1) How many electrons do 6.94 kg of water contain?

2) Two identically charged particles separated by a distance of 7.8 m repel each other with a force of 9 N. What is the magnitude of the charges?

3) In solid sodium chloride (table salt), chloride ions have one more electron than they have protons, and sodium ions have one more proton than they have electrons. These ions are separated by about 0.28 nm. Calculate the electrostatic force between a sodium ion and a chloride ion.

4) In gaseous sodium chloride, chloride ions have one more electron than they have protons, and sodium ions have one more proton than they have electrons. These ions are separated by about 0.24 nm. Suppose a free electron is located 0.86 nm above the midpoint of the sodium chloride molecule. What are the magnitude and the direction of the electrostatic force the molecule exerts on it? (Assume the chloride ion lies on the +x-axis and the sodium ion lies on the -x-axis, with the origin as the midpoint between the two ions. The free electron lies on the +y-axis.)

5) Identical point charges Q are placed at each of the four corners of a rectangle measuring 8.0 m by 9.0 m. If Q = 26 µC, what is the magnitude of the electrostatic force on any one of the charges?

6) Charge q₁ = 1.1 *10^-8 C is placed at the origin. Charges q₂ = -1.3 *10^-8 C and q₃ = 2.6 *10^-8 C are placed at points (0.18 m, 0 m) and (0 m, 0.24 m), respectively, as shown in the figure. Determine the net electrostatic force (magnitude and direction) on charge

q₃.

7) A small ball with a mass of 10 g and a charge of -0.2 µC is suspended from the ceiling by a string. The ball hangs at a distance of 5.0 cm above an insulating floor. If a second small ball with a mass of 60 g and a charge of 0.4 µC is rolled directly beneath the first ball, will the second ball leave the floor?

8) Four point charges, q, are fixed to the four corners of a square that is 15.0 cm on a side. An electron is suspended above a point at which its weight is balanced by the electrostatic force due to the four point charges, at a distance of 13 nm above the center of the square. (The square is horizontally flat, and the electron is suspended 13 nm vertically above the center of the square.) What is the magnitude of each fixed charge in coulombs? What is the magnitude of each fixed charge as a multiple of the electron's charge?