Online-Lab-1_Electric-Charges (2)

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PHYS 1402 Lab 1: Electric Charges Name: ___ ____________ Overview: On cold clear days rubbing almost any object seems to create a new kind of force. After being used, a plastic comb will pick up bits of paper, hair, or cork with it. (And when you think about it, this force is relatively strong. It can lift your hair up against the gravitational pull of the whole Earth downward on your hair!) Anyone who has walked across a carpet and then been shocked by touching a light switch or electrical appliance will attest to the presence of “electric forces.” You are going to begin a study of electrical phenomena by exploring the nature of the forces between objects that have been rubbed or pulled apart or come into contact with other objects that have had such interactions. These forces are attributed to a fundamental property of the constituents of atoms known as charge. The forces between charged particles that are not moving or are moving relatively slowly are known as electrostatic forces. You can start your study by exploring the circumstances under which electrostatic forces are attractive or repulsive. This should allow you to establish how many types of charge there are. Then you can proceed to a qualitative study of how the force between charged objects depends on the distance between the charged objects. This will lead you to a formulation of Coulomb’s law, the mathematical relationship that describes the vector force between two small, charged objects. Next you can carry out a more quantitative experiment on the repulsion between two charged objects when they are brought closer and closer together. Exploring the Nature of Electrical Interactions You can investigate the properties of electrical interactions between objects using the following materials: • roll of Scotch Magic© tape • hard rubber or hard plastic • polyester, felt or silk cloth Although the nature of electrical interactions is not obvious without careful experimentation and reasoning, let’s start by considering a plausible hypothesis: Hypothesis I: If the interaction between objects that have been rubbed or pulled apart is due to a property of matter called charge, then there are only two types of electrical charge. For the sake of convenience, we will call these charges positive charge and negative charge. 1

Try the activities suggested below. Mess around and see if you can design careful, logical procedures to demonstrate that there must be at least two types of charge. Carefully explain your observations and state reasons for any conclusions you draw. Activity 1-1: Test of Hypothesis I 1. You should tape a 10 cm or so, 2 strips of Scotch Magic © tape, onto a wooden table. The end of each tape should be curled under to make a non-stick handle. Peel your tapes off the table and bring the non-sticky side of the tapes toward the other non- sticky side. Hold both strips vertically. Question-1: Describe your observations. Do the strips attract, repel or not interact? The strips interact with each other when being peeled off from the wooden table. When these are peeled off, they become static charged. 2. Tape two more strips of tape with “handles” on the table and use a pen to label them “B” for bottom. Press a second strip of tape on top of each of the B pieces, and also give it a handle. Label these strips “T” for top. 3. Pull each pair of strips off the table. 4. Then pull each top and bottom strip apart. (Note: you will need to repeat this set of procedures several times to answer all the questions below.) Question-2: Describe the interaction between two top (T) strips when they are brought near each other. Do the strips attract, repel or not interact at all? When the strips are brought near each other, they repel. Question-3: Describe the interaction between two bottom (B) strips when they are brought near each other. Do the strips attract, repel or not interact at all? When the bottom strips are brough near each other they repel each other. 2

Question-4: Describe the interaction between a top (T) and a bottom (B) strip when they are brought near each other. Do the strips attract, repel or not interact at all? When a top strip and bottom strip are brought near each other, they attract to one another. These attract because the opposite charges are attracting to each other. Question-5: Are your observations of the tape strip interactions consistent with Hypothesis I, i.e., that there are two types of charge? Explain your answer carefully, in complete sentences, and using the results of all your observations. Yes, the results of the tap matches our hypothesis that there are two types of charges. This is true because it was shown in this experiment that the opposite charges attract, and the same charges repel. Question-6: Do like charges repel or attract each other? Do unlike charges repel or attract each other? Explain based on your observations. Like charges repel, while unlike charges attract. In the experiment, when I put the top of the tape near the other top of the tape, they repelled. On the contrary, when a top of the tape was brough near the bottom of the tape, these two were attracted to each other. Question-7: Based on your observations of the movement of the tapes, how does the strength of these forces compare to the gravitational force on the tapes near the surface of the earth? Hypothesis II: Conductors and Non-conductors There is a second hypothesis that we will consider that has to do with the properties of materials. Scientists believe that most matter is made up of atoms that contain positive and negative charges associated with protons and electrons, respectively. When electrons in an atom surround an equal number of protons the charges neutralize each other, and the atom does not interact with other charges outside the solid. In some types of solid materials, known as insulators, the electrons are tightly bound to the protons in the atoms and do not move away from their atoms. However, in other solids known as conductors, the electrons—but not the protons—are free to move under the influence of other charges. Hypothesis II: Charge moves readily on certain materials, known as conductors, and not on others, known as insulators. In general, metals are good conductors, while glass, rubber, and plastic tend to be insulators. To test Hypothesis II, watch the following video. Charged-Uncharged-Objects 3

