General Physics 2 Quarter 3 Week 3 ELECTRIC CHARGE The Origin of Electricity The electrical nature of matter is inherent in atomic structure. An atom consists of a small relatively massive nucleus that contains particles called protons and neutrons. A proton has a mass of 1.673 x 10-27 kg, and a neutron has a slightly greater mass of 1.675 x 10 -27 kg. Surrounding the nucleus is a diffuse cloud of orbiting particles called electrons. An electron has a mass of 9.11 x 10-31 kg. Like mass, electric charge is an intrinsic property of protons and electrons, and only two types of charge have been discovered, positive and negative. A proton has a positive charge, and an electron has a negative charge. A neutron has no net charge. Experiment reveals that the magnitude of the charge on the proton exactly equals the magnitude of the charge of the electron; the proton carries a charge + e, and the electron carries a charge of – e. The SI unit for measuring the magnitude of an electric charge is the coulomb (C), and has been determined experimentally to have a value e = 1.60 x 10-19 C. The symbol e represents only the magnitude of the charge on a proton or an electron and does not include the algebraic sign that indicates whether the charge is positive or negative. In nature, atoms are normally found with equal numbers of protons and electrons. Usually, then, an atom carries no net charge because the algebraic sum of the positive charge of the nucleus and the negative charge of the electrons is zero. When an atom, or any object, carries no net charge, the object is said to be electrically neutral. The neutrons in the nucleus are electrically neutral particles. The charge on the electrons or a proton is the smallest amount of free charge that has been discovered. Charges of larger magnitude are built up on an object by adding or removing electrons. Thus, any charge of magnitude q is an integer multiple on e; that is, q = Ne, where N is an integer. Because any electric charge q occurs in integers multiples of elementary, invisible charges of magnitude e, electric charge is said to be quantized. To show the quantized nature of electric charge, let us consider this example. How many electrons are there in one coulomb of negative charge? General Physics 2 Quarter 3 Week 3 The negative charge is due to the presence of excess electrons, since they carry negative charge. Because an electron has a charge whose magnitude is e = 1.60 x 10 -19 C, the number of electrons is equal to the charge e on each electron. Thus, the number N of electrons is Triboelectric Charging The presence of different atoms in an object provides different objects with different electrical properties. One property is known as electron affinity. The property of electron affinity refers to the relative amount of love that a material has for electrons. If atoms of a material have a high electron affinity, then that material will have a relatively high love for electrons. This property of electron affinity will be of utmost importance as we explore one of the most common methods of charging - triboelectic charging, also known as charging by friction or rubbing. Charging by Induction Induction charging is a method used to charge an object without touching the object to any other charged object. An understanding of charging by induction requires an understanding of the nature of a conductor and the polarization process. Charging Two-Sphere System Using A Negatively Charged Object One common demonstration performed to show how induction charging occur is by using two metal spheres. The metal spheres are supported by insulating stands so that any charge acquired by the spheres cannot travel to the ground. The spheres are placed side by side (see diagram i. below) so as to form a two-sphere system. Being made of metal (a conductor), electrons are free to move between the spheres - from sphere A to sphere B and vice versa. If a rubber balloon is charged negatively (perhaps by rubbing it with animal fur) and brought near the spheres, electrons within the two-sphere system will be induced to move away from the balloon. This is simply the principle that like charges repel. Being charged negatively, the electrons are repelled by the negatively charged balloon. And being present in a conductor, they are free to move about the surface of the conductor. Subsequently, there is a mass migration of electrons from sphere A to sphere B. This electron migration causes the two-sphere system to be polarized (see diagram ii. below). General Physics 2 Quarter 3 Week 3 Overall, the two-sphere system is electrically neutral. Yet the movement of electrons out of sphere A and into sphere B separates the negative charge from the positive charge. Looking at the spheres individually, it would be accurate to say that sphere A has an overall positive charge and sphere B has an overall negative charge. Once the two-sphere system is polarized, sphere B is physically separated from sphere A using the insulating stand. Having been pulled further from the balloon, the negative charge likely redistributes itself uniformly about sphere B (see diagram iii. below). Meanwhile, the excess positive charge on sphere A remains located near the negatively charged balloon, consistent with the principle that opposite charges attract. As the balloon is pulled away, there is a uniform distribution of charge about the surface of both spheres (see diagram iv. below). This distribution occurs as the remaining electrons in sphere A move across the surface of the sphere until the excess positive charge is uniformly distributed. https://www.physicsclassroom.com/class/estatics/Lesson-2/Charging-by-Induction The Law of Conservation of Charge The law of conservation of charge is easily observed in the induction charging process. Considering the example above, one can look at the two spheres as a system. Prior to the charging process, the overall charge of the system was zero. There were equal numbers of protons and electrons within the two spheres. In diagram ii. above, electrons were induced into moving from sphere A to sphere B. At this point, the individual spheres become charged. The quantity of positive charge on sphere A equals the quantity of negative charge on sphere B. If sphere A has 1000 units of positive charge, then sphere B has 1000 units of negative charge. Determining the overall charge of the system is easy arithmetic; it is simply the sum of the charges on the individual spheres. Overall Charge of Two Spheres = +1000 units + (-1000 units) = 0 units The overall charge on the system of two objects is the same after the charging process as it was before the charging process. Charge is neither created nor destroyed during this charging process; it is simply transferred from one object to the other object in the form of electrons. General Physics 2 Quarter 3 Week 3 CHARGING TWO-SPHERE SYSTEM USING A POSITIVELY CHARGED OBJECT What do you think will happen if there are two positively charged spheres? How would the movement of electron be changed? Study this figure: https://www.physicsclassroom.com/class/estatics/Lesson-2/Charging-by-Induction The positively charged balloon is brought near sphere A. Consider the graphic below in which a positively charged balloon is brought near Sphere A. The presence of the positive charge induces a mass migration of electrons from sphere B towards (and into) sphere A. This movement is induced by the simple principle