The complex permeability of ferrites is frequency dependent. The real part of the complex permeability deteriorates in a high frequency range and the imaginary part has a peak after staring the deterioration. This paper tries to reveal that the frequency characteristics for some ferrites can be approximately derived from a first-order linear differential equation for the magnetic field intensity and magnetic flux density. The first-order differential equation is expressed by the reciprocal of the complex permeability and provides first-order magnetic and electric circuits for ferrite cores. In contrast with the commonly used series Rs-Ls circuit for the cores, obtained from B=(μ'-jμ")H, the derived first-order electric circuit is composed of the inductance (L) and resistance connected in parallel. In this paper, it is demonstrated that the inductance L remains constant, whereas Ls decreases with the increase in frequency. In other words, the real part of the reciprocal of the complex permeability remains constant for the frequency increase. In addition, it is found that the imaginary part of the reciprocal of the complex permeability is approximately proportional to frequency.