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Current Density

Current density represents the current flowing per unit area of cross-section at a point

Physics [Science]

Central Board of Secondary Education [12]

2/11/2010Print This PageTell - A - FriendAdd to Wish ListReport Error

Current density represents the current flowing per unit area of cross-section at a point. To relate current, a microscopic quantity, to the microscopic motion of the charges, let's examine a conductor of cross-sectional 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/m2). 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 vd. then the displacement in a time interval t will be x = vdt, which implies



The speed vd at which the charge carriers are moving is known as the drift speed. Physically, vd 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

= nqd

Thus, we see that and d point in the same direction for positive charge carriers, in opposite directions for negative charge carriers.

2/11/2010Print This PageTell - A - FriendAdd to Wish ListReport Error

Related Words:

Current Density, Current Flowing, Microscopic Quantity, Microscopic Motion, Conductor of Cross-sectional 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

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