17.4 Coulomb's force law 6. EST (a) Earth has an excess of 6 X 10 electrons on each square centimeter of surface. Determine the electric charge of Earth in coulombs. (b) If, as you walk across a rug, about 10 2 kg of electrons transfer to your body, estimate the number of excess electrons and the total charge in coulombs on your body. 7. Determine the electrical force that two protons in the nucleus of a helium atom exert on each other when separated by 2.0 X 10 - m.12. ** Two objects with charges q and 4q are separated by 1.0 m. (a) Determine the charge (both sign and magnitude) and position of a third object that causes all three objects to remain in equilibrium (all three objects are on the same line). (b) Is the equilibrium stable or unstable? How do you know? (Hint: An object is in a stable equilibrium if a small displacement of the object from the equilibrium results in a force exerted on the object that points opposite to the direction of displacement.) 13. * Salt crystal Four ions (Na , Cl, Na , and Cl ) in a row are each separated from their nearest neighbor by 3.0 x 10 m. The charge of a sodium ion is te and that of a chlorine ion is -e. Determine the electric force exerted on the chlorine ion at the right end of the row due to the other three ions.38. ** The six objects shown in Figure P17.38 have equal-magnitude electric charge. Adjacent objects are separated by distance a. Write an expression in terms of q and a for the force that the five objects on the right exert on the positive charge on the left. FIGURE P17.38 + 9 + 9 - 9 +9 + + +