Testing the force-freeness in a 3D MHD model of a stable active region. Panel a: total current, I, along the magnetic field, B, where I is integrated over the cross-section of the expanding flux tube. Panel b: displays the angle γ between the current density, J, and B. Both I and γ are plotted for twelve selected field lines in an EUV loop over the normalized arc length (i.e., 0 and 1 are in the photosphere and the apex is at about 0.5). I and γ are defined in Eqs. (1) and (2), respectively. The hot coronal part of the loop (with temperatures above 1 MK) is found between the vertical dotted lines. Based on data from Bingert & Peter (2011). See Sect. 1.