ELECTRIC CHARGE Name Partners' Names Part 1 The Existence of Charge A) 1. Place an empty soda pop can on its side on a flat table. 2. Fully inflate a balloon. Hold the balloon next to the side of the can, but do not let the balloon and can touch. 10 111 000 3. Is there any evidence of interaction between the can and the balloon? 4. Rub the balloon with some wool, and again position it near the can. Record your observations of the interaction between the two objects. 5. Does the rubbed balloon exert a force on the can? How do you know? Is the force attractive or repulsive? UNIT 2 PARTICULATE NATURE OF MATTER II - 59 CALVIN COLLEGE
6. Bring a magnet near the can, and note if any interaction between the two occurs. Does the magnet exert a force on the can? B) 1. Tear or cut up some paper into very small pieces. (The pieces should be about the size of the upper case letters on this page.) 2. Hold a plastic comb a little ways above the paper, and see if the paper interacts with the comb. Do you see any interaction? 3. Rub the comb on the wool or through your hair, and bring it near the pieces of paper. Now does the comb interact with the paper? If so, how? 4. How do you know this is not a magnetic interaction? The interactions between the rubbed balloon and the metal can, and between the rubbed comb and the paper are examples of electrical force. Electrical forces result from the interactions of charges. We can say that when we rubbed the balloon we gave it an electrical charge. One property of the electric force worthy of explicit mention is that the force is stronger when charged objects are closer together, and it is weaker when they are further apart. Electrical forces can also act over a distance. That is, objects do not have to be touching in order to exert this force. (Gravity and magnetic forces can also do this.) UNIT 2 PARTICULATE NATURE OF MATTER II - 60 CALVIN COLLEGE
Determine whether or not the objects listed in the table below can be charged by testing whether or not they will attract the small paper bits you have made. Test these objects first before you have rubbed them, and then test them again after rubbing them with wool or silk. Find a few additional items around the room, and test them for charge as well. If you test glass objects, pay special attention to edges (e.g., the top of a jar rather than its side). Object Charged before rubbing? Charged after rubbing? balloon Plastic ruler Aluminum can Glass rod Styrofoam Wooden pencil Before going on to Part II, discuss your results with your professor. Part II Attraction and Repulsion Cut out two rectangular wedges from a large Styrofoam cup. (The wedges should each be about 2 inches wide.) Poke a small hole near one end of each wedge, and tie a string through the hole. You should be left with two dangling Styrofoam wedges as shown to the right. Rub both wedges with a piece of wool, and then hold the wedges up by the ends of the strings. While only holding onto the strings, try to bring the wedges together. Describe what happens. UNIT 2 PARTICULATE NATURE OF MATTER II - 61 CALVIN COLLEGE
What is the fundamental difference between the way these Styrofoam wedges reacted and the way all the materials in Part I reacted? You will now use one of your hanging Styrofoam wedges as an electroscope (a device which you can use to examine attractive and repulsive forces between charged objects). Rub your Styrofoam wedge with wool in order to charge it. Next, charge the following objects, one at a time, by rubbing them with wool or silk. Bring each object close to the Styrofoam wedge (but do not let them touch!), and record whether they attract or repel the Styrofoam wedge. You probably should recharge your Styrofoam wedge occasionally during the course of these tests. Test some additional objects as well, and record them in the spaces below, but please limit yourself to testing objects that you know can be charged. Plastic Ruler Balloon Object Attracts (A), Repels (R), or No Effect (NE) Comments Glass Rod Plastic Comb Scientists say that objects can have either a positive charge, a negative charge, or no charge at all. Like charges repel, but opposite charges attract. That is, a positively charged object will repel other positively charged objects, but it will attract negatively charged objects. Similarly, a negatively UNIT 2 PARTICULATE NATURE OF MATTER II - 62 CALVIN COLLEGE
charged object will repel other negatively charged objects, but it will attract positively charged objects. The existence of charge can be explained by extending the particle model to include the individual parts of an atom. Specifically, each atom consists of a nucleus made of protons and neutrons, and an outer region made of detachable electrons whirling about the nucleus. Each proton is positively charged, and each electron is negatively charged. Protons and electrons each carry the same magnitude of charge, so that an atom containing an equal number of protons and electrons is electrically neutral. (It is the number of protons in an atom which determines what kind of atom is present.) Most atoms are electrically neutral, i.e., they contain the same number of electrons as protons, but it is not uncommon for atoms to have a slight electrical imbalance. When a plastic ruler is rubbed with wool, electrons are rubbed off of the wool onto the ruler, giving the ruler a net negative charge and the wool a net positive charge. On the other hand, if glass is rubbed, electrons are pulled off the glass, giving the glass a net positive charge. Assume the Styrofoam wedge became negatively charged when you rubbed it with wool. What were the charges on the other objects you tested? (Go back and place a + or - beside each). Devise a way to test whether objects you have identified as negatively charged repel each other but attract objects you have identified as positively charged. Do the same for positively charged objects. Describe your test and your results below and on the next page: UNIT 2 PARTICULATE NATURE OF MATTER II - 63 CALVIN COLLEGE
UNIT 2 PARTICULATE NATURE OF MATTER II - 64 CALVIN COLLEGE
Part III Forces between charged and uncharged objects A) Recharge your Styrofoam wedge electroscope, and bring an uncharged aluminum can close to it. Is there an attractive or repulsive force? B)Bring other uncharged objects close to your electroscope, and record your results. Object Plastic Ruler Wooden Pencil Attracts (A), Repels (R), or No Effect (NE) Comments Glass Rod Plastic Comb 1. Were any of the forces repulsive? 2. How do these results compare to the forces you observed in Part I of these activities? In the particle model, even uncharged objects contain charged protons and electrons. Normally there are equal numbers of each, so the positive and negative charges cancel each other out, and there is no net charge on the object. The protons are held in place in the nuclei of the atoms, but the electrons are more mobile. (In some materials they can move anywhere within the object; in other materials they can only align themselves on different sides of the nuclei, but in either case they are more mobile than the protons.) UNIT 2 PARTICULATE NATURE OF MATTER II - 65 CALVIN COLLEGE
3. Can you use these ideas about the mobility of the electrons and what you know about how force falls off with distance to explain how there can be an attractive force between a charged and an uncharged object? Explain your answer below. You should find it helpful to include diagrams. UNIT 2 PARTICULATE NATURE OF MATTER II - 66 CALVIN COLLEGE