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
Slide 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 Define property and type of force Slide 4-19
Slide 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 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 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 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 Charging objects with insulators and conductors Demonstrations Slide 20-3
Slide Polarization model Insulator and Conductor simulation Colorado Phet and_Circuits and_Circuits Slide 20-3
Slide Charge Diagrams: Polarization, Grounding, and Charging Insulator and Conductor simulation Grounding Simulation Shocked by the door – John Travoltage Grounding a Positively charged-electroscope Grounding a negatively charged-electroscopes Balloons and Charge Slide 20-3
Slide 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 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