Exer 1 Forces of Electrostatic -Charging by Conduction and Charging by Induction Abstract The processes of charging by conduction and induction were observed and determined by several experiments. Plastic sheets and strips involved in many tests were all rubbed with tissue. First, the rubbed plastic sheet (-) became capable of attracting tissue bits (+). Second, the two rubbed plastic strips (-) were observed to repel each other. Third, the rubbed plastic sheet caused the pith balls to repel each other.Fourth, a balloon rubbed in a hair(-) became capable of attracting tissue bits (+). Using electroscope, the rubbed plastic sheet (-) when placed near the metal knob, caused the metal strips to diverge, and when touched, weaker repulsion was observed. As it was removed, the repulsion gradually decreased and stopped. Otherwise, when it was placed in contact with the knob, greater repulsion was observed. Thus, like charges repel and opposite charges attract; and in charging by conduction, contact between charged and neutral objects is necessary for the electrons to flow, giving a neutral body a charge similar to the stimulus, while in charging by induction the object being charged is polarized by the stimulus and gains a charge opposite of that of the stimulus.
I. Introduction Electrostatics is the interaction of electric charges that are at rest. The fundamental law of electrostatics states that like charges repel and opposite charges attract. And this force which charges exert to repel or attract one another is called electrostatic force. According to Coulomb’s law, the magnitude of force between two charged particles is directly proportional to the product of the charges of these particles and inversely proportional to the square of the distance between them. This electrostatic force, F, is
(1) where k is the proportionality constant and is equal to 8.988x109 Nm2/C2, q1 and q2 are the charges, and r is the distance between these point charges[1]. A neutral body contains equal amounts of positive and negative charges. A neutral body can be charged by conduction and induction. In induction, a medium in which electrons may transfer from one substance to another is required. In conduction, direct contact between substances is needed and no medium is required. Since charging by conduction involves direct contact, it is also called charging by contact. In terms of the charge gained by the body, in conduction, the charge gained by the body is similar to that of the stimulus while in induction, the charge gained by the body is opposite of that of the stimulus[1].. In the laboratory, electroscope is a device used to detect the presence of a charged body. Electroscope is normally composed of a metal knob, a conducting base and a pair of metal strips. Since all the parts of an electroscope can conduct, electrons can flow from the metal knob to the pair of metal strips[2]. The experiment aims to demonstrate the interaction between charged bodies, to show how an
electroscope can be used in charging by induction and conduction, and to facilitate in understanding electrostatic forces.
II. Methodology The experiment consist of five individual set-ups namely, (1) the plastic sheet and tissue paper set-up, (2) the plastic-plastic set-up, (3) the pith balls set-up, (4) the electroscope and (5) the balloon- tissue set-up. The first four set-ups made use of a rubbed plastic sheet, this was done to introduce charge on the sheet and demonstrate attraction and repulsion among charged bodies. The last set-up made use of the balloon, similar to the plastic sheet, the balloon was rubbed in human hair. This was done so that the balloon would gain the negative charge present on the human hair. The first part of the experiment made use of the plastic sheet – tissue set-up. A plastic sheet and tissue papers were obtained. A part of the tissue paper was torn into small crumpled bits and the remaining tissue paper was used to rub the plastic sheet vigorously towards one direction. The rubbed plastic sheet was then placed near the tissue bits. The reaction of the tissue bits towards the rubbed plastic sheet was recorded. In the second part of the experiment, two plastic sheets were used. The plastic strips were each rubbed with tissue paper and were suspended facing each other. Observations were noted. The next experimental set-up consists of two pith balls. Pith balls are small and made of cork-like materials which are lightweight and coated with a conductive material such as graphite, aluminum etc. The pith balls were suspended on separate string on a metal rod. The state of the pith balls before experimentation were observed and recorded. A plastic sheet was then obtained and was rubbed vigorously in one direction with a tissue paper. The rubbed plastic sheet was placed near the pith balls. The reaction of the pith balls towards the rubbed plastic sheet was recorded. The piths were then put into contact with the rubbed sheet and results were noted. The fourth part of the experiment used the electroscope set up. An electroscope is an instrument for detecting the presence of an electric charge or electrostatic phenomena. The electroscope consisting of a pair of thin leaves suspended on a metal rod that leads to a metal knob outside of an insulating enclosed container. To observe the effect of electrostatic, a plastic sheet rubbed with tissue paper was placed near the knob of the electroscope. Observations were noted. Without removing the sheet, the metal knob was touched momentarily and was observed. The plastic sheet placed near the metal knob was then removed and its effect on the set-up was recorded. The electroscope was then discharged by touching the metal knob with one’s fingers. After discharging, a plastic sheet rubbed with tissue paper was briefly put into contact with the knob of the electroscope. Observations were done and recorded. The last part of the experiment made use of a rubber balloon and the tissue paper bits. Similar to the first part of the experiment, a tissue paper was torn into small crumpled bits. The rubber balloon was then rubbed vigorously in hair and was place atop but not touching the bits of tissue paper. The interaction between the balloon and the tissue bits were recorded.
