Chapter 4 Steady Electric CurrentsElectriccurrent,ElectromotiveforcePrincipleofcurrentcontinuity,Energy dissipation1. Current & Current Density2.Electromotive Force3.Principle of Current Continuity4.Boundary Conditionsfor Steady Electric Currents5.EnergyDissipationinSteadyElectricCurrentFields6.ElectrostaticSimulation
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&CurrentDensityClassification:Conduction current and convention currentThe conduction currentis formed by the free electrons(or holes)in a conductoror theionsin an electrolyteThe convectioncurrentis resultingfromthe motion oftheelectron, the ions, or the other charged particlesin vacuum, asolid,a liquid ora gas
1. Current & Current Density Classification: 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 timeis called the electric currentintensity or electric current, and it isdenoted byI. The unit of electric current is ampere (A)The relationship between electric current I and electric chargeqisdqdtThe current density is a vector, and it is denoted as J. Thedirection ofthe current densityis the same as the flowing directionof the positive charges, and the magnitude is the amount of chargethrough unit cross-sectional area per unit time.The relationshipbetween the current element dl across a directedsurface element dS and the current densityJisdI = J.dsUV
The amount of charge flowing across a given area per unit time is called the electric current intensityor 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 is t q I d d = The 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 dI = J dS
The electric current across the area SisI =[J.dsWhich states that the electric current across an area is the flux of thecurrentdensitythrough the area.In most conducting media, the conduction current density J at apoint is proportional to the electric field intensity E at that point so thatJ=oEwhere is called the conductivity, andits unit is S/m. A largeOmeans that the conducting ability of the medium is strongerThe above equation is called the differential form of thefollowingOhm'slawU=IRUV
The electric current acrossthe area S is = S I J dS 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 intensityE at that pointso that J =E 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 followingOhm’s law U = IR
A conductorwith infinite is calleda perfect electric conductoror p.e.c..In aperfect electricconductor,electriccurrentcan beproducedwithout the influence of an electric fieldThere is no steady electric fieldin a perfect electric conductor.Otherwise, an infinite current will be generated, and it resultsin aninfiniteenergyAmedium without any conductivityis called a perfect dielectricoraninsulatorIn nature there exists no any p.e.c. or perfect dielectric
A 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 electric conductor, electric current can be produced without the influence of an electric field. There is no steady electric field in a perfect electric conductor. Otherwise, an infinite current will be generated, and it results in an infinite energy. In nature there exists no any p.e.c. or perfect dielectric