p240_ct12_f07 - Physics 240 Fall 2007 Lecture #12 No class...

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Physics 240 Fall 2007 Lecture #12 Dr. Dave Winn 2405 Randall winn@umich.edu No class on Tuesday October 16, no MP due until Oct. 18
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Electrical resistance For many materials V = IR Units of resistance = Vs/C = Ohms (sym: Ω ) Where the resistance R depends on resistivity ρ and geometry, for example R = ρ (L/A) Power dissipated in resistors P=I 2 R=V 2 /R Current Voltage I = (1/R)*V Symbol for resistors:
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Table of the combinations of: V = IR and P = IV EE Belt Buckle…
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Some properties of resistors Temperature dependence of resistance: R = R 0 [1 + α (T – T 0 )] Power is dissipated in transmission through a resistor: P = I 2 R = V 2 /R Resistance Temperature R T Demo Demo
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R T Sudden transition to superconductivity Room Temperature
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Some properties of resistors Temperature dependence of resistance: R = R 0 [1 + α (T – T 0 )] Power is dissipated in transmission through a resistor: P = I 2 R = V 2 /R Resistance Temperature R T Demo Demo
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Other materials… Ohm’s law is not a good approximation for the conductivity of some materials. For these the relation between I and V can be more complex Semiconductor diodes are an example An example: a “diode”, or one way current device. Demo: negative temperature coefficient
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EMF generators To maintain a current in a circuit, something must ‘pump’ positive charge from low to high potential This ‘electromotive
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p240_ct12_f07 - Physics 240 Fall 2007 Lecture #12 No class...

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