Atreyi SahaPhysics 132Prof. HatchFebruary 17, 2016Unit 2, Lecture 1: Intro to ElectrostaticsElectric Forces and Electric Fields: Now, we are looking at what IS light NOT what it does- Important Findings from Ben Franklino Charges: Like charges repel and opposite charges attract Two types of charges: positive, plus (+) and negative, minus (-)o protons (+), electrons (-): these are only negative because they are opposite chargeof the protons, neutrons - Demo: Rods, fur, silk, pith balls, and paper demo: like charged items repelled and oppositely charged items were attracted to each othero What is the nature of this force? We see an acceleration so there is a change in motion because the object went from rest to movemento Is it a familiar or a new force? new force - electric fieldo Long range or contact force? Don't need physical contact to feel the force - therefore, it must be a field forceo Like charges repel, opposite charges attract- Miscellaneous Factso When you touch an object, you are grounding it. Grounding an object means you are restoring it to neutral. Neutral means that there is a balance of charges: equal positives and negatives. Definition of Grounding: Physically connect it to the Earth. This is because the Earth is like a massive sink for charges. You can pour and draw out a lot of charges without altering the charged state of the Earth.o If you take an object and rub it, it will become "charged up." A practical example is that you rub your feet on the carpet and touch something and then receive a static shock because you have charged yourself up. When something is "charged," it has an imbalance of positives and/or negative charges. When rubbing an object, friction force creates enough energy that causes the breakage of molecular bonds that result in ions being formed.o Electrons are not the only ones being exchanged....IONS can be transferred between materialso Take home point: If you charge something, you have created a situation where there is an imbalance of charges (protons or electrons). - You are handed an object. How can you determine if it is neutral, positive or negative?o Neutral objects experience an attractive force towards charged objects - must be attractive towards a) positive charges b) negative charges c) another neutral object would do NOTHING KEY: neutral objects are attracted towards other objects as wello Positive objects experience an attractive force towards a) neutral object b) a negatively charged object. It would repel another positively charged object.o Negative objects experience an attractive force towards a) neutral object b) a positively charged object. It would repel anothernegatively charged object.- Conductors and Insulatorso Insulator (glass, rubber, air) - all electrons are tightlybound to their respective molecules the valence shell is more tightly boundo Conductor (metals) - some valence electrons areunbound from their respective molecules and movefreely through the materials Free flow of valence electrons through the metal Valence electrons are not tightly bound - they arelike a sea of electrons that are shared andfree to move Valence electrons move in response toforces- Metal Sphere and Charged Rod Exampleo Rod is positively charged. The metal has freeelectrons.o When you bring up a positively charged rod tothis sphere, electrons on the metal sphere willmove towards the plus because they are attracted to it.o When you remove the rod, metal ball will be left with plusses. This is because theelectrons centered around the charged plastic and then you took it away and stole the electrons in the process. The positive charges will be equally distributed because the valence electrons are free to move in response to positive charges until the positivecharges are distributed throughout. You have NOT neutralized themetal. Rather, you have CHARGEDit because you took away electrons.- Insulator exampleo The plastic rod in between is an insulatoro So, the charged plastic rod will cause thefirst metal ball to acquire charge (see above)sue to electron stealing.o However, nothing will happen to the secondmetal ball because no charge can betransferred via an insulator. - Metal rod exampleo Both metal balls will acquire a charge because electrons are valence electrons are free to flow through the connecting metal rod.o Equal distribution of charges across both spheres. Electrons came off leaving a netPLUS charge behindo Did not add protons, you took away
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