Spring constant with point charges

  • Thread starter mujadeo
  • Start date
  • #1
103
0

Homework Statement


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?





Homework Equations





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.
please help
 

Answers and Replies

  • #2
Dick
Science Advisor
Homework Helper
26,263
619
For a) the units of E field strength are not N. For b) that looks right, except you forgot to include the units, which are newtons. For c) figure out the potential energy stored in the compressed spring and equate it to the change of potential energy of the moved charges. You can't assume that the force is a constant.
 
  • #3
103
0
Dont I need the spring constant to get the PE stored in the spring though?
PE (in spring) = ( 1/2 k (delta s)^2)
 
  • #4
Dick
Science Advisor
Homework Helper
26,263
619
The change in the PE of the charges gives you the change in the PE of the spring. That gives you the spring constant.
 

Related Threads on Spring constant with point charges

  • Last Post
Replies
6
Views
9K
Replies
14
Views
824
  • Last Post
Replies
1
Views
468
  • Last Post
Replies
3
Views
6K
  • Last Post
Replies
1
Views
735
  • Last Post
Replies
1
Views
922
  • Last Post
Replies
3
Views
2K
Replies
3
Views
7K
  • Last Post
Replies
2
Views
1K
Top