4. A drop of oil is floating between two parallel plates with a separation of 8.3 mm. If the mass of the oil drop is 2.7 x 10-5 kg, and its charge is -9.612 x 10-19 C, determine the voltage across the charged plates and determine which plate is positive. Electrostatic + + + + attraction F - Eq Oil droplet Gravitational attraction microscope F - mg