电磁场与电磁波课件(电子科大)_FWEJ-Ch.04.ppt

* Chapter 4 Steady Electric Currents Electric current, Electromotive force Principle of current continuity, Energy dissipation. 1. Current Current Density 2. Electromotive Force 3. Principle of Current Continuity 4. Boundary Conditions for Steady Electric Currents 5. Energy Dissipation in Steady Electric Current Fields 6. Electrostatic Simulation 1. Current Current DensityClassification: Conduction current and convention current.The conduction current is formed by the free electrons (or holes) in a conductor or the ions in an electrolyte.The convection current is resulting from the motion of the electron, the ions, or the other charged particles in vacuum, a solid, a liquid or a gas.The amount of charge flowing across a given area per unit time is called the electric current intensity or electric current, and it is denoted by I. The unit of electric current is ampere (A)The relationship between electric current I and electric charge q isThe current density is a vector, and it is denoted as J. The direction of the current density is the same as the flowing direction of the positive charges, and the magnitude is the amount of charge through unit cross-sectional area per unit time.The relationship between the current element dI across a directed surface element dS and the current density J is The electric current across the area S is Which states that the electric current across an area is the flux of the current density through the area.In most conducting media, the conduction current density J at a point is proportional to the electric field intensity E at that point so that where ? is called the conductivity, and its unit is S/m. A large ? means that the conducting ability of the medium is stronger.The above equation is called the differential form of the following Ohm’s lawA conductor with infinite is called a perfect electric conductor, or p.e.c..A medium without any conductivity is called a perfect dielectric or an insulator.In a perfect