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3 - Potential of a Charged Disk

# 3 - Potential of a Charged Disk - MasteringPhysics...

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Potential and Field Due: 11:55pm on Monday, April 11, 2011 Note: You will receive no credit for late submissions. To learn more, read your instructor's Grading Policy [Switch to Standard Assignment View ] Resistance and Geometry Resistivity is the property that measures a material's ability to provide "obstacles" to the flow of electrons caused by an external electric field. Such a flow of electrons is called an electric current . Resistivity is defined as the ratio of the magnitude of the electric field to the magnitude of the current density : . Resistance is a closely related concept: It is a measure of an object's ability to provide "obstacles" to electric current. The resistance of a conductor (often, a metal wire of some sort) is defined as the ratio of the voltage between the ends of the wire to the current through the wire: . It has been discovered that for many conductors, this ratio tends to stay fairly constant; this empirical fact is known as Ohm's law . Ohm's law is not a fundamental law of physics; it is valid under certain conditions (mostly, metal conductors in a narrow range of temperatures). Still, Ohm's law is a very useful tool, since many circuits operate under these conditions. The unit of resistance is the ohm (its symbol is ). One ohm is the resistance of a conductor in which a current of one ampere is produced by a potential of one volt across its terminals. Therefore, 1 ohm is equal to one volt per ampere. The resistance of a conducting object depends on several factors: the resistivity of the material, the geometry of the object, and the temperature . For a wire with resistivity , length , and cross-sectional area , resistance can be found as . The questions below will help you better understand the concept of resistance and practice using the equations introduced above. The first series of questions refers to the following table, describing the voltage across and a current through several conductors, labeled A through E. ConductorVoltage (V)Current (A) A 12 2 B 0.1 0.04 C 100 20 D 20 100 E 12 0.04 Part A Which conductor in the table has the greatest resistance? ANSWER: Correct A B C D E There is not enough information given. Part B Which conductor in the table has the greatest resistivity? ANSWER: A B C D E There is not enough information given. [ Print ] Page 1 of 12 MasteringPhysics: Assignment Print View 4/11/2011 http://session.masteringphysics.com/myct/assignmentPrintView?assignmentID=1571100

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Recognizing when there is insufficient information to draw conclusions is an important skill for a scientist. It does take some courage to choose "not enough information" as an answer; however, you should trust your judgment. Many mistakes have been made when scientists (and nonscientists) rushed to conclusions without enough evidence to support them.
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