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

7、are(1)the square ends(with dimensions 1.2 cm x 1.2 cm)and(2)two rectangular sides(with dimensions 1.2 cm x 15 cm)?Solution:(a)(b)Ohms LawOhms law:the current through a device is always directly proportional to the potential difference applied to the device：i VA conducting device obeys Ohms law when

8、the resistance R of the device is independent of the magnitude and polarity of the applied potential difference V.A conducting material obeys Ohms law when the resistivity of the material is independent of the magnitude and direction of the applied electric field E.A plot of semiconducting pn juncti

9、on diodeA plot of 1000 resistorV is applied to the terminals of a device,establishing iSample Problem-Mean free time and mean free distanceProblem:The number of conduction electrons per unit volume n in copper is 8.49x1028 m3,its is 1.69x10-8,the electron mass m is 9.1x10-31kg,(a)What is the mean fr

10、ee time between collisions for the conduction electrons in copper?(b)The mean free path of the conduction electrons in a conductor is the average distance traveled by an electron between collisions.What is for the conduction electrons in copper,assuming that their effective speed veff is 1.6x106 m/s

11、?Solution:(a)(b)This is about 150 times the distance between nearest-neighbor atoms in a copper lattice.Thus,on the average,each conduction electron passes many copper atoms before finally hitting one.Sample Problem-Rate of energy dissipation in a wire carrying currentProblem:You are given a length

12、of uniform heating wire made of a nickelchromiumiron alloy called Nichrome;it has a resistance R of 72.At what rate is energy dissipated in each of the following situations?(1)A potential difference of 120 V is applied across the full length of the wire.(2)The wire is cut in half,and a potential dif

13、ference of 120 V is applied across the length of each half.Solution:(1)(2)SemiconductorsSome Electrical Properties of Copper and SiliconPropertyCopperSiliconType of materialMetalSemiconductorCharge carrier density,m-38.49 x 10281 x 1016Resistivity,.m1.69 x 10-82.5 x 103Temperature coefficient of resistivity,K-1+4.3 x 10-3-70 x 10-3Homework#3 of p700#26#38