Above all else, I don\'t know how to set this problem up properly- I understand

Question

Above all else, I don't know how to set this problem up properly- I understand that the two spheres in the problem can be treated as point charges by Gauss' law, but I don't know how to approach this problem:

(a) Calculate the potential energy of a system of two small spheres, one carrying a charge of 2.10 HC and the other a charge of -3.60 HC with their centers separated by a distance of 0.250 m. Assume zero potential energy when the charges are infinitely separated 2.72 (b) Suppose that one of the spheres is held in place and the other sphere, which has a mass of 1.50 g is shot away from it. What minimum initial speed would the moving sphere need in order to escape completely from the attraction of the fixed sphere? (To escape, the moving sphere would have to reach a velocity of zero when it was infinitely distant from the fixed sphere.) X m/s

Explanation / Answer

a)
Use the formula for potential energy of 2 charges,
U = K*q1*q2/r
= (9*10^9)*(2.1*10^-6)*(-3.6*10^-6) / (0.25)
= -0.272 J
Answer: - 0.272 J

b)
final energy will be zero when they are far apart
So initial energy must also be 0
use:
initial potential energy + initial kinetic energy = 0
-0.272 + 0.5*m*v^2 = 0
-0.272 + 0.5*(1.5*10^-3)*v^2 = 0
v = 19.05 m/s
Answer: 19.05 m/s

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