lecture11 - Physics 2102 Gabriela Gonzlez Physics 2102...

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Physics 2102 Current and resistance Physics 2102 Gabriela González Georg Simon Ohm (1789-1854)
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In a conductor, electrons are free to move. If there is a field E inside the conductor, F=qE means the electrons move in a direction opposite to the field: this is an electrical current. E We think of current as motion of imaginary positive charges along the field directions. = = dt i q dt dq i , Ampere second Coulomb [i] : Units = Andre-Marie Ampere 1775-1836 Electrical current
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Wasn’t the field supposed to be zero inside conductors? Yes, if the charges were in equilibrium . The reasoning was “electrons move until they cancel out the field”. If the situation is not static, that is, if electrons are moving, then the field can be nonzero in a conductor, and the potential is not constant across it! However, “somebody” has to be pumping the electrons: this is the job of the battery we put across a circuit. If there is no source creating the electric field, the charges reach equilibrium at E=0. Electrical current E
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Current is a scalar, NOT a vector, although we use arrows to indicate direction of propagation . Current is conserved, because charge is conserved! 1 i 2 i 3 i 3 2 1 i i i + = Electrical current: Conservation “junction rule”: everything that comes in, must go out.
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given current through it. The larger the resistance, the larger the potential we need to drive the same current. ) (abbr. Ohm Ampere Volt [R] : Units = Georg Simon Ohm (1789-1854) "a professor who preaches such heresies is unworthy to teach science.” Prussian minister of education 1830 i V R iR V R V i = = and : therefore and Ohm’s laws Devices specifically designed to have a constant value of R are called resistors, and symbolized by Electrons are not “completely free to move” in a conductor. They move erratically, colliding with the nuclei all the time: this is what we call “ resistance”. Resistance
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lecture11 - Physics 2102 Gabriela Gonzlez Physics 2102...

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