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lect09-1

# lect09-1 - Physics 227 Lecture 9 Current Resistance...

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Physics 227: Lecture 9 Current, Resistance, Resistivity Lecture 8 review: Energy Stored = U = Q 2 /2C = CV 2 /2 Energy stored problems can be confusing - is charge or voltage held constant? For constant charge, adding a dielectric of constant K increases capacitance a factor K, decreases voltage a factor of K, and decreases energy stored a factor K. (For constant voltage, energy stored increases a factor of K.) Force on dielectrics: pulled into constant charge capacitor, pushed out of constant voltage capacitor. Monday, October 3, 2011

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Fields in Conductors Earlier we have said that there is no field inside a conductor. If there were a field, the electrons would be forced to move, and would do so until they arranged in a pattern that made the internal field vanish. But there has been a somewhat hidden assumption here. We have no energy source that continues to supply energy to move the electrons around. For example... + + + + + - - - - - Capacitor Connect a wire between the plates of a capacitor. There is a voltage across it. Electrons will move until the capacitor is neutral. Use instead a battery and electrons move for a long time. In a conductor, electric field current. Monday, October 3, 2011
Moving Charges In conductors and many semiconductors, the moving charges are the electrons. The positive ions (atoms missing an electron) are generally fixed. In some semiconductors, positive ``holes’’ are the charge carriers. In solutions and gases, the charge carriers can be positive charges, or negative charges, or both. The motion is not the smooth cartoon pictured. Electrons scatter from the more or less fixed atoms. You might picture a small drunk person weaving through a crowd of still large people. e - e - e - e - +V -V E Monday, October 3, 2011

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Current iClicker A. The electric field E is only nonzero outside the conductor, so the electron velocity is constant inside. B. When the electrons scatter, on average their velocity is 0, so they never get that fast between scatterings. C. The current does not increase when the electrons move faster. D. When the electrons scatter, some of them keep going backwards. E. The question is wrong - it does happen. If there is an E field, there is continually acceleration. The electrons must then accelerate and get increasing speed, leading to a continually increasing current.
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lect09-1 - Physics 227 Lecture 9 Current Resistance...

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