Question Detail
Question
A positive charged particle with mass m=10 mg and charge Q=0.10 mC is placed just near the surface of a non-conducting table. What should be the charge density on the table to keep the particle above it in a stationary position? (g=9.8 m/s2)
Correct Answer
Option C is the correct answer.
Detailed Explanation
For equilibrium, the electric force from the charged table surface balances gravity: QE = mg => E = mg/Q. m = 10 mg = 1x10-5 kg, Q = 0.10 mC = 1x10-4 C. E = (1x10-5)(9.8)/(1x10-4) = 9.8x10-5/1x10-4 = 0.98 N/C. For a large flat non-conducting sheet, E = sigma/(2eps0), so: sigma = 2eps0*E = 2(8.85x10-12)(0.98) = 1.73x10-11 C/m2.
Hint
For equilibrium, the electric force from the charged table surface balances gravity: QE = mg => E = mg/Q.