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Current Resistivity Current – any motion of ___________ from one region to another. For some materials, especially metals, the current density (J) is directly proportional to electric field (E) - _______ “Law” ____ is the standard symbol used for current. ρ= dQ definition of current I= dt I definition of current density J= A Current units: ___________ 1 A = 1 C/s E J definition of resistivity Units: (V/m)/(A/m2) = Ω·m (Ω =V/A, Ω - Ohm) _______________ is the inverse of resistivity Current density units: ___________ Current ‘flows’ from _________ potential to _________ potential. DC - ______________ current – always in the same direction AC - ______________ current – continuously changes directions 1 EF 152 Fall, 2009 Lecture 4-5 Resistivity and temperature 2 EF 152 Fall, 2009 Lecture 4-5 Resistance R= R= V I ρL definition of resistance relationship between resistivity and resistance A R – units of Ω (Ohms) ρ (T ) = ρ 0 [1 + α (T − T0 )] EF 152 Fall, 2009 Lecture 4-5 temperature dependence of resistivity ρ0 and T0 are reference values α – temperature coefficient of resistivity 100 m of 12-gauge copper wire 100 W light bulb R=140 Ω 3 EF 152 Fall, 2009 Lecture 4-5 R=0.5 Ω 4 a b EF 152 Fall, 2009 Lecture 4-5 Find: Vad, Vbc, rate of energy use Example EF 152 Fall, 2009 Lecture 4-5 Sum of potential changes for a complete circuit is ________ Potential changes around a circuit EF 152 Fall, 2009 Lecture 4-5 Terminal voltage: Vab = E – Ir Internal resistance: r Sources of emf: _______________________________________ emf is not really a force – it is an energy/charge quantity like potential. Electromotive force (emf, E) causes current flow from lower to higher potential. To have a steady current you must have a ___________ circuit. Electromotive Force 9 7 5 Measures Internal Connect EF 152 Fall, 2009 Lecture 4-5 Vab R 2 Power delivered to a resistor Power - Rate at which energy is delivered to or extracted from a circuit element. Units of Watts (W) 1 W = 1V·A = (1 J/C)(1 C/s) = 1 J/s P = Vab I = ( IR) I = P = Vab I Energy and power in circuits EF 152 Fall, 2009 Lecture 4-5 Ohmmeter Ammeter Voltmeter Device Symbols 8 6