Chapter 5 Direct Currents in Electric circuits Conduction of Electricity a Ohm's law Non inear resistors Resistors connected Kirchhoff's laws Substitution theorem Mesh current
Chapter 5 Direct Currents in Electric Circuits ◼ Conduction of Electricity ◼ Ohm’s Law ◼ Non-linear Resistors ◼ Resistors Connected ◼ Kirchhoff’s Lawss ◼ Substitution Theorem ◼ Mesh Current
Voltage and current Sources a Thevenin theorem a transient in rc circuits If the electric field e is maintained in conductor by an external source say, a battery, then charges drift in the field and there is an electric current
◼ Voltage and Current Sources ◼ Thevenin Theorem ◼ Transient in RC Circuits If the electric field E is maintained in conductor by an external source, say, a battery, then charges drift in the field and there is an electric current
5.1 Conduction of electricity The current density vector J Its magnitude is the amount of charge flowing thru a surface in unit time. and has unit collom ampere second. meter meter It points in the direction of flow of positive charge If negative charges flow such as conduction electrons in a conductor, then j points in the opposite direction of flow of negative charges. ( See fig5-1 Seme-conductors contain one or two types of mobile charge, namely electrons and holes For ordinary conductors it holds that J=FE
Here o is the conductivity of the material. (see Table 5-1 for various common materials. Microscopically speaking, current in conductor forms due to the drift of conduction electrons =ne, where n--the conduction electron density the magnitude of the electron charge, the drift velocity of the electron charge For example, in copper, n=8.5 x 102elctron/M if =100 A/M, then the drift velocity ==74×10-5M/s~0.26M/h en he drift velocity is low different from thermal motion( 10 M s
Conservation of Charge Now consider a volume t bounded by a surface s inside conducting material. Let da be a small element of area on S J. da is the charge flowing out thru da per unit time Then /J·da is the charge flowing out of s per unit time, which is also the charge lost by the volume T per unit time dt r. Thus, one has the conservation of charge d Is J. da 1分md T di