Pre-LAB 1 Preparation: Electric Charge & Electric Interactions

Transcription

1 Name: Lab Partners: Date: Pre-LAB 1 Preparation: Electric Charge & Electric Interactions Directions: Read over the lab handout and then answer the following questions. Question 1 What are the two hypotheses that we will investigate in this lab? Question 2 Given that forces describe interactions between objects, would you think that the force exerted by one charged object on another gets larger or smaller as the objects are moved farther apart? PHYS-204: Physics II Laboratory i

2 Name: Lab Partners: Date: Objectives LAB 1 Electric Charge & Electric Interactions To understand that net charge may be positive or negative. To investigate the nature of the forces between charges and to describe these forces in a qualitatively and quantitatively way. To recognize that charges move readily on a conductor, but do not move about on an insulator. Overview Rubbing many objects together leaves them in a state where they are attracted or repelled from other objects. This phenomena is most pronounced on cold, clear days when the humidity is low. You may have noticed that after you have use a plastic comb, small pieces of paper or hair will stick to it. If you rub a balloon it will stick to your clothes. How many times have you cursed the phenomenon known as static cling. when you have emptied your dryer? Our study of electrical phenomana begins with an exploration of the nature of the forces between objects that have been rubbed or that have come into contact with objects that have been rubbed. These forces are attributed to a fundamental property known as charge. The forces between particles that are not moving or that are moving relatively slowly are known as electrostatic forces. We begin our study by investigating the circumstances under which electrostatic forces are attractive and under which they are repulsive. This will allow you to divide charged objects into two classes. Those with a positive charge, and those with a negative charge. Next, we will observe that charges can move easily throughout some materials, called conductors, while other materials, called insulators, block the movement of charge. These labs have been adapted from the Real Time Physics Active Learning Laboratories [1]. The goals, guiding principles and procedures of these labs closely parallel the implementations found in the work of those authors [1, 2]. Investigation 1: Exploring the Nature of Electrical Interactions The nature of electrical interactions can be deduced by careful experimentation and reasoning. We will first state two hypotheses about electrical interactions. We will then observe some electrical interactions and determine whether our observations are consistent with these hypotheses. Hypothesis 1 The interaction between objects that have been rubbed is due to a property of matter that we will call charge. We can divide charged objects into two categories: those with a positive charge and those with a negative charge. Hypothesis 2 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. PHYS-204: Physics II Laboratory 1

3 To test these hypotheses you will use the following items: 4 pieces of Scotch tape, approx. 10 cm long 2 small rod stands 2 threaded Styrofoam balls (with low mass) 2 threaded, metal-coated Styrofoam balls (with low mass) 1 PVC rod (gray) and wool cloth or fur 1 acrylic rod (clear) and polyester or silk cloth 1 paper towel Note: When you are answering the questions below, do not simply state results that you have memorized or learned previously. You must devise a sound and logical set of reasons based on your observations to support the hypotheses. Credit will only be given for reasoning based on your observations. Activity 1.1: Interactions of Scotch Tape Strips Step 1: You and your partners should place four strips of Scotch tape, 10 cm in length, on the lab table with the sticky side down (These strips of tape may already have been placed at your lab station). Curl over the end of each tape strip to make a non-stick handle. Peel your tape off the table and bring the non-sticky side of the tape toward your partner s strip. Question 1.1 Closely observe the strips of tape. What happens? How does the distance between the tapes affect the interaction between them? Step 2: Place two strips of tape on the table, sticky side down, and label them B for bottom. Then press two strips of tape, one on top of each of the B pieces; label these strips T for top. Pull each combination of top and bottom strips off the table. Then pull the top and bottom strips apart. Keep the pieces of tape from touching anything, so that they do not lose their electric charge. Bring the non-sticky sides of the two top strips together. Question 1.2 Describe the interaction between the two top strips when they are brought toward one another. Step 3: Bring the non-sticky sides of the two bottom strips toward one another. Question 1.3 Describe the interaction between two bottom strips when they are brought toward one another. PHYS-204: Physics II Laboratory 2

4 Step 4: Bring the non-sticky sides of a top and a bottom strip toward one another. Question 1.4 Describe the interaction between a top and a bottom strip. Question 1.5 Are your observations of the interactions of the tape strips consistent with the hypothesis that there are two types of charge? Please explain your answer carefully in complete sentences, and cite the outcomes of all your observations. Historically, scientists charged objects by rubbing a rubber rod with fur or by rubbing a glass rod with silk. It turns out that paper rubbed on acrylic (a synthetic polymer) works the same as silk on glass, and PVC (polyvinyl cholride) works the same as rubber. In the next activity you can get more experience studying the interactions between charged objects using techniques developed by early investigators. Note: In the activity that follows, your observations may be misleading if you touch the rods or any part of the pieces of tape other than the ends after charging them. Handle the pieces of tape and the rods carefully. Activity 1.2: Charging Acrylic and PVC Rods Step 1: If necessary, make another set of electrically charged bottom and top strips of tape, as you did in the previous activity. One member of the group should hold a top strip of tape in one hand, and a bottom strip in the other. Rub a PVC rod vigorously with fur or wool, and then bring it close to each of the strips. Try to keep the rod and the strip from touching each other. Question 1.6 What happens to each of the strips? Step 2: Rub an acrylic rod (the clear plastic rod) with paper, and then bring it close to each of the strips. Keep the rod and the strip from touching each other. Question 1.7 What happens to each of the strips? Question 1.8 Recalling the interactions between like and unlike charged objects that you observed before, can you explain your observations? PHYS-204: Physics II Laboratory 3