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Question-8 : Use your knowledge of how like and unlike charges interact with each other, and your observations to explain how this activity supports Hypothesis II. What do you think happened to the charges on the metal plate when the charged balloon was brought near it? Question-9 : Describe what happened with each interaction: charged balloon and wooden board and charged balloon and metal plate. Was there any difference in the behaviors? Activity 2: PhET Simulations Discussion When you walk across a carpet and touch a metal doorknob, you might feel a small shock. The friction between the different materials in your footwear and the carpet gives you a static charge. (Physicists call it a triboelectric charge, meaning a charge from frictional contact.) The metal doorknob is a conductor, so the excess charge on your body jumps to the knob rapidly, creating the spark. Your nerves are sensitive to electrical impulses, so you feel the shock. In the simulation, John Travoltage’s foot can be rubbed across a carpet so that his body collects charge. His arm can be rotated so that a spark will jump from his finger to the doorknob. 1. Open PhET simulation John Travoltage (https://phet.colorado.edu/en/simulation/john-travoltage). 2. Rub John Travoltage’s foot back and forth against the carpet until a spark jumps from his finger to the doorknob. Question-10: Explain what happens when John rubs his foot on the carpet. When John rubs his foot on the carpet, the negative charges enter his foot and up his body from the carpet. Question-11: What do the blue spheres represent? The blue spheres represent negative charges, which is an electron. Question-12: How does John Travoltage react when the spark jumps? When the spark jumps, John gets a shock and it hurts him. 4

3. Move John Travoltage’s hand close to the doorknob, then rub his foot on the carpet again. Question-13: Compared to the process you observed in Step 2, what happened differently this time? If you move John’s hand close to the doorknob and rub his foot on the carpet, the negative charges move through his body very quickly to the knob, and John doesn’t react when there is a spark this time. Question-14: How can you get the maximum charge built up on John Travoltage without producing a spark? You can get the maximum charge built up on John without producing a spark by keeping his hand away from the door and rubbing the foot on the ground. Question-15: Under what circumstances—if any—will a spark jump from John Travoltage’s foot to the doorknob? Why do you suppose this is (in terms of the physics involved)? Through the foot to the door sparks may not take place. Summing up: Under which conditions is a spark most likely to jump between John Travoltage and the doorknob? Describe in terms of the amount of charge on his body and the distance between his hand and the doorknob. The condition s of when his hand is close to doorknob and when his foot is rubbed quickly then we will see this. 4. Open the PhET simulation Balloons and Static Electricity (https://phet.colorado.edu/en/simulation/balloons-and-static-electricity) Question-16: What happens to the electrons (minus signs) on the wool sweater as you put the balloon near it? When you put the balloon near the wool sweater, there is no effect of electrons because the balloon doesn’t have a net charge. 5

Question-17 : What do you have to do to remove all electrons from the sweater? How can we transfer charges? It isn’t possible to remove all electrons from the sweater, because if you try to it will break the bonds between the atoms of wool. Question-18: What happens when you release the balloon? When you release the balloon, there will be no electrostatic effect because this is a neutral charge. Question-19: Click on “Reset Balloon”. What happens when the balloon is moved closer to the wall? Now bring the balloon close to the sweater. Once the balloon acquires negative charges, would the balloon be attracted to a neutral surface like the wall? It will have no effect because the balloon and wall have neutral charges. Question-20: Does an object have to have an electrical charge to be attracted to an electrical charge? Give an example. Yes, objects have to have an electrical charge to be attracted to an electrical charge. For example in the lab we just did, the balloon is attracted by the sweater, and after the balloon is rubbed with the sweater, the balloon now has a negative charge and the sweater gets the positive charge, and now attract. Question-21 : Summarize the important properties of charges. Alike charges repel, and opposite charges attract. Reference: 6

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Sokoloff, David R.. RealTime Physics Active Learning Laboratories Module 3 Electricity & Magnetism, Wiley Higher Ed. 7

Online-Lab-1_Electric-Charges (2)

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