Excellent responses; very good work.
Pick one of the pieces and determine whether the charged pipe attracts or repels it. It repels.
· Predict whether this piece of tape will attract or repel each of the hanging pieces. Then observe, and note your results. They repel each other
· Predict whether the other piece of tape just stripped will be attracted to or repelled from the charged pipe, and from each of a hanging pieces, then test your prediction and note your results. The strip of tape and charged pipe repel each other because they have the same charge. 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; pipe and other single tapes are negatively charged.
· Explain how you can reliably make such predictions. 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 pipe 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.
· Note the behavior of the tape, then predict what will happen when you bring the same end close to the other piece of hanging tape. The aluminum foil is charged by induction as the pipe approached and touched the surfaces. The closest surface of the uncharged object would have a net charge that is opposite to the charged pipe, and the side furthest from the charged pipe would acquire the same magnitude but opposite charge as the near surface. The near surface is oppositely charged to the pipe and results in a net force of attraction toward the pipe because its closer distance to the pipe 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 pipe. Initially the particle was attracted because it was charged by induction. After contact charge migrated from the pipe to the materials in contact with the pipe resulting in repulsion between the materials and the pipe.
· Can the behavior you observe be explained by the existence of a third type of charge? If a third charge existed it would attract both positive and negative charges.
· Explain what happened in terms of two types of charge. The aluminum cylinder is full of free charges that can migrate freely anywhere on the cylinder. The outside charge attracts opposite charges, which build to greatest density on regions of the aluminum cylinder nearest it. This process must continue until there is no electric field in the aluminum, because if there is an electric field in the aluminum then charges will move in response to it. Movement of charges must continue until the field is eliminated. This effectively 'shields' all point inside the cylinder from the effect of external charges.
· Predict whether your finger will attract or repel the other piece of tape, then test and note your results. Attract because of the charges.
· How is the effect of the pipe on the tape different than if the aluminum foil is removed? Explain what is going on. The free charges from the aluminum foil will not be present.
· Bring the charged pipe (without the foil on the end) near the tape while keeping it outside the cylinder. Explain what happens. 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.
· Explain any difference in the behavior of the tape due to the aluminum cylinder. When the charged plastic rod is brought near there is a redistribution of charge on the conducting rod and the force on the charged object will change. This redistribution of charges causes the end of the pipe near the rod to take on a charge opposite to that of the rod. The charge near that end will experience a force in the same direction as if the rod was near.
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