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Chapter 17 Electric Potential Warm Up • When a battery is connected to a capacitor, why do the plates acquire charges of the same magnitude? Closure • When a battery is connected to a capacitor, why do the plates acquire charges of the same magnitude? Objectives: The students will be able to: •Explain how and why capacitance is effected when adding a dielectric. •Explain what is meant by an electric dipole and determine the magnitude of the electric dipole moment between two point charges. •Solve problems involving dielectrics. 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) 17.8 Dielectrics Dielectric strength is the maximum field a dielectric can experience without breaking down. Water as a Dielectric DIELECTRIC CONSTANT: K = C / Co = ratio of the capacitances V = Vo / K Effect of a dielectric between the plates of a parallel plate capacitor. Note – the charge is constant ! A dielectric is added between the plates of a charged capacitor (battery not connected): Q = Qo, therefore Q = C V and Qo = Co Vo Co Vo = C V, and if Vo decreases to V, Co must increase to C to keep equation balanced, and V = Vo Co/C Definition of DIELECTRIC CONSTANT: K = C / Co = ratio of the capacitances V = Vo / K C 2012 J. F. Becker Video 17.8 Dielectrics The molecules in a dielectric tend to become oriented in a way that reduces the external field. The charges induced on the surface of the dielectric (insulator) reduce the electric field. “Polarization” of a dielectric in an electric field E gives rise to thin layers of bound charges on the dielectric’s surfaces, creating surface charge densities +si and –si. A neutral sphere B in the electric field of a charged sphere A is attracted to the charged sphere because of polarization. 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. “Polarization” of a dielectric in an electric field E. 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) 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! 17.9 Storage of Electric Energy Heart defibrillators use electric discharge to “jump-start” the heart, and can save lives. 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. 17.10 Cathode Ray Tube: TV and Computer Monitors, Oscilloscope The electrons can be steered using electric or magnetic fields. 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. 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. 17.11 The Electrocardiogram (ECG or EKG) Video clip The electrocardiogram detects heart defects by measuring changes in potential on the surface of the heart. #43 page 490 What is the capacitance of a pair of circular plates with a radius of 5.0 cm separated by 3.2 mm of mica? #44 page 490 A 3500-pF air gap capacitor is connected to a 22-V battery. If a piece of mica is placed between the plates, how much charge will flow from the battery? #47 on page 490 A cardiac defibrillator is used to shock a heart that is beating erratically. A capacitor in this device is charged to 5.0 kV and stores 1200 J of energy. What is its capacitance? Homework: #42 and #46 Page 490 Kahoot 17-7 through 17-8 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: Summary of Chapter 17 • Equipotential: line or surface along which potential is the same • Electric potential of a point charge: • Electric dipole potential: Summary of Chapter 17 • Capacitor: nontouching conductors carrying equal and opposite charge •Capacitance: • Capacitance of a parallel-plate capacitor: Summary of Chapter 17 • A dielectric is an insulator • Dielectric constant gives ratio of total field to external field • Energy density in electric field: