How to obtain x, y, and z component of a vector field

Dear Sir,

I am required to model poisson nernst plank equation ussing coefficient form of PDE. For this, I need to enter x and y component of velocity field u. For instance, u=Ai+Bj+Ck is vector with A B C components along x y z direction, What is process of obtaining the values of A B C of dependent variable vector like u?

Thanks & Regrads,

Vikas Nandal