chapter 17 physics: principles with applications, 7th edition …€¦ · 17.9 storage of electric...

© 2014 Pearson Education, Inc.

This work is protected by United States copyright laws and is provided solely forthe use of instructors in teaching their courses and assessing student learning.Dissemination or sale of any part of this work (including on the World Wide Web)will destroy the integrity of the work and is not permitted. The work and materialsfrom it should never be made available to students except by instructors usingthe accompanying text in their classes. All recipients of this work are expected toabide by these restrictions and to honor the intended pedagogical purposes andthe needs of other instructors who rely on these materials.

Lecture PowerPoints

Chapter 17Physics: Principles withApplications, 7th edition

Giancoli

Chapter 17

Electric Potential

© 2014 PearsonEducation, Inc.

Contents of Chapter 17

• Electric Potential Energy and Potential Difference

• Relation between Electric Potential and Electric Field

• Equipotential Lines and Surfaces

• The Electron Volt, a Unit of Energy

• Electric Potential Due to Point Charges

• Potential Due to Electric Dipole; Dipole Moment


Contents of Chapter 17

• Capacitance

• Dielectrics

• Storage of Electric Energy

• Digital; Binary Numbers; Signal Voltage

• TV and Computer Monitors: CRT, Flat Screens

• Electrocardiogram (ECG or EKG)


17.1 Electric Potential Energy andPotential Difference

The electrostatic force isconservative—potential energycan be defined

Change in electric potentialenergy is negative of work doneby electric force:


(17-1)

Electric potential is defined as potential energy per unitcharge; analogous to definition of electric field as forceper unit charge:

Unit of electric potential: the volt (V).

1 V = 1 J/C.



(17-2a)

Only changes in potential can be measured, allowingfree assignment of V = 0.



(17-2b)


Analogy between gravitational and electrical potentialenergy. Just as the more massive rock has morepotential energy, so does the larger charge:


17.2 Relation between Electric Potential andElectric Field

Work is charge multiplied by potential:

Work is also force multiplied by distance:


Solving for the field,

In general, the electric field in a given direction at anypoint in space is equal to the rate at which the electricpotential decreases over distance in that direction.

17.2 Relation between Electric Potential andElectric Field


(17-4b)

17.3 Equipotential Lines and Surfaces


An equipotential is a line or surfaceover which the potential is constant.

Electric field lines are perpendicularto equipotentials.

The surface of a conductor is anequipotential.

17.3 Equipotential Lines and Surfaces


Equipotential lines of an electric dipole:

17.4 The Electron Volt, a Unit of Energy

One electron volt (eV) is the energy gained by anelectron moving through a potential difference ofone volt.


17.5 Electric Potential Due to Point Charges

The electric potential due to a point charge can bederived using calculus.


(17-5)



These plots show the potentialdue to (a) positive and (b)negative charge.


Using potentials instead of fields can make solvingproblems much easier—potential is a scalar quantity,whereas the field is a vector.


17.6 Potential Due to Electric Dipole;Dipole Moment

The potential due to an electric dipole is just thesum of the potentials due to each charge, and canbe calculated exactly.



Approximation for potentialfar from dipole:


(17-6a)


Or, defining the dipole moment p = Ql,


(17-6b)

17.7 Capacitance

A capacitor consists of two conductors that are close butnot touching. A capacitor has the ability to store electriccharge.


17.7 Capacitance


Parallel-plate capacitor connected to battery. (b) is acircuit diagram.

17.7 Capacitance

When a capacitor is connected to a battery, the chargeon its plates is proportional to the voltage:

The quantity C is called the capacitance.

Unit of capacitance: the farad (F)

1 F = 1 C/V


(17-7)

17.7 Capacitance

The capacitance does not depend on the voltage; it is afunction of the geometry and materials of thecapacitor.

For a parallel-plate capacitor:


(17-8)

17.8 Dielectrics

A dielectric is an insulator, and is characterized by adielectric constant K.

Capacitance of a parallel-plate capacitor filled withdielectric:


(17-9)

Dielectric strength is themaximum field a dielectriccan experience withoutbreaking down.

17.8 Dielectrics


The molecules in a dielectric tend to become oriented ina way that reduces the external field.

17.8 Dielectrics


This means that the electric field within the dielectric isless than it would be in air, allowing more charge to bestored for the same potential.

17.8 Dielectrics


A charged capacitor stores electric energy; the energystored is equal to the work done to charge the capacitor.

17.9 Storage of Electric Energy


(17-10)


The energy density, defined as the energy per unitvolume, is the same no matter the origin of the electricfield:

The sudden discharge of electric energy can be harmfulor fatal. Capacitors can retain their charge indefinitelyeven when disconnected from a voltage source—becareful!


(17-11)


Heart defibrillators use electric discharge to “jump-start” the heart when its beats become irregular, and cansave lives.


17.10 Digital; Binary Numbers; SignalVoltage


Analog signal voltages vary continuously.



Digital signals use binarynumbers to represent numericalvalues.


In order to convert an analog signal to digital, the signalmust be sampled. A higher sampling rate reproduces thesignal more precisely.



Before it is sent to a loudspeaker or headset, a digitalaudio signal must be converted back to analog.



Noise can easily corrupt ananalog signal; a digitalsignal is much lesssensitive to noise.


17.11 TV and Computer Monitors: CRTs,Flat Screens

A cathode ray tubecontains a wire cathodethat, when heated, emitselectrons.A voltage source causesthe electrons to travel tothe anode.



The electrons can be steered using electric ormagnetic fields.


CRT monitors have a large cathode ray tube as theirdisplay. Variations in the field steer the electrons ontheir way to the screen.



Flat screens contain tiny pixels in red, green, and bluewhose brightness can be changed.



The array of pixels thencreates an image; thisexample has very lowresolution. HD screenshave 1080 × 1920 pixels.



The electrocardiogram detectsheart defects by measuringchanges in potential on thesurface of the heart.

17.12 Electrocardiogram (ECG or EKG)


• Electric potential is potential energy per unit charge:

• Electric potential difference: work done to movecharge from one point to another

• Relationship between potential difference and field:

Summary of Chapter 17


(17-2a)

(17-4a)

• Equipotential: line or surface along which potential isthe same

• Electric potential of a point charge:

• Electric dipole potential drops off as 1/r2



(17-5)

• Capacitor: nontouching conductors carrying equaland opposite charge

• Capacitance:

• Capacitance of a parallel-plate capacitor:



(17-7)

(17-8)

• A dielectric is an insulator

• Dielectric constant gives ratio of total field to externalfield

• Energy density in electric field:

• Digital electronics convert analog signal to digitalapproximation using binary numbers



(17-11)

chapter 17 physics: principles with applications, 7th edition …€¦ · 17.9 storage of electric...

Documents