# Chapter 20 Lecture Presentation Electric Fields and Forces © 2015 Pearson Education, Inc.

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Chapter 20 Lecture Presentation Electric Fields and Forces © 2015 Pearson Education, Inc.

Slide 20-2 Suggested Videos for Chapter 20 Prelecture Videos Charges and Forces Coulomb’s Law Electric Fields Class Videos Charges and Forces: Demonstrations Charges and Forces: Warm-Ups Electric Field Video Tutor Solutions Electric Fields and Forces Video Tutor Demos Charged Rod and Aluminum Can Electroscope in Conducting Shell © 2015 Pearson Education, Inc.

Slide 20-3 Suggested Simulations for Chapter 20 ActivPhysics 11.1–11.6, 11.9, 11.10 PhETs John Travoltage Balloons and Static Electricity Conductivity Charges and Fields Electric Field Hockey Microwaves Optical Tweezers and Applications Electric Field of Dreams © 2015 Pearson Education, Inc.

Slide 20-4 In Sections 3.1–3.3, you studied how a vector could be resolved into its component vectors. Electric forces and electric fields are vectors, so you will need to use vector components to solve electric force and field problems. You learned that a vector can be represented as the sum of its component vectors and. Chapter 20 Preview Looking Back: Vectors and Components © 2015 Pearson Education, Inc.

Slide 20-5 Chapter 20 Preview Stop to Think The tension in the rope is 100 N. Given that sin 30° = 0.50 and cos 30° = 0.87, the x- and y-components of the tension are A.–87 N, 50 N B.87 N, 50 N C.–50 N, 87 N D.50 N, –87 N E.87 N, –50 N © 2015 Pearson Education, Inc.

Slide 20-6 General Force Model Newton 0th Law Objects are dumb - They have no memory of the past and cannot predict the future. Objects only know what is acting directly on them right now Newton's 1st Law An object that is at rest will remain at rest and an object that is moving will continue to move in a straight line with constant speed, if and only if the sum of the forces acting on that object is zero. a x = 0 m/s 2 IFF Sum F x = 0 N (IFF => if and only if) Newton's 3rd Law Recall that a force is an interaction between two objects. If object A exerts a force on object B then object B exerts a force on object that is in the opposite direction, equal in magnitude, and of the same type. Visualizations: Force Diagrams System Schema

Slide 20-7 Net Force Model Slide 4-19 Newton's 2nd Law acceleration of an object = sum of forces acting on that object / the mass of the object a x = 1/m * (Sum F x )

Slide 20-8 Lists types of forces you used in Physics 1 Slide 4-19

Section 20.1 Charges and Forces © 2015 Pearson Education, Inc.

Slide 20-10 Experimenting with Charges The major tools in a modest laboratory for studying electricity include: A number of plastic and glass rods, each several inches long. These can be held in your hand or suspended by threads from a support. Pieces of wool and silk Small metal spheres, an inch or two in diameter, on wood stands © 2015 Pearson Education, Inc.

Slide 20-11 X-force Demonstrations Balloon and wall/ceiling Rubbing the rod Rod and Paper Rod and Rod Attraction Repulsion Comb and Hair Transfer of Charge © 2015 Pearson Education, Inc.

Slide 20-12 X-force Demonstrations What is the nature of this force? Is it familiar or a new type of force? How does magnitude of x-force compare to gravity? Do magnets have x-property? How do we tell if an object is charged? © 2015 Pearson Education, Inc.

Slide 20-13 Results of Sticky Tape Warm-up: Part 1 Top TapeBottom TapePaperFoil Top TapeRepulsionAttraction Bottom TapeAttractionRepulsionAttraction PaperNo interactionNo Interaction FoilNo Interaction Table of interactions between Top Tape, Bottom Tape, Foil strips, and Paper strips Also, the effect is stronger if there is more charge and the interaction, i.e. the force gets weaker as distance between the objects increases. The interaction between foil and the charged tapes was stronger than the interaction of paper with the charged tapes.

Slide 20-14 Define property and type of force Slide 4-19

Slide 20-15 Summarize what we know about Electric Forces and Charge Slide 4-19 Write ideas on Board Identify Which are correct? Which are incorrect? Which are duplicates?

Slide 20-16 Historical models for electric charge Fluid models of electric charge Too much electric fluid in an object makes it positively charged and too little makes it negatively charged. Particle model of electric charge Electric charge is carried by microscopic particles. Removing or adding these particles to an object changes the charge of the object. How could we test these models? © 2014 Pearson Education, Inc.

