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# lecture10 - Resistance Is Futile! Physics 2102 Spring 2007...

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Unformatted text preview: Resistance Is Futile! Physics 2102 Spring 2007 Jonathan Dowling Physics 2102 Spring 2007 Lecture 10 Current and Resistance Georg Simon Ohm (1789-1854) What are we going to learn? A road map Electric charge Electric force on other electric charges Electric field, and electric potential Moving electric charges : current Electronic circuit components: batteries, resistors, capacitors Electric currents Magnetic field Magnetic force on moving charges Time-varying magnetic field Electric Field More circuit components: inductors. Electromagnetic waves light waves Geometrical Optics (light rays). Physical optics (light waves) Resistance Electrons are not "completely free to move" in a conductor. They move erratically, colliding with the nuclei all the time: this is what we call "resistance". The resistance is related to the potential we need to apply to a device to drive a given current through it. The larger the resistance, the larger the potential we need to drive the same current. Ohm's laws R! V i and therefore : i = V R and V = iR Georg Simon Ohm (1789-1854) Units : [R] = Volt " Ohm (abbr. !) Ampere "a professor who preaches such heresies is unworthy to teach science." Prussian minister of education 1830 Devices specifically designed to have a constant value of R are called resistors, and symbolized by Vector : Current density and drift speed r r r J Same direction as E such that r i = ! J " dA The current is the flux of the current density! If surface is perpendicular to a constant electric field, then i=JA, or J=i/A J dA Units: [J ] = Ampere m2 E i Drift speed: vd :Velocity at which electrons move in order to establish a current. L A E i Charge q in the length L of conductor:q = ( n A L) e n =density of electrons, e =electric charge q n ALe i J L i= = = n A e vd vd = = t= L t n Ae n e vd r vd r J = n e vd Resistivity and resistance Metal "field lines" These two devices could have the same resistance R, when measured on the outgoing metal leads. However, it is obvious that inside of them different things go on. Resistivity is associated with a material, resistance with respect to a device constructed with the material. Example: A L V r r E resistivity: ! = or, as vectors, E = ! J J ( resistance: R=V/I ) Conductivity : " = 1 ! V E= , L i J= A V != L=RA i L A R=! L A - + Makes sense! For a given material: Longer ! More resistance Thicker ! Less resistance Resistivity and Temperature Resistivity depends on temperature: = 0(1+ (T-T0) ) At what temperature would the resistance of a copper conductor be double its resistance at 20.0C? Does this same "doubling temperature" hold for all copper conductors, regardless of shape or size? b Power in electrical circuits a A battery "pumps" charges through the resistor (or any device), by producing a potential difference V between points a and b. How much work does the battery do to move a small amount of charge dq from b to a? dW= -dU= -dq V=(dq/dt) dt V= iV dt The battery "power" is the work it does per unit time: P=dW/dt=iV P=iV is true for the battery pumping charges through any device. If the device follows Ohm's law (i.e., it is a resistor), then V=iR and P=iV=i2R=V2/R 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 , what might the fatal voltage be? (Ans: 75 V) Example Two conductors are made of the same material and have the same length. Conductor A is a solid wire of diameter 1.0 mm. Conductor B is a hollow tube of outside diameter 2.0 mm and inside diameter 1.0 mm. What is the resistance ratio RA/RB, measured between their ends? A R=L/A B AA= r2 RA/RB= AB/AA= 3 AB= ((2r)2-r2)=3r2 Example A 1250 W radiant heater is constructed to operate at 115 V. (a) What will be the current in the heater? (b) What is the resistance of the heating coil? (c) How much thermal energy is produced in 1.0 h by the heater? Formulas: P=i2R=V2/R; V=iR Know P, V; need R to calculate current! P=1250W; V=115V => R=V2/P=(115V)2/1250W=10.6 i=V/R= 115V/10.6 =10.8 A Energy? P=dU/dt => dU=P dt = 1250W 3600 sec= 4.5 MJ Example A 100 W lightbulb is plugged into a standard 120 V outlet. (a) What is the resistance of the bulb? (b) What is the current in the bulb? (c) How much does it cost per month to leave the light turned on continuously? Assume electric energy costs 6/kWh. (d) Is the resistance different when the bulb is turned off? Resistance: same as before, R=V2/P=144 Current, same as before, i=V/R=0.83 A We pay for energy used (kW h): U=Pt=0.1kW (30 24) h = 72 kW h => \$4.32 (d): Resistance should be the same, but it's not: resistivity and resistance increase with temperature. When the bulb is turned off, it is colder than when it is turned on, so the resistance is lower. Example An electrical cable consists of 105 strands of fine wire, each having 2.35 resistance. The same potential difference is applied between the ends of all the strands and results in a total current of 0.720 A. (a) What is the current in each strand? [0.00686] A (b) What is the applied potential difference? [1.61e-08] V (c) What is the resistance of the cable? [2.24e-08 ] EMF devices and single loop circuits b The battery operates as a "pump" that moves positive charges from lower to higher electric potential. A battery is an example of an "electromotive force" (EMF) device. a These come in various kinds, and all transform one source of energy into electrical energy. A battery uses chemical energy, a generator mechanical energy, a solar cell energy from light, etc. i - + d b c The difference in potential energy that the a device establishes is called the EMF i and denoted by . Va+-iR=Va = iR Va a b c iR d=a Circuit problems b Given the emf devices and resistors in a circuit, we want to calculate the circulating currents. Circuit solving consists in "taking a walk" along the wires. As one "walks" through the circuit (in any direction) one needs to follow two rules: When walking through an EMF, add + if you flow with the current or - otherwise. How to remember: current "gains" potential in a battery. When walking through a resistor, add -iR, if flowing with the current or +iR otherwise. How to remember: resistors are passive, current flows "potential down". Example: Walking clockwise from a: +-iR=0. Walking counter-clockwise from a: -+iR=0. a Ideal batteries vs. real batteries If one connects resistors of lower and lower value of R to get higher and higher currents, eventually a real battery fails to establish the potential difference , and settles for a lower value. One can represent a "real EMF device" as an ideal one attached to a resistor, called "internal resistance" of the EMF device: E true E true E R rint R E true = E ! i rint The true EMF is a function of current. i R1 Resistors in Series E R2 + E ! iR1 ! iR2 = 0 ! E = i ( R1 + R2 ) = iRtot Rtot = R1 + R2 Behave like capacitors in parallel! n If you have n resistors in series : Rtot = ! Ri i =1 Resistors in parallel + E i1 a i3 i2 R1 R2 E R1 E Outer loop : E - i3 R2 = 0 ! i3 = R2 Node a : i1 = i2 + i3 Left loop : E ! i2 R1 = 0 ! i2 = i1 = + i1 E E &1 1 # + = \$ + !E R1 R2 % R1 R2 " 1 a Rtot Rtot = &1 1 # \$ + ! % R1 R2 " 1 &n 1# \$' ! % i =1 Ri " Same as capacitors in series. For n resistors : Rtot = Example Bottom loop: (all else is irrelevant) i= 12V 8 V 12V = = 1.5 A R 8! ...
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## This note was uploaded on 11/18/2011 for the course PHYSICS 2102 taught by Professor Dowling during the Fall '10 term at LSU.

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