

Current Density 
Current density represents the current flowing per unit area of crosssection at a point 

Current density represents the current flowing per unit area of crosssection at a point. To relate current, a microscopic quantity, to the microscopic motion of the charges, let's examine a conductor of crosssectional area A, as shown in given figure. A microscopic picture of current flowing in a conductor The total current through a surface can be written as where is the current density (the SI unit of current density are A/m^{2}). If q is the charge of each carrier, and n is the number of charge carriers per unit volume, the total amount of charge in this section Q = q(nAx). Suppose that the charge carriers move with a speed v_{d}. then the displacement in a time interval t will be x = v_{d}t, which implies The speed v_{d} at which the charge carriers are moving is known as the drift speed. Physically, v_{d} is the average speed of the charge carriers inside a conductor when an external electric field is applied. Actually an electron inside the conductor does not travel in a straight line; instead, its path is rather erratic as shown. Motion of an electron in a conductor From the above equations, the current density can be written as = nq_{d} Thus, we see that and _{d} point in the same direction for positive charge carriers, in opposite directions for negative charge carriers. 

Related Words: 
Current Density, Current Flowing, Microscopic Quantity, Microscopic Motion, Conductor of Crosssectional Area, Microscopic Picture, Current Flowing In A Conductor, Si Unit of Current Density, Total Amount of Charge, Drift Speed, Electric Field, Electron, Motion of An Electron In A Conductor, Positive Charge, Negative Charge



