I am having trouble with Problem 9 only. Two point charges (+10.0 nC and -10.0 nC) are located 8.00 cm apart. For each problem, let U=0 when all of the charges are separated by infinite distance. Problem 9: What is the potential energy if a third pint charge q = - 4.2 nC is placed at point b? The answer is negative 11.2 micro Joules or - 11.2?J. How do they get the negative?

Problems 7-10. Two point charges (+10.0 nC and -10.0 nC) are located 8.00 cm apart. For each problem, let U = 0 when all of the charges are separated by infinite distances. 7. What is the potential energy for these two charges? 8. What is the potential energy if a third point charge q = -4.2 nC is placed at point a? 9. What is the potential energy if a third point charge q = -4.2 nc 8.00 cm 8.00 cm is placed at point b? a b 10. What is the potential energy if a -4.00 - 4.00 - 4.00- third point charge q = -4.2 nC cm cm cm is placed at point c? Problems 7-10