You ll get a charge out of this

Transcription

1 HPP Activity 63v1 You ll get a charge out of this A pacemaker is installed in a human being to enable their heart to maintain a regular beat. Electrodes are placed on patients everyday that allow measurements of heart, brain, and muscle activity. Cochlear implants stimulate nerves in the cochlea with electrical signals allowing the recipient to hear. Muscles of quadriplegics are stimulated with electrical signals to keep them from atrophying. Magnets are placed on various parts of the body because of claims that they ease pain or promote healing or delay damage as in the case of macular degeneration. Electronic devices are being developed to allow the blind to see. Do these devices work and if so, how? Can we describe the basis for these electronic devices in terms of forces or maybe work and energy? Exploration GE Place a 15 cm or so long strip of tape on the lab table with the sticky side down, with the end of each tape curled over to make a non-stick handle. Vigorously peel your tape off the table. Repeat the procedure with another piece of tape. Bring the non-sticky side of one of the two tapes toward the non-sticky side of the second tape. What happens as the tapes get closer together? Place two strips of tape on the table sticky side down with a non-stick handle and label them "B" for bottom. Press another tape (with a non-stick handle) on top of each of the "B" pieces; label these strips "T" for top. Pull each pair of strips off the table. Then vigorously pull the top and bottom strips apart. 2. Describe the interaction (a force!) between two top strips. 3. Describe the interaction between two bottom strips. 4. Describe the interaction between a top and a bottom strip 5. Slowly move each pair of tapes closer together. How does the distance between the tapes, affect the interaction between them? i.e. Does the interaction increase, decrease, or stay the same? We say that the tapes are charged. This is clearly something acquired by the tapes or changed in the tapes when you pull them from the table. The charges interact, i.e. exert forces on each other from a distance! Two charged objects need not touch to experience an interaction. We describe the interactions between charged objects with the words attract and repel. Supported in part by NSF-CCLI Program under grants DUE # and DUE #

2 HPP Activity 63v1 2 Invention Based on experiments similar to the preceding ones or others that involve rubbing dissimilar objects together suggest the two charge model of electrical interactions. Matter can have the property of electric charge, and there are two kinds of charge, which we call positive and negative, thanks to Ben Franklin. If a sample of matter has equal amounts of positive and negative charge then it is electrically neutral. When two samples of dissimilar materials are rubbed together, negative charge on one sample travels to the other sample leaving them oppositely charged. The two charge model also says that the charges interact according to the following rules: 1. Like charges repel (exert repulsive forces). 2. Unlike charges attract (exert attractive forces). Application GE The figure below shows two tapes. Draw the net electric force vector on each tape. 2. What kind of charge could be on the tapes? 3. The figure below shows two tapes.

3 HPP Activity 63v1 3 Does there appear to be an electric force on either tape? 4. Assuming the tapes are neutral, does this mean there is no electric charge in the tape? GE Charge up a balloon by rubbing it on your shirt. Suppose you bring the balloon close to a neutral wall or whiteboard. What do you think you will observe? 2. Perform the experiment and describe what you see. 3. Can you explain your observations in terms of electric charge? Invention Materials differ in how well they allow electric charge to move in them. Generally we classify a material as a conductor if it allows easy movement of charge. Usually conductors have lots of electrons in them that are free to move around, even when the object is neutral. If charge cannot move in the material then it is called an insulator. While electric charge cannot roam freely around an insulator it can move small distances, on the order of the interatomic distance. This leads to the polarization phenomenon. Application Imagine the following scenario, or set it up if possible.

4 HPP Activity 63v1 4 Two metal balls are touching each other. A charged rod is brought near the left one. While the rod is near, the right ball is taken away. Finally the rod is taken away. At the end of this procedure, the lefthand metal ball has a negative charge. GE Describe what you think is happening to charge in the metal balls when the positive rod is brought nearby. 2. Why is the left ball negatively charged at the end?

5 HPP Activity 63v1 5 This is called charging by induction.