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SM_chapter20 - Electric Potential and Capacitance CHAPTER...

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545 Electric Potential and Capacitance CHAPTER OUTLINE 20.1 Potential Difference and Electric Potential 20.2 Potential Differences in a Uniform Electric Field 20.3 Electric Potential and Electric Potential Energy Due to Point Charges 20.4 Obtaining Electric Field from Electric Potential 20.5 Electric Potential Due to Continuous Charge Distributions 20.6 Electric Potential of a Charged Conductor 20.7 Capacitance 20.8 Combinations of Capacitors 20.9 Energy Stored in a Charged Capacitor 20.10 Capacitors with Dielectrics 20.11 Context Connection The Atmosphere as a Capacitor ANSWERS TO QUESTIONS Q20.1 When one object B with electric charge is immersed in the electric field of another charge or charges A , the system possesses electric potential energy. The energy can be measured by seeing how much work the field does on the charge B as it moves to a reference location. We choose not to visualize A ’s effect on B as an action-at-a- distance, but as the result of a two-step process: Charge A creates electric potential throughout the surrounding space. Then the potential acts on B to inject the system with energy. Q20.2 The potential energy increases. When an outside agent makes it move in the direction of the field, the charge moves to a region of lower electric potential. Then the product of its negative charge with a lower number of volts gives a higher number of joules. Keep in mind that a negative charge feels an electric force in the opposite direction to the field, while the potential is the work done on the charge to move it in a field per unit charge. Q20.3 To move like charges together from an infinite separation, at which the potential energy of the system of two charges is zero, requires work to be done on the system by an outside agent. Hence energy is stored, and potential energy is positive. As charges with opposite signs move together from an infinite separation, energy is released, and the potential energy of the set of charges becomes negative. Q20.4 If there were a potential difference between two points on the conductor, the free electrons in the conductor would move until the potential difference disappears. Q20.5 A sharp point in a charged conductor would imply a large electric field in that region. An electric discharge could most easily take place at that sharp point. Q20.6 Use a conductive box to shield the equipment. Any stray electric field will cause charges on the outer surface of the conductor to rearrange and cancel the stray field inside the volume it encloses.
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546 Electric Potential and Capacitance Q20.7 The grounding wire can be touched equally well to any point on the sphere. Electrons will drain away into the ground and the sphere will be left positively charged. The ground, wire, and sphere are all conducting. They together form an equipotential volume at zero volts during the contact.
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