that opposites attract. Negatively charged electrons throughout the two-sphere system are attracted to the positively charged balloon. This movement of electrons from sphere B to sphere A leaves sphere B with an overall positive charge and sphere A with an overall negative charge. The two-sphere system has been polarized. With the positively charged balloon still held nearby, sphere B is physically separated from sphere A. The excess positive charge is uniformly distributed across the surface of sphere B. The excess negative charge on sphere A remains crowded towards the left side of the sphere, positioning itself close to the balloon. Once the balloon is removed, electrons redistribute themselves about sphere A until the excess negative charge is evenly distributed across the surface. In the end, sphere A becomes charged negatively and sphere B becomes charged positively. The Importance of a Ground in Induction Charging In the charging by induction cases discussed above, the ultimate charge on the object is never the result of electron movement from the charged object to the originally neutral objects. The balloon never transfers electrons to or receive electrons from the spheres; nor does the glass rod transfer electrons to or receive electrons from the spheres. The neutral object nearest the charged object (sphere A in these discussions) acquires its charge from General Physics 2 Quarter 3 Week 3 the object to which it is touched. In the above cases, the second sphere is used to supply the electrons to sphere A or to receive electrons from sphere A. The role of sphere B in the above examples is to serve as a supplier or receiver of electrons in response to the object that is brought near sphere A. In this sense, sphere B acts like a ground. Activity 1: Charge it! Directions: Follow the procedures as stated in the activity. Objective: To demonstrate the transfer of electric charge form one object to another. Materials: 2 small rubber balloons Small piece of wool cloth Small pieces of paper Procedure: 1. Blow one balloon and tie it. 2. Rub one side of the balloon with the scrap of wool. 3. Move a finger toward the balloon in the charged spot. What do you observe? _________________________________________________________________ _________________________________________________________________ __________________________________ 4. Recharge your balloons, and try holding the charged parts near each other. What do you observe? Explain your observations. _________________________________________________________________ _________________________________________________________________ __________________________________ Rub one of the balloon and put on a table top (or the floor) and try gently rolling it? What do you observe? Explain you observations. _________________________________________________________________ _________________________________________________________________ _________________________________________________________________ ___________________________________________________ Prepare the small bits of paper. Place it on top of a table. Recharge your balloon and hold it slightly above the small bits of paper. What do you observe? Explain your observation. _________________________________________________________________ _________________________________________________________________ 5. 6. General Physics 2 Quarter 3 Week 3 Guide questions: 1. What happened on the part of the balloon that you rubbed with the scrap of wool? _________________________________________________________________ _________________________________________________________________ 2. What is the role of the rubbing process in the activity? ______________________________________________________________ ______________________________________________________________ Activity 2 : Charging by Induction Objective: Describe how the presence of negatively charge object induces movement of electrons. Materials: 2 pcs Styrofoam cups 1 pcs rubber ballon 2 pcs softdrinks empty cans scotch tape/double-sided tape Procedure: 1. Label the softdrink cans as can A and can B. 2. Mount the softdrink can on top of the styro cup using a scotch tape or doublesided tape. 3. Place the can side by side. 4. Charge the ballon by rubbing it with animal fur or hair (this will make the rubber ballon negatively charged) 5. Place the negatively charged balloon near to one of the cans. 6. Follow the figure below: 7. Observe what happens. Write your observation in your notebook. General Physics 2 Quarter 3 Week 3 Guide questions: 1. What happens to the can when you brought the negatively charged rubber ballon near it? ________________________________________________________________ ________________________________________________________________ ________________________________________________________________ 2. Describe the movement of the electrons in the experimental set-up. _________________________________________________________________ _________________________________________________________________ 3. 4. Would you expect that can A would be attracted by the negatively charged balloon? Explain why or why not? _________________________________________________________________ _________________________________________________________________ _________________________________________________________________ _________________________________________________________________ What is the role of the balloon in the activity? _________________________________________________________________ _________________________________________________________________ _________________________________________________________________ _________________________________________________________________ Activity 3 : 10 minute Video-Tutorial Charging by Induction Video Tutorial Objective: Explain how charging by induction happens. Using your this smartphones or laptops with internet go to link: https://www.youtube.com/watch?v=763tiBXvTGw&feature=youtu.be The Charging by Induction Video Tutorial describes what charging by induction is and explains how and why it occurs. Numerous examples, animations, and illustrations are provided. After watching the video lesson you will be able to answer the following questions: 1. 2. What is charging by induction and how does it occur? How can the results of charging by induction be predicted and explained? General Physics 2 Quarter 3 Week 3 Reflection: Write your answer on the following: 1. I learned that _________________________________________________________________ _________________________________________________________________ _________________________________________________________________ _________________________________________________________________ 2. I enjoyed the lesson most on _________________________________________________________________ _________________________________________________________________ _________________________________________________________________ _________________________________________________________________ 3. I want to learn more on _________________________________________________________________ _________________________________________________________________ _________________________________________________________________ References: Cutnell, J.D. and K. W. Johnsons. (2016). Physics, 9th Edition. https://www.physicsclassroom.com/class/estatics/Lesson-2/Charging-by-Induction https://www.youtube.com/watch?v=763tiBXvTGw&feature=youtu.be
