大学物理下26-Current-and-Resistance课件.ppt
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1、Current and Resistance Dr.Huaqiu Deng E-mail:Electric CurrentElectric current i:a stream of moving charges qUnit of i:ampere,or A,or C/sThe current i through the conductor has the same value at planes aa,bb,and ccMoving charges q:Properties of CurrentCharge is conserved:the current into the junction
2、 must equal the current out:Current is a scalar:its direction is drawn in the direction in which positive charge carriers would move,even if the actual charge carriers are negative and move in the opposite directionCurrent DensityIf the current is uniform across the surface and parallel to dADrift S
3、peed(漂移速度)(漂移速度)The current i:The drift speed vd:Example:in the copper conductors of household wiring,electron drift speeds are perhaps 105 or 104 m/s,whereas the random-motion speeds are around 106 m/s.Resistance and ResistivityUnit of R:ohm or,and 1 ohm=1 =1 V/A Resistor:a conductor whose function
4、 in a circuit is to provide a specified resistanceResistivity of Some Materials at RoomTemperature(20oC)MaterialResistivity,(.m)Temperature Coefficient of Resistivity,(K-1)Typical metalsSilver1.62x10-84.1x10-3Copper1.69x10-84.3x10-3Gold2.35x10-84.0 x10-3Aluminum2.75x10-84.4x10-3Manganin(锰铜锰铜)4.82x10
5、-80.002x10-3Tungsten(钨钨)5.25x10-84.5x10-3Iron9.68x10-86.5x10-3Platinum(铂铂)10.6x10-83.9x10-3Typical semiconductorsSilicon,pure2.5x103-70 x10-3Silicon,n-type8.7x10-4Silicon,p-type2.8x10-3Typical insulatorsGlass1010 1014Fused quartz 1016Sample Problem-A material has resistivity,a block of the material
6、has resistanceProblem:A rectangular block of iron has dimensions 1.2cm x 1.2cm x 15cm.A potential difference is to be applied to the block between parallel sides and in such a way that those sides are equipotential surfaces(as in Figure).What is the resistance of the block if the two parallel sides
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