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Lect_11_note - Chapter 27 - 2 Current and Resistance Ohms...

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Chapter 27 - 2 Current and Resistance
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Ohm’s law E J = ? ( E ) J J = σ E 2 2 1 e e nq τ m σρ m σ nq τ == = J = σ E Experiments Model Analysis
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Ohm’s law E J 2 2 1 e e nq τ m σρ m σ nq τ == = J = σ E For copper: n=8.5 x 10 28 /m 3 q=e=1.6 x 10 -19 C V = 1.57 x 10 6 m/s v d = 0.0043 m/s d= 3.9 x 10 -8 m
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Ohm’s law fails, why? ± When moving charges collide with atoms, the kinetic energy is transferred to the atoms. ± The conductor is getting warm ± n , τ varies with temperature. 2 2 1 e e nq τ m σρ m σ nq τ == =
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Nonohmic Materials ± If the resistivity of a material varies with the temperature, It will show Nonohmic behavior.
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Nonohmic Materials ± Not all materials follow Ohm’s law ± Materials that do not obey Ohm’s law are said to be nonohmic ± Ohm’s law is not a fundamental law of nature, but based on some observations ± Ohm’s law is an empirical relationship valid only for certain materials
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Nonohmic Materials ± Nonohmic materials are those whose resistance changes with voltage or current ± The current-voltage relationship is nonlinear
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Metal Resistivity ±
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This note was uploaded on 01/28/2011 for the course PHYS 011 taught by Professor Nianlin during the Fall '08 term at HKUST.

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Lect_11_note - Chapter 27 - 2 Current and Resistance Ohms...

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