Some questions on drift-diffusion equation

[Tomxuandj]

For example: I want to know how to use drift-diffusion equation to solve velocity of electron. In "momentum.C", I see "J[i]=N[i]U[i]" and in driftDiffusion.C, this relation "J[i] == (-mu_[i](Tifvc::grad(plasmaConstants::KBEN_[i]) + plasmaConstants::KBE*N_[i]*fvc::grad(Ti) - N_[i]*z_[i]*E)) ".This "==" can be saw as an equation or this is equal to "="