current and resistance 1

current and resistance 1 - Georg Simon Ohm(1789-1854...

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1 1 Georg Simon Ohm (1789-1854) Current and Resistance – I "a professor who preaches such heresies is unworthy to teach science.” Prussian minister of education 1830 This lecture: HRW 26.1-26.5 For next time: HRW 26.6-26.9 Today Moving charges through conductors Current and current density Resistance and resistivity Conservation of charge in circuits 2 Where are the electrons in an isolated piece of copper wire in a static electric field? Naïve view: Copper is a conductor Ä electrons are free to move. They move until E = 0 inside the wire. Once E=0, there is no more motion. Reality: Electrons are hot Ä they always move at ~10 6 m/s !! Their directions are randomized after frequent collisions with vibrating nuclei. Conductors in electrostatic equilibrium E = 0 only on average! 3 Non-electrostatic conductors E Suppose V is set up across a copper wire, e.g., by a battery. Then E 0 in the wire . Electrons drift toward increasing potential, much more slowly than their random motion. Electron drift gives a net flow of charge, a current. dq I = , q = I dt dt () Coulomb Units: [i] = Ampere A second 0 +V As electrons move to cancel the field, the battery replenishes them to maintain the field. I Current is a scalar whose sign indicates direction. + (-) charge flows in the + (-) I direction.

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This note was uploaded on 04/05/2008 for the course PHYS 212L taught by Professor Huang,tai-yin during the Spring '07 term at Penn State.

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current and resistance 1 - Georg Simon Ohm(1789-1854...

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