Physics Charge?

You hold a ruler that has a charge on it's tip 2 cm above a small piece of tissue paper to see if it can be picked up. The ruler has -10 microcoloumbs of charge. The tissue has 5 g of mass. What is the minimum charge required to pick up the tissue paper?


The weight of the paper is (0.005 kg)(9.8 m/s^2) = 0.049 N. Set that force equal to the electrostatic attraction, letting "Q" be the charge that would be needed (on the paper) to lift the paper: 0.049N = kQ(10^(-5) C)/(0.02 m)^2 => Q = (0.049N)(0.02 m)^2 / [(10^(-5)C)*(9 x 10^9 Nm^2/C^2)] = 2.178 x 10^(-10) C; I would give the answer as 220 pC.


The ruler is charged so it induces an opposite charge on the paper. The paper rises up and transfers some charge to the ruler, gets charged with the same charge and gets repelled down, where it discharges to Earth and then the cycle repeats.