Electrostatic charging resulted by sliding contacts of particles and wall is ubiquitous in gas-solid systems. However, the mechanism of sliding electrification involved the initial charge is still unclear. In the current paper, to accurately obtain triboelectric charge, the initial charge and the final charge were measured. A quantitative comparison of the experimental results and the value of theoretical model based on the condenser model is provided. And the results suggest the final charge of the grain is linear with the initial charge for a short sliding distance, whereas the experimental data become scattered for a long sliding length. The longer sliding length generation was found to generate more triboelectric charge, and the influences of the normal load on the triboelectric charge density were analyzed by considering the contact area. It provides a comprehensive perspective that the triboelectrification is fundamentally described as a synergistic effect of the sliding contact area and the real contact area. (C) 2017 Elsevier B.V. All rights reserved.