1. A metal needle of length 7.6 cm does not have any excess charge on it. It is brought to a distance d=9.2 cm from the center of a negatively charged sphere containing N= 4.7x1015 excess electrons. As a result the needle becomes polarized, with +8.8 nC on the end that is close to the sphere, and -8.8 nC on the end that is far.
What is the magnitude of the total force acting on the needle?
2. Two charges, Q1=4.9 nC and Q2=-5.5x10-9 C are situated at the diagonal corners A and C of a rectangle of sides AB=DC=22.6 cm and AD=BC=4.3 cm. What is the magnitude of the electric field at point D?