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Copyright 2010 Pearson Education, Inc.Lecture OutlineChapter 19

Physics, 4th EditionJames S. Walker

Chapter 19Electric Charges, Forces, and FieldsCopyright 2010 Pearson Education, Inc.2Units of Chapter 19 Electric Charge Insulators and Conductors Coulombs Law The Electric Field Electric Field Lines Shielding and Charging by Induction Electric Flux and Gausss LawCopyright 2010 Pearson Education, Inc.319-1 Electric ChargeThe effects of electric charge were first observed as static electricity:

After being rubbed on a piece of fur, an amber rod acquires a charge and can attract small objects.Copyright 2010 Pearson Education, Inc.419-1 Electric ChargeCharging both amber and glass rods shows that there are two types of electric charge; like charges repel and opposites attract.

Copyright 2010 Pearson Education, Inc.519-1 Electric Charge

All electrons have exactly the same charge; the charge on the proton (in the atomic nucleus) has the same magnitude but the opposite sign:Copyright 2010 Pearson Education, Inc.619-1 Electric ChargeThe electrons in an atom are in a cloud surrounding the nucleus, and can be separated from the atom with relative ease.

Copyright 2010 Pearson Education, Inc.719-1 Electric ChargeWhen an amber rod is rubbed with fur, some of the electrons on the atoms in the fur are transferred to the amber:

Copyright 2010 Pearson Education, Inc.819-1 Electric ChargeWe find that the total electric charge of the universe is a constant:Electric charge is conserved.Also, electric charge is quantized in units of e.The atom that has lost an electron is now positively charged it is a positive ionThe atom that has gained an electron is now negatively charged it is a negative ionCopyright 2010 Pearson Education, Inc.9

19-1 Electric ChargeSome materials can become polarized this means that their atoms rotate in response to an external charge. This is how a charged object can attract a neutral one.Copyright 2010 Pearson Education, Inc.1019-2 Insulators and ConductorsConductor: A material whose conduction electrons are free to move throughout. Most metals are conductors.Insulator: A material whose electrons seldom move from atom to atom. Most insulators are non-metals.Copyright 2010 Pearson Education, Inc.1119-2 Insulators and ConductorsIf a conductor carries excess charge, the excess is distributed over the surface of the conductor.

Copyright 2010 Pearson Education, Inc.1219-2 Insulators and ConductorsSemiconductors have properties intermediate between conductors and insulators; their properties change with their chemical composition.Photoconductive materials become conductors when light shines on them.Copyright 2010 Pearson Education, Inc.1319-3 Coulombs LawCoulombs law gives the force between two point charges:

The force is along the line connecting the charges, and is attractive if the charges are opposite, and repulsive if the charges are like.Copyright 2010 Pearson Education, Inc.1419-3 Coulombs Law

The forces on the two charges are action-reaction forces.Copyright 2010 Pearson Education, Inc.1519-3 Coulombs LawIf there are multiple point charges, the forces add by superposition.

Copyright 2010 Pearson Education, Inc.1619-3 Coulombs LawCoulombs law is stated in terms of point charges, but it is also valid for spherically symmetric charge distributions, as long as the distance is measured from the center of the sphere.Copyright 2010 Pearson Education, Inc.1719-4 The Electric FieldDefinition of the electric field:

Here, q0 is a test charge it serves to allow the electric force to be measured, but is not large enough to create a significant force on any other charges.Copyright 2010 Pearson Education, Inc.18

19-4 The Electric FieldIf we know the electric field, we can calculate the force on any charge:

The direction of the force depends on the sign of the charge in the direction of the field for a positive charge, opposite to it for a negative one.Copyright 2010 Pearson Education, Inc.19

19-4 The Electric FieldThe electric field of a point charge points radially away from a positive charge and towards a negative one.Copyright 2010 Pearson Education, Inc.2019-4 The Electric FieldJust as electric forces can be superposed, electric fields can as well.

