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,  A, nA, pA) Scalar, Positive current=flowing direction of positive charges

Resistance and Ohm’s Law R: resistance, L: length, A:cross section,  : resistivity Unit(R): Ohm(  ), 1  = 1V/A; Unit(  ): ohm.meter ( .m) Temperature dependence: R T = R 0 [1+  (T-T 0 )] T in Celsius, R 0 = R(T=T 0 =20 o C)  : temperature coefficient (0.003~0.005 o C -1 ) L A

Resistivities of Different Materials

Superconductivity & Non-ohmic Conductors

Electromotive Force & Circuits A pump is needed to keep the water circulating  pump~electromotive force (emf) Circulating without pump  No emf~imposiple

emf Source (  ) Terminal voltage: V ab =  V ab =  = IR (ideal emf) V ab =  r (with internal resistance, r)

Voltage and Current Measurements

Potential drops and rises in circuit

Power: the energy consumed per unit time. Power Output of a Source Ir 2 is the energy dissipated inside the source  heat Energy and Power in Electric Circuits

Resistors in Series & Parallel (in series) (in parallel)

Combinations

Kirchhoff’s Rules Junction Rule Loop Rule

Kirchhoff’s Rules

Electrical Measuring Instruments Ammeters Galvanometer Voltmeters

Resistance-Capacitance Circuits and Transient Effects

Summary: Current

Summary: Resistance & Ohm’s Law

Summary: emf

Summary: Energy & Power

Summary: Resistors in Parallel & Series (in series) (in parallel)

Summary: Kirchhoff’s Rules

Summary: Charging & Discharging

Homework Ch19: 1, 5, 7, 12, 24, 31, 33, 42, 48, 52, 57, 61, 63

Problem 57

Problem 61

Problem 52 & 53