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Tribo-electrification between aluminum target and grass beads has been studied experimentally based on the current generated by the electrification of grass beads. Effect of tangential velocity of impact on the current is investigated. As far as the velocity is low enough, the current shows positive value. However, for higher velocity, the current changes from positive to negative as the time elapses. Sudden increase or decrease in the velocity gives a respective current corresponding to each velocity. The fact means that the polarity of the current is solely determined by the tangential velocity as far as the target is not exchanged. Preservation conditions of the metal target affect on the polarity of the current. If the target is preserved in atmospheric condition for a day, positive current is generated firstly, decreased with time elapsed and finally negative current is generated. Two different experiments support that the polarity change found in this study is not related to the surface oxidization. Effect of water molecules adsorbed on the target surface has been investigated through heat treatment of the target. Higher temperature of the treatment causes the sharper decrease in the initial current, which suggests that the water molecules adsorbed on the target surface causes the change in the polarity of the tribo-electrification. Contact potential difference between particles and target also changes by heat treatment of the target.

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