Copyright 2010 Pearson Education, Inc.2119-4 The Electric Field

Copyright 2010 Pearson Education, Inc.2219-4 The Electric Field

Copyright 2010 Pearson Education, Inc.2319-4 The Electric Field

Copyright 2010 Pearson Education, Inc.2419-5 Electric Field LinesElectric field lines are a convenient way of visualizing the electric field.Rules:Point in the direction of the field vector at every pointStart at positive charges or infinityEnd at negative charges or infinityAre more dense where the field is strongerCopyright 2010 Pearson Education, Inc.2519-5 Electric Field Lines

The charge on the right is twice the magnitude of the charge on the left (and opposite in sign), so there are twice as many field lines, and they point towards the charge rather than away from it.Copyright 2010 Pearson Education, Inc.2619-5 Electric Field LinesCombinations of charges. Note that, while the lines are less dense where the field is weaker, the field is not necessarily zero where there are no lines. In fact, there is only one point within the figures below where the field is zero can you find it?

Copyright 2010 Pearson Education, Inc.2719-5 Electric Field LinesA parallel-plate capacitor consists of two conducting plates with equal and opposite charges. Here is the electric field:

Copyright 2010 Pearson Education, Inc.2819-5 Electric Field Lines

Copyright 2010 Pearson Education, Inc.2919-5 Electric Field Lines

Copyright 2010 Pearson Education, Inc.30

At equilibrium under electrostatic conditions, any excess charge resides on the surface of a conductor.At equilibrium under electrostatic conditions, the electric field is zero at any point within a conducting material.Copyright 2010 Pearson Education, Inc.3119-6 Shielding and Charge by Induction

The conductor shields any charge within it from electric fields created outside the conductor.The electric field just outside the surface of a conductor is perpendicular to the surface at equilibrium under electrostatic conditions.Copyright 2010 Pearson Education, Inc.3219-6 Shielding and Charge by InductionThe electric field is stronger where the surface is more sharply curved.

Copyright 2010 Pearson Education, Inc.3319-6 Shielding and Charge by Induction

A charge +q is suspended at the center of a hollow, electrically neutral, spherical conductor. This charge induces

a charge of q on the interior surface and

(b) a charge of +q on the exterior surface of the conductor.Copyright 2010 Pearson Education, Inc.3419-6 Shielding and Charge by InductionA conductor can be charged by induction, if there is a way to ground it.

This allows the like charges to leave the conductor; if the conductor is then isolated before the rod is removed, only the excess charge remains.Copyright 2010 Pearson Education, Inc.3519-7 Electric Flux and Gausss LawElectric flux is a measure of the electric field perpendicular to a surface:

Electric flux= product of electric field at any point on the Gaussian surface and the area of the surfaceCopyright 2010 Pearson Education, Inc.36 = 0o = 90o = ~30oCopyright 2010 Pearson Education, Inc.19-7 Electric Flux and Gausss LawGausss law states that the electric flux through a closed surface is proportional to the charge enclosed by the surface:

Copyright 2010 Pearson Education, Inc.3819-7 Electric Flux and Gausss LawGausss law can be used to find the electric field in systems with simple configurations.

Outside the shell, the Gaussian surface encloses all of the charge.(b) Inside the shell, the Gaussian surface encloses no charge.

Copyright 2010 Pearson Education, Inc.39Summary of Chapter 19 Electrons have a negative charge, and protons a positive charge, of magnitude

Unit of charge: Coulomb, C Charge is conserved, and quantized in units of e Insulators do not allow electrons to move between atoms; conductors allow conduction electrons to flow freelyCopyright 2010 Pearson Education, Inc.40Summary of Chapter 19 The force between electric charges is along the line connecting them Like charges repel, opposites attract Coulombs law gives the magnitude of the force:

Forces exerted by several charges add as vectorsCopyright 2010 Pearson Education, Inc.41Summary of Chapter 19 A spherical charge distribution behaves from the outside as though the total charge were at its center Electric field is the force per unit charge; for a point charge:

Electric fields created by several charges add as vectorsCopyright 2010 Pearson Education, Inc.42Summary of Chapter 19 Electric field lines help visualize the electric field Field lines point in the direction of the field; start on + charges or infinity; end on charges or infinity; are denser where is greater Parallel-plate capacitor: two oppositely charged, conducting parallel plates Excess charge on a conductor is on the surface Electric field within a conductor is zero (if charges are static)

Copyright 2010 Pearson Education, Inc.43Summary of Chapter 19 A conductor can be charged by induction Conductors can be grounded Electric flux through a surface:

Gausss law:

Copyright 2010 Pearson Education, Inc.44