Slide 20-17 Contemporary model for electric charge Two objects start as neutral— the total electric charge of each is zero. During rubbing, one object gains electrons and becomes negatively charged. The other object loses an equal number of electrons and with this deficiency of electrons becomes positively charged. © 2014 Pearson Education, Inc.

Slide 20-18 Charge Conservation © 2015 Pearson Education, Inc.

Slide 20-19 Charges, Atoms, and Molecules An atom has a dense, positively charged nucleus, containing positively charged protons and neutral neutrons. The nucleus is surrounded by the much-less-massive orbiting negatively charged electrons that form an electron cloud. Charge, like mass, is an inherent property of electrons and protons. © 2015 Pearson Education, Inc.

Slide 20-20 An Atomic View of Charging Electrons and protons are the basic charges in ordinary matter. There are no other sources of charge. An object is charged if it has an unequal number of protons and electrons. Most macroscopic objects have an equal number of protons and electrons. Such objects are electrically neutral. © 2015 Pearson Education, Inc.

Slide 20-21 An Atomic View of Charging Objects gain a positive charge not by gaining protons, but by losing electrons. Protons are extremely tightly bound within the nucleus, but electrons are bound much more loosely. Rubbing causes charge separation – takes energy to separate charges © 2015 Pearson Education, Inc.

Slide 20-22 Conceptual Exercise 14.2 You pull your sweater and shirt off together, and then pull them apart. You notice that they attract each other—a phenomenon called "static" in everyday life. Explain the mechanism behind this attraction and suggest an experiment to test your explanation. © 2014 Pearson Education, Inc.

Slide 20-23 How do charged objects and neutral objects interact? Part A A small ball with zero net charge is positively charged on one side, and equally negatively charged on the other side. The ball is placed near a positive point charge as shown. Would the ball be attracted toward, repelled from, or unaffected by the positive point charge? Explain. Is your answer consistent with what you observed in the tape activity? Explain. Through careful observations of physical phenomenon, scientists develop models or mental pictures to account for what is observed. These scientific models can also be used to predict physical behavior. From observations of electrical we can develop a model for electric charge.

Slide 20-24 How do charged objects and neutral objects interact? Part B Use your model for electric charge to account for the electrical attraction between a charged tape and an uncharged metal ball. As part of your answer, draw a sketch of the charge distribution on the tape and ball both before and after they are brought near one another.

Slide 20-25 Charge Model: Electric forces can be attractive or repulsive Objects with the same sign of charge repel each other Objects with the opposite sign of charge attract each other Neutral objects are polarized by charged objects which creates attractive forces between them There are two kinds of charges, positive (protons) and negative (electrons). In solids, electrons are charge carriers (protons are 2000 time more massive). A charged object has a deficit of electrons (+) or a surplus of electrons (-). Neutral objects have equal numbers of + and – charges F e gets weaker with distance: F e α 1/r 2 F e between charged tapes are > F e between charged tapes & neutral objects Rubbing causes some objects to be charged by charge separation Charge can be transferred by contact

Slide 20-26 Charging objects with insulators and conductors Demonstrations Slide 20-3

Slide 20-27 Polarization model Insulator and Conductor simulation http://regentsprep.org/Regents/physics/phys03/ainsvscon/default.htm http://regentsprep.org/Regents/physics/phys03/ainsvscon/default.htm Colorado Phet http://phet.colorado.edu/simulations/index.php?cat=Electricity_Magnets_ and_Circuits http://phet.colorado.edu/simulations/index.php?cat=Electricity_Magnets_ and_Circuits Slide 20-3

Slide 20-28 Charge Diagrams: Polarization, Grounding, and Charging Insulator and Conductor simulation http://regentsprep.org/Regents/physics/phys03/ainsvscon/default.htm http://regentsprep.org/Regents/physics/phys03/ainsvscon/default.htm Grounding Simulation Shocked by the door – John Travoltage Grounding a Positively charged-electroscope Grounding a negatively charged-electroscopes Balloons and Charge http://phet.colorado.edu/en/simulation/balloons Slide 20-3

Slide 20-29 Triboelectric series => Triboelectric Effect When two of the following materials are rubbed together under ordinary circumstances, the top listed material becomes positively charged and the lower listed material becomes negatively charged. MORE POSITIVE 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

Slide 20-30 A Question of Charge (Warm-up for next class) If I give you an object, how can you tell if it is positively or negatively charged? (assume a top tape is + charged.) Suppose a newspaper article claims that scientists in California have found a third type of charged object. What experiments would you perform to check their claim? Slide 20-3