Find out the relieved heat, in time unity and volume unity, in a copper wire, in which the current density is j=300KA/m^2

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Current density represents the load which passes through cross-unit area, in unit time.

j=Q/(Scross*delta t)=l/Scross

Current density is a size vector, targeted by the drift speed of load carriers

q=RI^2/(Sl)=[ro*l/(S^2L)]j^2*s^2=ro *j^2=1.5kW/m^3

This is the "local" form of Joule's law P=RI^2

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