Slide 1Slide 2Lightning Review15.2 Current and Drift Speed15.2 Current and Drift Speed (2)Slide 615.2 Current and Drift Speed - ExampleSlide 8Slide 915.2 Current and Drift Speed - CommentsMini-quiz17.3 Resistance and Ohm’s Law - Intro17.3 Definition of Resistance17.3 Resistance - Units17.3 Ohm’s LawSlide 16Slide 17Definition: resistorExample: Resistance of a Steam Iron17.4 Resistivity - Intro17.4 Resistivity - Definition17.4 Resistivity - Remarks17.4 Resistivity - UnitsSlide 24Slide 25Slide 26Slide 27Slide 28Slide 2917.5 Temperature Variation of Resistance - ExamplePlatinum Resistance Thermometer A resistance thermometer, which measures temperature by measuring the change in the resistance of a conductor, is made of platinum and has a resistance of 50.0 W at 20oC. When the device is immersed in a vessel containing melting indium, its resistance increases to 76.8 W. Find the melting point of Indium.1101/14/1901/14/19General Physics (PHY 2140)Lecture 8Lecture 8 ElectrodynamicsElectric current current and drift speed resistance and Ohm’s law resistivity temperature variation of resistance electrical energy and powerChapter 17http://www.physics.wayne.edu/~apetrov/PHY2140/2201/14/1901/14/19Hours of operationHours of operation::Mondays, Tuesdays, Wednesdays 11 AM to 6 PMThursdays and Fridays 11 AM to 3 PMUndergraduate students taking PHY2130-2140 will be able to get assistance in this Center with their homework, labwork and other issues related to their physics course. The Center will be open: Monday, September 22 to Wednesday, December 10, 2003.Department of Physics and Astronomyannounces the Fall 2003 opening ofThe Physics Resource CenterThe Physics Resource Centeron Monday, September 22 in Room 172Room 172 of Physics Research Building. of Physics Research Building.3301/14/1901/14/19Lightning ReviewLightning ReviewLast lecture: 1.1.Capacitance and capacitorsCapacitance and capacitorsCapacitors with dielectrics (CCapacitors with dielectrics (C↑ if ↑ if ↑↑))2.2.Current and resistanceCurrent and resistanceElectric currentElectric currentCurrent and drift speedCurrent and drift speedReview Problem: A parallel-plate capacitor is attached to a battery that maintains a constant potential difference V between the plates. While the battery is still connected, a glass slab is inserted so as to just fill the space between the plates. The stored energya. increasesb. decreases c. remains the same0 0,AC C Cdke k= =QItD=DdI nqv A=221 12 2 2QU QV CVC= = =4401/14/1901/14/1915.2 Current and Drift Speed15.2 Current and Drift SpeedConsider the current on a conductor of cross-sectional Consider the current on a conductor of cross-sectional area A. area A. Avdqvdt5501/14/1901/14/1915.2 Current and Drift Speed (2)15.2 Current and Drift Speed (2)Volume of an element of length Volume of an element of length x is : x is : V = A V = A x.x.Let n be the number of carriers per unit of volume.Let n be the number of carriers per unit of volume.The total number of carriers in The total number of carriers in V is: n A V is: n A x.x.The charge in this volume is: The charge in this volume is: Q = (n A Q = (n A x)q.x)q.Distance traveled at Distance traveled at drift speed drift speed vvdd by carrier in time by carrier in time t: t: x = x = vvd d t.t.Hence: Hence: Q = (n A Q = (n A vvd d t)q.t)q.The current through the conductor: The current through the conductor: I = I = Q/ Q/ t = n A t = n A vvd d qq..6601/14/1901/14/1915.2 Current and Drift Speed (3)•In an isolated conductor, charge carriers move randomly in all directions.•When an external potential is applied across the conductor, it creates an electric field inside which produces a force on the electron. •Electrons however still have quite a random path.•As they travel through the material, electrons collide with other electrons, and nuclei, thereby losing or gaining energy.•The work done by the field exceeds the loss by collisions. •The electrons then tend to drift preferentially in one direction.7701/14/1901/14/1915.2 Current and Drift Speed - Example15.2 Current and Drift Speed - ExampleQuestion:Question:A copper wire of cross-sectional area 3.00x10A copper wire of cross-sectional area 3.00x10-6-6 m m22 carries a current of carries a current of 10. A. Assuming that each copper atom contributes one free electron to 10. A. Assuming that each copper atom contributes one free electron to the metal, find the drift speed of the electron in this wire. The density of the metal, find the drift speed of the electron in this wire. The density of copper is 8.95 g/cmcopper is 8.95 g/cm33..8801/14/1901/14/19Question:A copper wire of cross-sectional area 3.00x10-6 m2 carries a current of 10 A. Assuming that each copper atom contributes one free electron to the metal, find the drift speed of the electron in this wire. The density of copper is 8.95 g/cm3.Reasoning: We know:•A = 3.00x10-6 m2 •I = 10 A.•  = 8.95 g/cm3.•q = 1.6 x 10-19 C.•n = 6.02x1023 atom/mol x 8.95 g/cm3 x ( 63.5 g/mol)-1•n = 8.48 x 1022 electrons/ cm3.9901/14/1901/14/19Question:A copper wire of cross-sectional area 3.00x10-6 m2 carries a current of 10 A. Assuming that each copper atom contributes one free electron to the metal, find the drift speed of the electron in this wire. The density of copper is 8.95 g/cm3.Ingredients:A = 3.00x10-6 m2 ; I = 10 A.;  = 8.95 g/cm3.; q = 1.6 x 10-19 C.n = 8.48 x 1022 electrons/ cm3.( ) ( ) ( )22 3 19 6 2610.0 /8.48 10 1.6 10 3.00 102.46 10 /dI C svnqAelectrons m C mm s- --= =� � �= �101001/14/1901/14/1915.2 Current and Drift Speed - Comments15.2 Current and Drift Speed - CommentsDrift speeds are usually Drift speeds are usually very smallvery small..Drift speed much smaller than the average speed Drift speed much smaller than the average speed between collisions. between collisions. Electrons traveling at 2.46x10Electrons traveling at 2.46x10-6-6 m/s would would take 68 min to m/s would would take 68 min to travel 1m.travel 1m.So why does light turn on so quickly when one flips a So why does light turn on so quickly when one flips a switch?switch?The info (electric field) travels at roughly 10The info (electric field) travels at roughly 1088 m/s… m/s…111101/14/1901/14/19Mini-quizMini-quizConsider a wire has a long conical shape. How does the Consider a wire has a long