5 Benjamin Franklin arbitrarily assigned the term negative to the nature of the charge on the PVC rod (or, in his day, an amber rod) when the PVC rod is rubbed with fur. Conversely, the nature of the charge found on the acrylic rod (or a glass rod) after it is rubbed with paper silk (or with silk) is defined as positive. Although the term negative could just as well have been assigned to the charge on the acrylic rod; scientists have kept the same choice ever since. Investigation 2: Using Induction to Demonstrate that Negative Charges Move Easily Prediction 2.1 Will anything happen when you bring a charged object close to a neutral object? If so, what will happen? Will the neutral object be attracted or repelled, or will it depend on whether the charged object is positive or negative? Activity 2.1: Interaction of charged and neutral objects Below you will test the prediction you have just made. Step 1: Take one of the Styrofoam balls with a string attached. Touch the entire surface of the Styrofoam ball with your hands. This lets any charge flow off so that the ball is electrically neutral. Hold the string so that the ball hangs down and can swing freely. Again charge up the PVC rod by rubbing it with the wool or fur. Bring the rod close to the neutral ball, but keep the rod and ball far enough apart that they do not touch. Question 2.1 What happens to the neutral ball? Is there an interaction between the rod and the ball? Is this what you predicted? Prediction 2.2 Will a neutral ball behave the same way if it is approached by a charged acrylic rod instead of the PVC rod? Step 2: Charge up the clear acrylic rod by rubbing it with the piece of paper. Bring it close to the neutral Styrofoam ball. Do not let the rod and the ball touch. Question 2.2 Describe the interaction between the clear acrylic rod and the neutral ball. Is it similar to the interaction between the PVC rod and the ball, or is it opposite? Is this what you predicted? PHYS-204: Physics II Laboratory 4

6 Figure 1: The behavior of the neutral object in the vicinity of a charged one requires some explaining. It is one observation that leads us to believe that a neutral object is made up of charged particles-if a neutral object had no charged particles within it, there should not be any interaction between the neutral object and a charged one. But, as you have seen, there can be a significant interaction. If we recognize that all objects contain charged particles, namely the electrons and protons in the atoms in the object, we can visualize what will happen to these charges when an electrically charged object is brought near. If a negatively charged rod is brought close to a neutral ball, the atoms become distorted, with electrons repelled away from the rod and the protons attracted toward the rod. The attractive force on the positive charges is greater than the repulsive force one the negative charges because the negative charges are farther away. Question 2.3 What sequence of observations would you have to make in order to tell if an object is electrically neutral. Be sure the sequence you describe is consistent with all the observations you have made. Activity 2.2: Using Induction to Demonstrate that Charges Move Easily on Conductors and not on Non-conductors Matter is made 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. The process by which external charges can cause electrons to rearrange themselves so that the positive and negative charges no longer neutralize each other is known as induction. In this activity you will study an interaction that involves induction in a conductor and in an insulator. You should observe the interaction between a small uncharged insulator that is hanging freely and a charged rod. You should also observe the interaction between the charged rod and an uncharged hanging conductor. Step 1: Charge the PVC rod with excess electrons by rubbing it with fur. Touch each of your hanging balls to make sure they are not charged. Bring the negatively charged PVC rod near an uncharged insulator consisting of a single Styrofoam ball. PHYS-204: Physics II Laboratory 5

7 (a) (b) Figure 2: Depiction of how excess electrons on a rod can influence a neutral insulator and a neutral conductor. (a) The electrons in the Styrofoam insulator are repelled from those in the rod but stay with their atoms. (b) Electrons in a metal-coated ball are repelled by the electrons in the rod and are free to move as far away from the rod as possible. PHYS-204: Physics II Laboratory 6

8 Question 2.4 What happens before the two objects touch? Question 2.5 What happens after they touch? Step 2: Repeat step 1, using an uncharged metal-coated Styrofoam ball. Question 2.6 What happens before the two objects touch? Question 2.7 What happens after they touch? Question 2.8 Use Hypothesis 2, which claims that charges move easily on conductors, to explain why the metal-covered ball is attracted to the rod before touching it and is repelled after touching it, showing that it has picked up a significant amount of charge from the rod. Question 2.9 Use Hypothesis 2, which claims that if an object is an insulator, its electrons will stay in the vicinity of their atoms, to explain why the uncoated Styrofoam ball is still attracted to the rod, or only weakly repelled, after touching it. This laboratory exercise has been adapted from the references below. References [1] David R. Sokoloff, Priscilla W. Laws, Ronald K. Thornton, and et.al. Real Time Physics, Active Learning Laboratories, Module 3: Electric Circuits. John Wiley & Sons, Inc., New York, NY, 1st edition, PHYS-204: Physics II Laboratory 7

9 [2] Priscilla W. Laws. Workshop Physics Activity Guide, Module 4: Electricity and Magnetism. John Wiley & Sons, Inc., New York, NY, 1st edition, PHYS-204: Physics II Laboratory 8