Two charges Qa = 3 µC and Qb = -3 µC are placed on the x-axis with a separation of a = 21 cm.
(a) Find the net electric field at point P, a distance d = 13 cm to the left of charge Qa.
This is no prob = -1364069.13N
(b) Find the force on Qb due to Qa .
No prob again = -1.837
The charges Qa and Qb are now attached to the ends of a spring whose unstretched length is s0 = 21 cm. With the charges attached, the spring compresses to an equilibrium length s1 = 9 cm.
(c) Calculate the spring constant ks of the spring.
this is what i cant figure out?
The Attempt at a Solution
heres are the steps i did (its wrong though)
1. Spring constant is Fsp = -k delta s
2. i already know the force that the 2 charges excert on each other (= 1.837N)
3. delta s would be 21cm - 9cm = 12cm = .12m
4. then i just plugged in:
1.837 = -k(.12m)
k = -15.308
this is wrong.