Phy 103: Fundamentals of PhysicsElectric CurrentResistanceOhm’s LawGeorg Simon Ohm (1789-1854)Electric PowerDirect Current (DC) & Alternating Current (AC)Alessandro Volta (1745-1827)Nikola Tesla (1856-1943)Electric CircuitsSlide 11Phy 103: Fundamentals of PhysicsChapter 23: Electric CurrentLecture NotesElectric CurrentAn electric potential difference causes electric charges to moveThe flow of electric charge is called electric currentPositive charge accelerates toward lower electric potentialNegative charge accelerates toward higher electric potentialThe rate of flow of electric charge (I) through a conducting material is the amount of charge (Q) that flows divided by the time (t) it takes to flow, orI = Q/tSI units are coulombs per second (C/s), called amperes (A), where 1 coulomb/sec = 1 AmpereBy convention, electric current is defined as the flow of positive charge flowing from high potential (+) to low potential (-)ResistanceThe physical property of a material to “impede” the flow of electric charge is called electrical resistanceAn object’s resistance (R) depends on:Its inherent ability to conduct electricity, its resistivity ()The surface area (A): the wider the area the more room for current to flowThe length (L) of the object: the longer the object the more material the current must be pushed throughR ~ L/AConductors (like metals) have low resistanceInsulators (plastics & non-metals) have high resistanceOhm’s LawFor a given potential difference (V), the magnitude of electric current (I) depends on the physical properties of the conductor (dimensions and material)The electric current that will flow through a circuit is proportional to the potential difference (V)Inversely proportional to the resistance of the circuitI = V/R {this is Ohm’s Law}For many substances, R is constant:R = V/I = a constant valueGeorg Simon Ohm (1789-1854)German physicistOriginally a secondary school (gymnasium) teacherPursued research to obtain a university postStudied electricity and the physics of hearingMost famous work (on “Ohm’s Law”) was published in a book in 1827Electrical studies were strongly influenced by Fourier’s work on heat conductionElectric PowerIt takes effort and energy (work) to drive electric charge through a circuit (against its resistance)The rate of energy (power or P) required to drive electric current through a circuit (or part of a circuit) is proportionalTo the potential difference (V) across To the electric current (I) that flows through a circuitP=VIThe SI units of power are _____Volts (V) times amperes (A)Joules per second (J/s)Watts (W)Direct Current (DC) & Alternating Current (AC)When the power source running an electric circuit moves charge only one direction it is a direct current (DC) circuitCurrent flows from the high potential terminal (+) to the low potential terminal (-)In DC circuits, the power source supplies the electronsBatteries and photoelectric cells produce DC currentWhen the power source driving an electric circuit moves charge back-and-forth it is a an alternating current (AC) circuitIn AC circuits, you supply the electronsOur wall sockets typically fluctuate between +170 V and –170 V at a rate of 60 HzThe fluctuating voltage has a sinusoidal waveform:Alessandro Volta (1745-1827)Italian physicist & inventorFirst person to isolate methaneFascinated with electricity at an early agePioneered the field of electrochemistryConstructed the first battery to produce electricity (called a voltaic pile)Nikola Tesla (1856-1943)Serbian-American inventor & engineerA major rival of Thomas EdisonPioneered the use of AC current for commercial powerSold many of his patents to Westinghouse CorporationNotable inventions:RadioFluorescent lightsWireless communicationAlternating current transmissionTesla coil transformerAn electric automobile (the Pierce Arrow)Over 700 patents“Science is but a perversion of itself unless it has as its ultimate goal the betterment of humanity”Electric CircuitsPower supply: provides the electric potential difference between its terminalsA device that transforms energy from one form of energy (such as chemical) into electrical energyCharacterized by its electromotive force (V)The potential difference or voltage between the terminalsConnecting wires: provide a path for electric current to flow (their resistance is usually very small ~ 0 )Consumer (or load): any electrical device connected to the circuit (characterized by its resistance, R)Electric CircuitsTypes of electrical connections:Series:components are connected head-to-tailParallel:Components are connected head-to-head & tail-to-tail+ -+