This work presents a numerical study of natural convection in a laterally heated cavity filled with an electrically conductive fluid (Pr = 0.0321) in the presence of an external magnetic field and an internal heat source. The finite volume method with the SIMPLER algorithm is used to solve the system of equations governing the magnetohydrodynamics flow. The influence of volumetric heating S_Q on the flow structure and on the heat transfer within the cavity for Gr = 10⁴, 10⁵, and 10⁶ was examined. The effects of aspect ratio (A = 1, 0.5, and 2), Prandtl number (low Prandtl number fluids), and magnetic field (Ha = 0 to 100) were determined in the steady state with internal heat generation. Two orientations of the magnetic field were considered in order to have better control of the flow. The strongest stabilization of the flow field with internal heat generation is found when the magnetic field is oriented horizontally.