GEN PHYS FOR TECH ST
GEN PHYS FOR TECH ST PHYS 2102
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This 22 page Class Notes was uploaded by Elva Fahey on Tuesday October 13, 2015. The Class Notes belongs to PHYS 2102 at Louisiana State University taught by Staff in Fall. Since its upload, it has received 7 views. For similar materials see /class/223005/phys-2102-louisiana-state-university in Physics 2 at Louisiana State University.
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Date Created: 10/13/15
Physics 2 9 5 Aurora Boregllls Christi n BUth Physics 2102 Lecture 20 Version 03042009 Review Lectures 1119 Lecture 11 Capacitance 1 0 Any two charged conductors form a capacitor 0 Capacitance C QV 0 Simple Capacitors Parallel plates C 80 Ad Spherical C 431 80 abba Lecture 12 Capacitance 2 Capacitors in series same charge not necessarily equal potential equivalent capacitance 1 Ce 61 C 1 1C2 Capacitors in parallel same potential not necessarily same Charge equivalent capacitance CeqC1C2 0 Energy in a capacitor UQ22CCV22 0 Energy density u80E22 Lecture 13 Current and Resistance 1 Capacitor with a dielectric capacitance increases C KC 0 Dielectric consists of molecules which align in eld yields surface charges which reduce the eld between the plates 0 Battery creates potential difference which leads to a current in a closed circuit 0 Current arrow is drawn in direction in which positive Charge carriers would move Drift speed vd speed at which electrons move to establish a current Vd nAe me gt Jnevd i J Lecture 14 Current and Resistance 2 0 A resistor is a conductor whose resistance does not change w1th the voltage K R p A 0 A linear I chrve is said to be Ohmic otherwise nonOhmic Resistivity is associated with a material resistance with respect to a device constructed with the material 0 Conductivity 0 i p Resistivity depends on temperature p p0 10 T T 0 Reason for resistance conduction electrons collide with stationary ionic lattice Resistance of 21 Rod A 4 L gt 4 gt V Makes sense For a given material EZ Ji L A V A Pl RZ A L R pA Longer a More resistance Thicker a Less resistance Lecture 15 DC Circuits 1 Electromotive fore devices emf maintain a potential between their terminals 0 Kirchhoff s loop rule KLR KLR The algebraic sum of the changes in potential encountered in a complete traversal of any loop in a circuit is equal to zero When walking through an emf add E if you ow with the current or E otherwise When walking through a resistor add z R if owing with the current or z R otherwise Lecture 16 DC Circuits 2 The potential of ideal emf devices does not depend on the current real emf devices have internal resistance 0 Kirchhoff s junction rule KJR The sum of the currents entering any junction is equal to the sum of the currents leaving the junction Resistors in parallel replace by equivalent resistance 1 n l lie 1 eq JI1 j 0 Resistors in series replace by equivalent resistance 0 Ammeter measures current voltmeter measures voltage Lecture 17 DC circuits RC circuits 0 Technique to solve multiloop circuits 1 Simplify compile circuits 2 Apply loop rule 3 Equations to unknowns 0 RC circuits simple circuit with timevarying current time constant is 17 RC Charging a capacitor qt CE1 e tRC Discharging a capacitor qtCEe tRC Step I Simplify Compile Circuits Resistors V Key formula Vz R In series same current ReqZRj vw W In parallel same voltage IReq ZIRj Wv W Capacitors QC V f w 81 RIEEliI R2 1 R l same Charge b E ICeq EllC f a s23 1 l same voltage L CquZC39j b RC Circuits Discharging a Capacitor I 3 Assume the switch has been closed I R on a for a long time the capacitor all will be charged with QCE Then close the switch on 92 charges nd their way across the circuit establishing a current VRVC0 itRqtC0 gt dqdtRqtC0 i R Solution qtq0e tRCCEe VRC 1 t dqdt q RC e tRC ER e tRC C o a H Exponential discharge 0 E ER r gr Lecture 18 Magnetic elds 1 0 Magnetic elds exert forces on moving charges F B q v xB the force is perpendicular to the eld and the velocity 0 Magnetic eld lines are used to visualize magnetic elds 0 Magnetic eld B has SI unit tesla 0 Right hand rule for vector products cross products 0 Cathode ray tube crossed electric and magnetic elds lead to discovery of electron 0 Hall effect reveals that charge carriers in metals are negative electrons Lecture 19 Magnetic elds 2 0 Hall effect reveals that charge carriers in metals are negative electrons 0 Charged particle with velocity perpendicular to magnetic eld on circular path mvi 2mm r quB lqlB 0 Generally linear and Circular motion superimposed a helix with radius r and pitch p TV 0 Charged particles trapped in magnetic bottle natural phenomenon aurora borealis Cyclotrons and synchrotrons to accelerate electron and protons Physics 2102 Lecture 14 Current 11 Curren r is rhe Flow of posi rive charge Aq zE CSA 7Cs 3Cs 2Cs 6Cs 9 gtlt lt 9lt a b 6 d3 Current is conserved lt 1A 2A 2A 2A BAT 4AT i A z q Cs A Resistance l gt The resistance is related to the potential we need to apply to a device to drive a given I current through it II The larger the resistance the larger the potential we need to drive the same current Georg Simon Ohm I 17891854 R VOlt Q quota professor who preaches such heresies A m a r is unworthy to teach science Prussian mpere minister of education 1830 Devices specifically designed to have a constant value of R are called resistors and symbolized by JV Example A human being can be electrocuted if a current as small as 50 mA passes near the heart An electrician working with sweaty hands makes good contact with the two conductors he is holding If his resistance is 1500 Q what might the fatal voltage be Ans 75 V 0 P 0 0 1 9 U Ohm s Law 0 A resistor is a conductor whose resistance does not change with the voltage 0 Plots b c is called I chrve 0 Conductor with linear I chrve is said to be Ohmic Fig b 0 Conductor semiconductor diode in Fig c is nonOhmic Cuncnl mA Currenl m 7 2 1 1 quot 0 2 1 I Potential difference V Potenlial liffvrem e V a I U Resistivity and resistance Two devices could have the same resistance R Metal gt eldlines Yet obvious that inside different things go on E a a Resistivity p 7 or as vectors E p 1 Resistance RVI Conductivity 0 1 Resistivity is associated with a material Resistance with respect to a device constructed with the material
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