A plastic rod rubbed with wool is found to have a negative charge of 8×10⁻⁷ C. The number of electrons transferred, and the direction of the transfer, is

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(4) 5×10¹², from wool to plastic rod. Given: charge on the rod q=-8×10⁻⁷ C; e=1.6×10⁻¹⁹ C. Idea: charge is quantised, |q|=ne, and rubbing only moves electrons from one body to the other. n=(|q|)/e=(8×10⁻⁷)/(1.6×10⁻¹⁹)=5×10¹² The rod is negative, so it has gained these electrons: they moved from the wool to the plastic rod, and the wool is left with an equal positive charge. So 5×10¹² electrons, from wool to plastic rod.

A plastic rod rubbed with wool is found to have a negative charge of 8×10⁻⁷ C. The number of electrons transferred, and the direction of the transfer, is

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