# HowTo Adding a new transport equation

Suppose you want to solve and additional scalar transport equation for the scalar ψ by adding it to an existing solver. The equation has the form

where ρ and Γ are defined as `dimensionedScalar` and are retrieved from a dictionary using the `lookup` member function. *S*_{φ} is the source term.

To implement the equation, just add the line:

dimensionedScalarField psi;

to the `createFields.H` file of the solver,

Then, solve the equation by adding the following lines in the point of the solver where you want the equation to be solved.

solve ( fvm::ddt(rho, psi) + fvm::div(phi, psi) - fvm::laplacian(gamma, psi) == S_psi );

In this example, the source term is treated explicitly. To manage it implicitly, OpenFOAM provides the `Sp` and the `SuSp` functions.
Suppose we can write it as *S*_{k} = Κψ. The code lines to solve the same equation with an implicit treatment of the source term are:

solve ( fvm::ddt(rho, psi) + fvm::div(phi, psi) - fvm::laplacian(gamma, psi) == fvm::Sp(kappa, psi) );

`SuSp(kappa, psi)` can be used to discretise the source term implicitly or explicitly according to the sign of `kappa`.