations     easy           moderate     difficult
            Reliability      excellent      good         fair
            Interest         excellent      good         fair
            Lab time         -1 class       1 class      +1 class
            Process Skill    A     B     C     D     E     F
            Reasoning        1     2     3     4     5     

	       

Charging by induction rqedistributes the charge on a neutral object. This is done by creating an opposite charge on the surface of an uncharged object that is nearest a charged object. This charge is temporary unless a path is provided so charge can leave or enter a surface that is charged by induction. Charging by conduction results in an uncharged object receiving the same charge as the charged object. A spark from a charged object can be considered as contact between the charged object and the object being charged. The instantaneously ionized air provides a path for the charge transfer similar to direct contact.

Students will likely have an understanding of "opposite charges attract" and "likes charges repel" but may have difficulty explaining the charging by induction. Charging by induction is observed in this exploration when charged objects are able to attract uncharged objects. Students will likely modify their present knowledge of electrostatics to include more than two possible charges to explain how oppositely charged pieces of tape are both attracted to uncharged objects. Some students may insist that only one charge actually exists - the excess or deficiency of electrons compared to the protons in the nucleus. Encourage a range of expressions that will provide explanations for observations of this lab. Virtually any student explanation will contain half-truths that can be helpful in making a smooth transition into the concept development of electrostatics. It would be most appropriate to explain that the assumed negative charge of the electron has meaning only when considered as identical in magnitude and opposite in charge to the proton that is locked in the nucleus.

Part A

1. The paper, salt, pepper, styrofoam, and the bits of aluminum foil were all charged by induction as the comb approached and touched the surfaces. The closest surface of the uncharged object would have a net charge that is opposite to the charged comb, and the side furthest from the charged comb would acquire the same magnitude but opposite charge as the near surface. The near surface is oppositely charged to the comb and results in a net force of attraction toward the comb because its closer distance to the comb provides a larger force than the force of repulsion from the far side of the surface. Sometimes uncharged particles are attracted, touch and then are repelled by the charged comb. Initially the particle was attracted because it was charged by induction. After contact charge migrated from the comb to the materials in contact with the comb resulting in repulsion between the materials and the comb.

2. Rubbing wool over the comb and observing the effect on paper may be the most dramatic, but the other materials are capable of producing a significant charge. When two surfaces are in close contact with each other, the material with the higher affinity for electrons will strip some of the outer electrons from the other material.

3. Electrons can be transferred from one atom to another in chemical reactions or in mechanical operations like rubbing two materials together. In this lab the comb likely is imparted a negative charge by stripping electrons from the cloth or fur. Protons can be added or subtracted from the nucleus only in nuclear reactions.

Part B

1. 1. The strip of tape and charged comb repel each other because they have the same charge.

2. The two separated pieces of tape are attracted to each other, so they have opposite charges. The top tape is positively charged while the bottom tape, comb and other single tapes are negatively charged.

3. Electrons removed from a surface results in a positive charge. Electrons added to a surface results in a negative charge. When rubbed with fur or cloth, plastic and rubber typically have a higher affinity for electrons so the comb is gaining electrons for a negative charge. No charge exists in a given material when atoms exist with equal numbers of protons and electrons. There were therefore only two different charges: positive and negative.

4. If a third charge existed it would attract both positive and negative charges.

5. After separating the tapes, the top is positively charged and the bottom is negatively charged. The negatively charged comb will then attract one and repel the other.

6. As explained above, uncharged objects can be charged inductively with the surface nearest the charging object acquiring an opposite charge and which results in an attractive force between the objects. This explains how an uncharged piece of paper could be attracted to both positively and negatively charged tape strips.