How many electrons must be removed from a neutral, isolated conducting sphere to give it a positive charge of 8.0 x 10-8 C?

Three charged marbles are glued to a nonconducting surface and are placed in the diagram as shown. The charges of each marble are q1 = 6.20 µC, q2 = 1.01 µC, and  q3 = −2.14 µC.  Marble q1 is a distance r1 = 3.00 cm to the left of the marble q2, while marble q3 is a distance r3 = 2.00 cm to the right of the marble q2, as shown. Calculate the magnitude of the electric field a distance r' = 1.00 cm to the left of the center marble. What is the electric field due to each individual marble? How do you find the total or "net" electric field when you have several electric fields?  Another marble is placed 1 cm to the left of the middle marble. If this new marble has a charge of −3.62 µC, calculate magnitude and direction of the force on it.

Two point charges are placed on the x-axis. 12 µC at x = 0.6 m, and -15 µC at x = -0.6 m. Determine the magnitude and direction of the electric field on the y-axis at y = 0.45 m.

A glass rod with a charge of  + 15.6 nC is held  2.50 cm from a piece of wool that is oppositely charged.  If the attractive force between the glass rod and wool is  - 3.50 X 10 -3 N,  what is the charge, in nanoCoulombs, on the piece of wool ?    (Ans:  - 15.6 nC)