no tagsLecture 002: Electric Charge and Coulomb's Law (Wolfson20.1-20.2)SteveSekula, 24 January 2011 (created 19 August 2010)Main Goals of this LectureDefine basic concepts in electricity: charge, units of charge, and forceDefine the mathematical relationship between charge and forceexerted by one charge on another (Coulomb's Law)Activities in this LectureDemonstrations of the electric force1.The origin of electric force: electric chargeStudent Discussion: what is electric charge? See if they have anyanswers. (2 minutes)a.2.Description of electric charge: fundamental measure, units,conventions for signs, examples of charges (electron, proton),conservation of charge3.Interactive Effort: using the atomic theory of matter, predict whatshould happen to a candle flame in the presence of an electric field?4.Electric ChargeWhat is "electric charge"?fundamentally, nobody really knows the answer to that yetdon't lose heart! Fundamentally, nobody really know what "mass" iseither - however, we are familiar with mass because we spend ourGeneral Physics - E&M (PHY 1308) LectureNotesGeneral Physics - E&M (PHY 1308) LectureNotesGeneral Physics - E&M (PHY 1308) - Lecture Notes file:///home/sekula/Dropbox/Documents/Notebook...1 of 4 01/24/2011 04:45 PMlives pushing things around.we understand "electric charge" in the same way that weunderstand "mass" - through observations of the natural world,careful measurement, and the use of the language of mathematicsto express observations, develop frameworks to describe the worldand make predictions about the outcomes of experimentsHere are some things we DO know about electric chargeIt comes in two varieties - positive and negativeBenjamin Franklin - yes, one of the Founders of the UnitedStates - is credited with devising this naming schemethis naming scheme is VERY useful, because it connectsdirectly to an observation: the total charge in any system isTHE ALGEBRAIC SUM OF THE INDIVIDUAL CHARGES INTHE SYSTEM. For instance,It is a property that is "carried" by particles like electrons andprotonsTotal electric charge in a defined system (e.g. an enclosed region)is CONSERVED - that is, no matter what happens to the system thetotal charge cannot change. Charged particles may be created, orthey may be annihilated, in various physical processes, but thoseprocesses cannot change the total electric charge.Electric Charge DemonstrationsVan de Graaf generator - sparks and lightning and shocksCapacitor + candle: demonstrate the behavior of ions in an electricforce fieldBalloon simulatorq :: total= q1+ q2+ q3+ : =XNi=1qiGeneral Physics - E&M (PHY 1308) - Lecture Notes file:///home/sekula/Dropbox/Documents/Notebook...2 of 4 01/24/2011 04:45 PMQuantities of ChargeAll electrons carry the same charge. All protons carry the same charge.The proton's charge has exactly the same MAGNITUDE as, but theopposite sign of, the electron's charge.Given that the electron and proton are so different in many other ways(e.g. mass, radius, etc.) this is a remarkable fact.The magnitude of the electron or proton charge is the elementarycharge, . Electrons have charge , and protons have charge . Electriccharge is quantized - that is, there is a smallest unit below which you canno longer subdivide a system of electric charge. Charge only comes indiscrete amounts.And what is ? The Standard International (SI) unit of electric charge isthe Coulomb, named after Charles-Augustin de Coulomb and denoted by acapital letter C. It is convention to define:Making the elementary charge:Exploring ChargeSee the supplementary slides for Lecture 002 for movies and images thatillustrate electric charge:The charged balloon demonstration and simulationThe charged plastic comb water-deflection demoThe beam tree as an example of charge and aestheticsArc flash injury simulation using a dummyFluffy carpet and the danger of touching grounded metal objectse Àe +e e 1C :25 0 = 6 Â 118elementary charges1e :60 0 C = 1 Â 1À19General Physics - E&M (PHY 1308) - Lecture Notes file:///home/sekula/Dropbox/Documents/Notebook...3 of 4 01/24/2011 04:45 PMElectric ForceAs some of these demonstrations indicate, electric charge is able to exert aforce. We tend not to notice this force most of the time because theelectrons and protons in our bodies, and in the work around us, are largelypaired up and thus electrically neutral (zero electric charge) on a humanscale.As the balloon demonstration and the funny video of the electric shockfrom the carpet illustrate, electric charge and force go hand-in-hand. Manyobservations and measurements of the relationship between:The magnitude of the charges involvedthe distance between the charges (it's direction AND magnitude)the sign of the chargesthe force exerted between chargeshave been carefully studied. The result is mathematical statement that hasbeen upheld by thousands upon thousands of repeated experiments carriedout over hundreds of years - a LAW. This law is known as "Coulomb's Law":where is the force VECTOR (magnitude and direction) that charge 1exerts on charge 2. is a constant, determined from repeatedexperimentation, whose value is:Let's draw a picture of this and illustrate the pieces of this formula. Itcombines two key areas of mathematics: standard algebra and vectoralgebra. The picture will help us to parse the meaning of this formula,considering two cases: a pair of like-signed charged, and a pair of opposite-signed charges.F r 12~=r2k Á q1Á q2^F 12~k k :0 0 N =C = 9 Â 19Á m2 2General Physics - E&M (PHY 1308) - Lecture Notes file:///home/sekula/Dropbox/Documents/Notebook...4 of 4 01/24/2011 04:45
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