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General Physics1 E1 Basic Electrostatics

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Electrostatics Complete the tutorial on electrostatics (both sides of handout) Tape activity www.sos.siena.edu/~rfinn/phys140s09/ tape.pdf

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Electrostatics electric charges that are not moving Definition of new symbols e - = electron p = proton n = neutron e + = positron

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Nature of charge Amber is a natural substance that can be naturally charged Few naturally occurring objects can be electrically charged Static electricity is easily shown with plastic, rubber, and synthetic fibers Electrical charges behave like positive and negative numbers

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Electric Force Similar form as gravitational force (weird, huh?), except positive and negative charges Like charges repel; opposite charge attract

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A proton is located just north of an electron. The net force that the electron feels due to the proton is in which direction? 1. North 2. South 3. West 4. East

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Coulomb’s Law - magnitude k = 9.0 x 10 9 N m 2 /C 2 Coulomb’s contant q 1, q 2 = charges measured in Coulombs (C) 1 C is a lot of charge! r = distance between q 1 and q 2

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Charge Carriers The electron The electron 1 e - = -1.602 x 10 -19 C smallest unit of charge, also referred to as e negatively charged Charge of 1 e - is -1e mass = 9.11 x 10 -31 kg The proton The proton 1 p = 1.602 x 10 -19 C positively charged Charge of 1 p is +1e mass = 1.67 x 10 -27 kg Charge is quantized!

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Coulomb’s Law with different constants (more relevant to future chapters) ε 0 =permittivity of free space

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Illustrations of Coulomb’s Law Comb & Paper Balloon on wall

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Illustrations of Coulomb’s Law Electroscope inductionconduction

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Charge Conservation The total charge of an isolated system is conserved. Rubbing a balloon on hair Triboelectric series Decreasing tendency to lose electrons

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Conductors and Insulators Charges aren’t free to move around in an insulator Why they may be charged If charge is left on them due to rubbing another material, the charge can’t go anywhere Examples – amber, plastics, synthetic materials Charges are free to move very easily in materials called conductors Examples – metals, salty water

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Conductor versus Insulator Conductor Insulator e - ’s travel charge separation within molecules

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Charging by Induction

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Net Force from Multiple Charges An electrostatic force exists between each pair of charges according to Coulomb’s law Add components of forces to get net force when adding multiple charges

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Adding Vectors Magnitude and direction of Net Force given by:

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The Atomic Model

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How objects become charged A macroscopic object is composed of a huge number of tiny atoms Each atom has a tiny nucleus (includes protons and neutrons) surrounded by a cloud of electrons Equal number of electrons and protons Avogadro’s number of protons or neutrons have a mass of ~ 1 g. Total mass of electrons is negligible Most of the volume of an atom is occupied by the electron clouds

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Coulomb’s Law – examples Review Example E1.2 Determine direction of force Review Example E1.3 Determine direction and magnitude of force

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Van de Graff Generator Demonstration In-class Pie plates Ion motor Extra Credit - see schedule and moodle

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Group Problems E1S.3 E1S.5 E1S.10

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Charges in a line What is the net force on the 48 C charge?

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Charges in a line - Solution Q1Q1 Q2Q2 Q3Q3 +x

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Charges in a line - Solution Q1Q1 Q2Q2 Q3Q3 +x Do signs make sense?!!

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Charges in a square Q1Q1 Q4Q4 Q3Q3 Q2Q2 Find net force on Q 1 +y +x

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Charges in a square Do signs of forces make sense? Does trig make sense? The x and y components of the net force are given by: Q1Q1 Q4Q4 Q3Q3 Q2Q2

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Charges in a square

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