Chapter 20

Lecture Presentation

Electric Fields and Forces

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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

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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

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• 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 PreviewLooking Back: Vectors and Components

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Chapter 20 PreviewStop 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

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General Force Model

Newton 0th LawObjects 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 LawAn 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.

ax = 0 m/s2 IFF Sum Fx = 0 N (IFF => if and only if)

Newton's 3rd LawRecall 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

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Net Force Model

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Newton's 2nd Lawacceleration of an object = sum of forces acting on that object / the mass of the object

ax = 1/m * (Sum Fx)

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Lists types of forces you used in Physics 1

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Section 20.1 Charges and Forces

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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

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X-force Demonstrations

• Balloon and wall/ceiling

• Rubbing the rod• Rod and Paper• Rod and Rod

• Attraction • Repulsion

• Comb and Hair

• Transfer of Charge

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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?

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Results of Sticky Tape Warm-up: Part 1

Top Tape Bottom Tape Paper Foil

Top Tape Repulsion Attraction Attraction Attraction

Bottom Tape Attraction Repulsion Attraction Attraction

Paper No interaction No Interaction

Foil No 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.

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Define property and type of force

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Summarize what we know about Electric Forces and Charge

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Write ideas on BoardIdentify • Which are correct?• Which are incorrect?• Which are duplicates?

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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?

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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.

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Charge Conservation

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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.

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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.

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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

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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.

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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.

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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.

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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

• Fe gets weaker with distance:  Fe α 1/r2

• Fe between charged tapes are > Fe between charged tapes & neutral objects

• Rubbing causes some objects to be charged by charge separation

• Charge can be transferred by contact

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Charging objects with insulators and conductors • Demonstrations

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Polarization model • Insulator and Conductor simulation

http://regentsprep.org/Regents/physics/phys03/ainsvscon/default.htm

• Colorado Phet

http://phet.colorado.edu/simulations/index.php?cat=Electricity_Magnets_and_Circuits

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Charge Diagrams: Polarization, Grounding, and Charging

• Insulator and Conductor simulation

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

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Triboelectric series => Triboelectric EffectWhen 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

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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?

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