current, resistance, and electromotive force
DESCRIPTION
Chapter 25. Current, Resistance, and Electromotive Force. Current. Current = Rate of flow of charge Unit: A ( Ampere ) 1 A = 1C/s. Current flows from high potential to low potential. Total Amount of Charge. - PowerPoint PPT PresentationTRANSCRIPT
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CURRENT, RESISTANCE, AND ELECTROMOTIVE FORCE
Chapter 25
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CURRENT
Current = Rate of flow of charge Unit: A (Ampere)1 A = 1C/s
Current flows from high potential to low potential
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TOTAL AMOUNT OF CHARGESuppose a wire carries current I. Find the total charge passing through the wire in time t.
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CONSERVATION OF CHARGE(KIRCHHOFF’S JUNCTION RULE)
Arrows: Directions of POSITIVE charge flow
We now know that current (in metals) is in fact caused by negatively charged electrons flowing in the opposite direction.
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POSITIVE OR NEGATIVE CHARGE CARRIERS
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CURRENT DENSITYCurrent Density: Current per unit areaUnit: A/m2
a b
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DIRECTION
Left or Right?
I
J
E Right
Right
Right
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DRIFT VELOCITY
Closing time at the bar
Drift velocity: vd
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DERIVATION OF DRIFT VELOCITY
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FIND THE DRIFT VELOCITYGiven n=8.49 × 1028 m-3 for copper, what is the drift velocity of electrons in a copper wire with radius r=900μm and I =17 mA?
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RESISTANCE
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OHM’S LAW
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RESISTOR
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RESISTIVITY ρ AND CONDUCTIVITY σ
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RESISTIVITY
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WHICH HAS THE HIGHER RESISTANCE?
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OHMIC MATERIAL
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NONOHMIC MATERIAL, GRAPH
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POWER
Power: Energy per unit timeUnit: W (Watt)1W=1J/s
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EXAMPLE
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KILOWATT-HOUR AND JOULES
Both measure energy1kWh is the amount of energy dissipated by a 1000 Watts light bulb in one hour.Therefore:1kWh = (1000J/s) (3600s)=3.6 × 106 J