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19 Current, Resistance, & DC Circuits. Electric Current. Current: net charge passing through a cross-section per unit time. Units: Ampere (A), 1 A = 1C/s ( mA , m A , nA , pA ) Scalar , Positive current=flowing direction of positive charges. Resistance and Ohm’s Law. A. L.
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Electric Current Current: net charge passing through a cross-section per unit time. Units: Ampere (A), 1 A = 1C/s (mA, mA, nA, pA) Scalar , Positive current=flowing direction of positive charges
Resistance and Ohm’s Law A L R: resistance, L: length, A:cross section, r: resistivity Unit(R): Ohm(W), 1W = 1V/A; Unit(r): ohm.meter (W.m) Temperature dependence: RT = R0[1+a(T-T0)] T in Celsius, R0 = R(T=T0=20oC) a: temperature coefficient (0.003~0.005 oC-1)
Electromotive Force & Circuits A pump is needed to keep the water circulating pump~electromotive force (emf) Circulating without pump No emf~imposiple
emf Source (e) Terminal voltage: Vab = eVab = e = IR (ideal emf) Vab = e -Ir (with internal resistance, r)
Energy and Power in Electric Circuits Power: the energy consumed per unit time. Power Output of a Source Ir2 is the energy dissipated inside the sourceheat
Resistors in Series & Parallel (in series) (in parallel)
Kirchhoff’s Rules Junction Rule Loop Rule
Electrical Measuring Instruments Ammeters Galvanometer Voltmeters
Summary: Resistors in Parallel & Series (in series) (in parallel)