Preliminary Concepts Current Density Defined 。 Consider charge carriers g in a volume V moving with drift velocity va: .If the density of charge carriers is n [#/unit volume],in a time At,the charge AQ that passes through area A is given by: △Q=n q va A△t —vaAt So that Area A a I=△Q/At=n a va 。 Current density J is defined as the density of charge carriers flowing through a unit area,infinitely thin J=I/A=△Q/(A△t)=nqva N or,in vector form: J=n q va where the direction of J is in the direction of va Then,current I is given by: I=J.n dA where n is the unit normal to A Basic Electronics-Special Lecture for TIPP 2011 7 Gary Drake,Argonne National Lab-Session 1Basic Electronics – Special Lecture for TIPP 2011 7 Gary Drake, Argonne National Lab – Session 1 Preliminary Concepts  Current Density Defined • Consider charge carriers q in a volume V moving with drift velocity vd:  If the density of charge carriers is n [#/unit volume], in a time t, the charge Q that passes through area A is given by: So that • Current density J is defined as the density of charge carriers flowing through a unit area, infinitely thin or, in vector form: Then, current I is given by: Q = n q vd A t I = Q / t = n q vd A vd vd t Area A J = I / A = Q / (A t) = n q vd J = n q vd where the direction of J is in the direction of vd I = f J . ň dA where ň is the unit normal to A vd