Class Monday and Wednesday no class Friday (trip to Pittsfield) • Covered the ground rules for course • Discussed “expert thinking” vs “novice thinking” • reviewed some ideas from mechanics • charge • material model conductor & insulator • static charge • electroscope o near charge o charged by contact o charged by induction P250-­‐L1-­‐W1 Jan 9, 2012 Knight Chapter 26 40%
Quizzes & Homework
Test1
15%
Test 2
15%
Final
30%
http://csma31.csm.jmu.edu/physics/giovanetti/PHYS250/ Most homework will be provided by the Lon Capa online system. There are bound to be problems associated with the use of any tool. I am typically very amenable to accommodating students if the problems are brought to my attention early. Chapter 26 • Electric Force o Exploration and discovery (static electricity) § charge & force • +, -­‐, neutral • attraction, repulsion • distance dependence § material properties • conductors & insulators o Formal definition § +, -­‐ , Neutral § Atoms and charge • electron, proton § fundamental unit of charge § neutral= Σq=0 § charge conservation § model for conductors and insulators o Charging § contact § polarization (close no contact) § induction § discharging o dipole o Coulombs law o Field § point charge o vector review Chapter 27 Electric fields Review basic ideas of mechanics: Mass (material)-­‐ there is a quantity called inertial mass that characterizes a materials motion by determining its response to forces or being an essential parameter in describing momentum. Gravitational mass= a quantity that determines the effects of other matter i.e. the force of gravity. No reason for these to be the same but they are in intro physics. Einstein resolved this paradox with General Relativity. Newton’ 3 laws. Force, momentum Energyè Kinetic (motion), Potential (stored), Work (transfer) Motionè Postion, velocity, acceleration (1-­‐d graphs and relations) (3-­‐d vectors) Gravity, external (person pushing), spring, friction, tension (rope forces) Matter is made up of point masses that can be assembled into rigid bodies, fluids and gases (billiard ball model). More detailed models will include rigid bodies that still can have some internal vibrations, complex gases that have vibrating and rotating constituents and other extensions. Matter has a remarkable feature in that the core elements are exactly identical. This has been tested to a very high precision. Approach This group in particular should appreciate that there are experts and novices. We need to take the expert approach-­‐ • Ideas and formulations need to be consistent • Limit the number of exceptions or special cases •
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Develop models Confront models and mathematical formulations Develop checks (units, considering extremes) Proper problem approach o diagrams !!! o List of ingredients o relevant physics (often several principles) o formulas o careful calculation (spreadsheet or calculator) I always marvel at the student that freely admits to having significant difficulties with physics but then stubbornly embraces his/her approach. If a friend who tended to insult people and behave rudely was complaining that they were frustrated that they couldn’t meet people then you might suggest a change of behavior. “Just try to be nice when you first meet someone. “ If the person refuses to follow this practical advice then one can expect no change. It is clear to you that the problem could be solved by a straightforward modification of behavior. Now implementing that change might offer significant challenges to the your friend but you would expect that either he/she take the advice and make an effort or suffer the consequences. In this course we will try to address both concepts and approach. I suggest that you work on both. Thinking like an expert will be the most important skill that you take away from a college education. Electroscope and static electricity Basic starting points basic units of charge +,-­‐ {use electron -­‐1, proton +1 NOT COULOMBS!!!} repulsion attraction (people models for 4th grade, hand out charges and move accordingly) How do things charge up? The key is electron affinity (Triboelectric Series ) MORE POSITIVE (less amenable to electron visits) • rabbit's fur • glass • mica • nylon • wool • cat's fur • silk • paper • cotton • wood • acrylic • cellophane tape • polystyrene • polyethylene • rubber balloon • saran wrap MORE NEGATIVE (more amenable for electron visits)
Based on some complex atomic and molecular physics (material science), regions of different material have slight more or less affinity for partially mobile charge. You can imagine kids occasionally climbing the neighbors fence to retrieve a baseball. Electrons in insulators are normally confined. But they can wander into adjacent regions. When two materials are in contact the “visiting” charges will spend more time in one material than is reciprocated (see series above). Separating the material can then result in charge transfer if there is an unequal level of charge. Take two materials in the series above that are maximally apart and the ability to transfer charge is high. Plastic and fur are two good candidates.