III. Results and Discussion
When the plastic sheet is rubbed in one direction with the tissue paper, the plastic becomes negatively charged through rubbing or friction. The tissue bits were attracted to the rubbed plastic sheet when they are put close to each other. This can be explained by the rearrangement of the charges within the atom [3]. While one side of the tissue paper becomes more positive, the other side becomes more negative. Since the plastic sheet is negatively charged, the slightly positive side of the tissue paper is attracted to the plastic sheet. As the plastic sheet becomes nearer to the tissue bits, their tendency to be attracted to each other increases. After touching the rubbed plastic sheet, one could observe that there is a weaker attraction between the plastic sheet and tissue bits compared to the former observed attraction. This is due to grounding. When the rubbed plastic sheet is touched, the electrons escaped to the ground since the ground has a high affinity for electrons. The transfer of electrons from the plastic sheet to the hand caused the deficiency of electrons in the sheet [4]. The fundamental law of electrostatics which states that like charges repel and unlike charges attract is further demonstrated in the second part of the experiment. By rubbing each plastic strips with tissue paper, the strips became charged through friction. When the two plastic strips are put side by side, facing each other, they repel each other. The two plastic strips being negatively charged repel each other since they have like charges. Pith balls conduct electricity due to the aluminum coat put to the pith, plant tissue that looks like Styrofoam [4]. When the rubbed plastic sheet is put near the pith balls, the pith balls separated and the balls are attracted to the plastic sheet. However if the balls touch the rubbed plastic sheet, the attraction between the two materials changed to repulsion. Consequently, the pith balls diverge less from each other. Electroscope is an instrument which demonstrates both charging by induction and charging by conduction. Initially, the electroscope is uncharged or it has equal amounts of negative and positive charge. In the experiment, it is first charged by induction. This is done by putting a negatively charged plastic sheet near the metal knob of the electroscope. The positive charges of the knob become attracted to the negative charges of the plastic sheet. At the same time, the negative charges of the knob repelled by the plastic sheet travels through the metal rod to the two leaves of the metal foil. Since the leaves receive the same negative charge, the leaves diverge from each other. By touching the knob with one’s fingers, grounding is done. There is lesser repulsion after touching the knob due to the negative charges repelled by the plastic sheet travelling through our body to the floor. After the charged plastic is removed near the knob, repulsion between the two metal leaves is again observed before the initial position of the metal leaves is reached. However, the repulsion is caused by the two positive charges in the leaves. As the plastic sheet is removed, the electroscope becomes deficient of negative charges and has more positive charges [4]. When the charged plastic sheet is put in contact with the knob of the electroscope, conduction happens. Negative charges will be left on the electroscope after the charged sheet is in contact with the electroscope. The charge present on the electroscope is the charge also present on the plastic sheet [6]. Again, the leaves diverge. In a place where the weather is cool and dry such as in the laboratory, an object is easily charged. When the balloon is rubbed in one’s hair, it gains extra electrons from the hair [7]. The positive side of the tissue paper becomes attracted to the negatively charged balloon. However, if the two balloons are put close to each other, the balloons repel because they are both negatively charged.
IV. Conclusion and Recommendation As stated in the fundamental law of matter, “Like signs repel; opposite signs attract”. That’s why the negatively charged plastic sheet and balloon attracted the tissue bits (+), and the negatively charged plastic strips repelled each other. Charging by conduction involves direct contact between the charged object and the neutral object which is to be charged, and the charge gained by the object to be charged is similar to that of the stimulus. Since, the charged object has higher charged it can cause the neutral object to gain a similar charge. For example, a positive stimulus, the charged object, when in contact with a neutral object, as the electrons of the neutral object are attracted with the positively charged stimulus, they transfer from the neutral object to the stimulus, resulting to the neutral object to be deficient in electrons; therefore, the neutral object becomes positively charged similar to the stimulus. Otherwise, when the stimulus is negatively charged, the excess electrons of that body transfer to the neutral body when in contact; thus, that neutral body gains more electrons and a negative charge similar to the stimulus. On the other hand, charging by induction involves a medium in which the electrons may flow from the ground to the object to be charged, and vice versa. Unlike conduction, induction does not need direct contact but the presence of a stimulus, charged body, is needed to facilitate polarization of the object to be charged, and that object to be charged gains a charge opposite of that of the stimulus. When the stimulus is positively charged, the electrons of the neutral body move to the side nearest to the stimulus, and when a medium from the ground is connected to the object to be charged, more electrons will flow to that body, making it negatively charged unlike the charge of the stimulus. When the stimulus is negatively charged, the electrons of the body to be charged will move to the side of that body farthest from the stimulus, and when the medium is connected to that body the electrons will flow through that to the ground; thus, the body will be deficient in electrons and be positively charged. In the experiment, the last listed in the methodology, the Van De Graaff generator wasn’t shown to the class because of the lack of apparatus. It would be better if the generator was present so the students would be able to further understand electrostatic phenomena with more generators. Temperature and humidity affect static charges; it may be helpful to find different sites where these factors vary. Care and safety must also be observed. The conditions of the place of experimentation affect the result of the experiment. Providing a positively charged object not only a negatively charged plastic sheet to charge the tissue bits, pith balls and the electroscope will give the experimenters more observations to be the bases of their studies.
V. References 1. H.D. Young, R.A. Freedman, University Physics 6th ed, Chapter 22, Pearson Education Pte Ltd, Singapore, 2000. 2. D.C. Giancoli, Physics 6th ed, Chapter 16, Pearson Education South Asia Pte Ltd, Singapore, 2007. 3. Hewitt, P.G., Conceptual Physics, Pearson Education, Inc., Boston, USA, 2005, pp.512 4.Manning, K., R. Webber, G. Weygand and M. White. College Physics, 5th ed. Cacho Hermanos, Inc. Philippines, 1977, pp. 571-572. 5. Hewitt, P.G., Conceptual Physics, Pearson Education, Inc., Boston, USA, 2005, pp.502
6. Childers,R.L, E.R. Jones, Contemporary Physics, 3rd ed., McGraw-Hill Book Co., New York, USA,2001. pp 498-499 7. Crisostomo, R., A.Padua, Practical and Explorational Physics Modular Approach, Quezon City, Philippines, 2003,pp 248.
