Chapter 17 Physics: Principles with Applications, 7th edition …€¦ · 17.9 Storage of Electric...
Transcript of Chapter 17 Physics: Principles with Applications, 7th edition …€¦ · 17.9 Storage of Electric...
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Lecture PowerPoints
Chapter 17Physics: Principles withApplications, 7th edition
Giancoli
Chapter 17
Electric Potential
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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
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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)
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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:
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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.
17.1 Electric Potential Energy andPotential Difference
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Only changes in potential can be measured, allowingfree assignment of V = 0.
17.1 Electric Potential Energy andPotential Difference
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17.1 Electric Potential Energy andPotential Difference
Analogy between gravitational and electrical potentialenergy. Just as the more massive rock has morepotential energy, so does the larger charge:
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17.2 Relation between Electric Potential andElectric Field
Work is charge multiplied by potential:
Work is also force multiplied by distance:
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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
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17.3 Equipotential Lines and Surfaces
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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
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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.
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17.5 Electric Potential Due to Point Charges
The electric potential due to a point charge can bederived using calculus.
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17.5 Electric Potential Due to Point Charges
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These plots show the potentialdue to (a) positive and (b)negative charge.
17.5 Electric Potential Due to Point Charges
Using potentials instead of fields can make solvingproblems much easier—potential is a scalar quantity,whereas the field is a vector.
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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.
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17.6 Potential Due to Electric Dipole;Dipole Moment
Approximation for potentialfar from dipole:
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17.6 Potential Due to Electric Dipole;Dipole Moment
Or, defining the dipole moment p = Ql,
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17.7 Capacitance
A capacitor consists of two conductors that are close butnot touching. A capacitor has the ability to store electriccharge.
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17.7 Capacitance
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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
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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:
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17.8 Dielectrics
A dielectric is an insulator, and is characterized by adielectric constant K.
Capacitance of a parallel-plate capacitor filled withdielectric:
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Dielectric strength is themaximum field a dielectriccan experience withoutbreaking down.
17.8 Dielectrics
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The molecules in a dielectric tend to become oriented ina way that reduces the external field.
17.8 Dielectrics
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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
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A charged capacitor stores electric energy; the energystored is equal to the work done to charge the capacitor.
17.9 Storage of Electric Energy
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17.9 Storage of Electric Energy
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!
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17.9 Storage of Electric Energy
Heart defibrillators use electric discharge to “jump-start” the heart when its beats become irregular, and cansave lives.
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17.10 Digital; Binary Numbers; SignalVoltage
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Analog signal voltages vary continuously.
17.10 Digital; Binary Numbers; SignalVoltage
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Digital signals use binarynumbers to represent numericalvalues.
17.10 Digital; Binary Numbers; SignalVoltage
In order to convert an analog signal to digital, the signalmust be sampled. A higher sampling rate reproduces thesignal more precisely.
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17.10 Digital; Binary Numbers; SignalVoltage
Before it is sent to a loudspeaker or headset, a digitalaudio signal must be converted back to analog.
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17.10 Digital; Binary Numbers; SignalVoltage
Noise can easily corrupt ananalog signal; a digitalsignal is much lesssensitive to noise.
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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.
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17.11 TV and Computer Monitors: CRTs,Flat Screens
The electrons can be steered using electric ormagnetic fields.
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CRT monitors have a large cathode ray tube as theirdisplay. Variations in the field steer the electrons ontheir way to the screen.
17.11 TV and Computer Monitors: CRTs,Flat Screens
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Flat screens contain tiny pixels in red, green, and bluewhose brightness can be changed.
17.11 TV and Computer Monitors: CRTs,Flat Screens
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The array of pixels thencreates an image; thisexample has very lowresolution. HD screenshave 1080 × 1920 pixels.
17.11 TV and Computer Monitors: CRTs,Flat Screens
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The electrocardiogram detectsheart defects by measuringchanges in potential on thesurface of the heart.
17.12 Electrocardiogram (ECG or EKG)
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• 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
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• Equipotential: line or surface along which potential isthe same
• Electric potential of a point charge:
• Electric dipole potential drops off as 1/r2
Summary of Chapter 17
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• Capacitor: nontouching conductors carrying equaland opposite charge
• Capacitance:
• Capacitance of a parallel-plate capacitor:
Summary of Chapter 17
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• 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
Summary of Chapter 17
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