Slide 1: Chapter 17
Electric Potential
Slide 2: Units of Chapter 17 Electric Potential Energy and Potential Difference
Relation between Electric Potential and Electric Field
Equipotential Lines
The Electron Volt, a Unit of Energy
Electric Potential Due to Point Charges
Potential Due to Electric Dipole; Dipole Moment
Slide 3: Units of Chapter 17 Capacitance
Dielectrics
Storage of Electric Energy
Cathode Ray Tube: TV and Computer Monitors, Oscilloscope
The Electrocardiogram (ECG or EKG)
Slide 4: Electrostatic Potential Energy and Potential Difference When a force is “conservative” ie gravitational and the electrostatic force a potential energy can be defined
Change in electric potential energy is negative of work done by electric force:
Slide 5: Electrostatic Potential Energy and Potential Difference Electric potential is defined as potential energy per unit charge: Unit of electric potential: the volt (V).
1 V = I J/C
Slide 6: Electrostatic Potential Energy and Potential Difference Only changes in potential can be measured, allowing free assignment of V = 0.
Slide 7: Electrostatic Potential Energy and Potential Difference Analogy between gravitational and electrical potential energy:
Slide 8: Relation between Electric Potential and Electric Field Work is charge multiplied by potential Work is also force multiplied by distance
Slide 9: Relation between Electric Potential and Electric Field Therefore If the field is not uniform, it can be calculated at multiple points:
Slide 10: Equipotential Lines An equipotential is a line or surface over which the potential is constant.
Electric field lines are perpendicular to equipotentials.
The surface of a conductor is an equipotential.
Slide 11: Equipotential Lines of Dipole
Slide 12: If you cross contour lines especially perpendicular you will be changing your gravitational potential and very rapidly if the lines are close together
Slide 13: The Electron Volt, a Unit of Energy One electron volt (eV) is the energy gained by an electron moving through a potential difference of one volt.
Example 17-2 : Example 17-2 PE
Speed ve ?
Electric Field from Potential : Electric Field from Potential a) Find E field
Slide 16: 17.5 Electric Potential Due to Point Charges The electric potential due to a point charge can be derived using calculus.
Slide 17: 17.5 Electric Potential Due to Point Charges These plots show the potential due to (a) positive and (b) negative charge.
Slide 18: 17.5 Electric Potential Due to Point Charges Using potentials instead of fields can make solving problems much easier – potential is a scalar quantity, whereas the field is a vector.
Slide 19: 17.6 Potential Due to Electric Dipole; Dipole Moment The potential due to an electric dipole is just the sum of the potentials due to each charge, and can be calculated exactly.
Slide 20: 17.6 Potential Due to Electric Dipole; Dipole Moment Approximation for potential far from dipole: (17-6a)
Slide 21: Or, defining the dipole moment p = Ql, (17-6b) 17.6 Potential Due to Electric Dipole; Dipole Moment
Slide 22: 17.7 Capacitance A capacitor consists of two conductors that are close but not touching. A capacitor has the ability to store electric charge.
Slide 23: 17.7 Capacitance Parallel-plate capacitor connected to battery. (b) is a circuit diagram.
Slide 24: 17.7 Capacitance When a capacitor is connected to a battery, the charge on its plates is proportional to the voltage: (17-7) The quantity C is called the capacitance.
Unit of capacitance: the farad (F)
1 F = 1 C/V
Slide 25: 17.7 Capacitance The capacitance does not depend on the voltage; it is a function of the geometry and materials of the capacitor.
For a parallel-plate capacitor: (17-8)
Slide 26: 17.8 Dielectrics A dielectric is an insulator, and is characterized by a dielectric constant K.
Capacitance of a parallel-plate capacitor filled with dielectric: (17-9)
Slide 27: 17.8 Dielectrics Dielectric strength is the maximum field a dielectric can experience without breaking down.
Slide 28: 17.8 Dielectrics The molecules in a dielectric tend to become oriented in a way that reduces the external field.
Slide 29: 17.8 Dielectrics This means that the electric field within the dielectric is less than it would be in air, allowing more charge to be stored for the same potential.
Slide 30: 17.9 Storage of Electric Energy A charged capacitor stores electric energy; the energy stored is equal to the work done to charge the capacitor. (17-10)
Slide 31: 17.9 Storage of Electric Energy The energy density, defined as the energy per unit volume, is the same no matter the origin of the electric field: (17-11) The sudden discharge of electric energy can be harmful or fatal. Capacitors can retain their charge indefinitely even when disconnected from a voltage source – be careful!
Slide 32: 17.9 Storage of Electric Energy Heart defibrillators use electric discharge to “jump-start” the heart, and can save lives.
Slide 33: 17.10 Cathode Ray Tube: TV and Computer Monitors, Oscilloscope A cathode ray tube contains a wire cathode that, when heated, emits electrons. A voltage source causes the electrons to travel to the anode.
Slide 34: 17.10 Cathode Ray Tube: TV and Computer Monitors, Oscilloscope The electrons can be steered using electric or magnetic fields.
Slide 35: 17.10 Cathode Ray Tube: TV and Computer Monitors, Oscilloscope Televisions and computer monitors (except for LCD and plasma models) have a large cathode ray tube as their display. Variations in the field steer the electrons on their way to the screen.
Slide 36: 17.10 Cathode Ray Tube: TV and Computer Monitors, Oscilloscope An oscilloscope displays en electrical signal on a screen, using it to deflect the beam vertically while it sweeps horizontally.
Slide 37: 17.11 The Electrocardiogram (ECG or EKG) The electrocardiogram detects heart defects by measuring changes in potential on the surface of the heart.
Slide 38: Summary of Chapter 17 Electric potential energy: Electric potential difference: work done to move charge from one point to another
Relationship between potential difference and field:
Slide 39: Summary of Chapter 17 Equipotential: line or surface along which potential is the same
Electric potential of a point charge: Electric dipole potential:
Slide 40: Summary of Chapter 17 Capacitor: nontouching conductors carrying equal and opposite charge
Capacitance: Capacitance of a parallel-plate capacitor:
Slide 41: Summary of Chapter 17 A dielectric is an insulator
Dielectric constant gives ratio of total field to external field
Energy density in electric field: