Need help solving electric forces and fields

[Anthem26]

These are the problems I have left to do, but I have no clue as to how to solve them.

Homework Statement

a. Two small metal spheres are 23.0 cm apart. The spheres have equal amounts of negative charge and repel each other with a force of 0.040 N. What is the charge on each sphere?

b.A tiny sphere with a charge of q = +8.2 µC is attached to a spring. Two other tiny charged spheres, each with a charge of -4.0 µC, are placed in the positions shown in the figure, in which b = 4.2 cm. The spring stretches 5.0 cm from its previous equilibrium position toward the two spheres. Calculate the spring constant. (diagram: http://imgur.com/FKcPp.gif )

c.What are the magnitude and direction of the electric field midway between two point charges, -13 µC and +15 µC, that are 7.3 cm apart?

d.Two equal charges (Q = +0.95 nC) are situated at the diagonal corners A and B of a square of side x = 1.0 m as shown in the diagram. What is the magnitude of the electric field at point D? (diagram: http://imgur.com/eervv.gif )

Homework Equations

For all problems, I used Coulumb's law: F = k|q1||q2|/r^2
k=8.99*10^9 Nm^2/C^2
1 nC = 10^-9 C

The Attempt at a Solution

a. equation setup: .04 N = 8.99*10^9 * (q1)(q2)/(.23m)^2
I got 1.2x10^-6 for q1 & q2 as my answer, but that's not correct. Was my setup wrong or was it just a simple alegbraic error?

b. I honesty have no clue how to solve this problem. Hopefully someone could give me a little insight on how to setup this problem.

c. This problem seemed easy enough and straight forward, but after several attempts I still get the wrong answer. Here's my equation setup: F = 8.99*10^9 * (-1.3*10^-3)(1.5*10^-8)/(.073m)^2 I got 3.3*10^-4 but apparently that's not the correct answer.

d. I honestly don't know have to solve this one either. :(

 

[ideasrule]

a. equation setup: .04 N = 8.99*10^9 * (q1)(q2)/(.23m)^2
I got 1.2x10^-6 for q1 & q2 as my answer, but that's not correct. Was my setup wrong or was it just a simple alegbraic error?

It's a simple algebraic error. Recalculate, and remember that q is negative.

b. I honesty have no clue how to solve this problem. Hopefully someone could give me a little insight on how to setup this problem.

It's asking for the spring constant, so the equation F=-kx should pop into mind. Since k=-F/x, what's F? What's x?

c. This problem seemed easy enough and straight forward, but after several attempts I still get the wrong answer. Here's my equation setup: F = 8.99*10^9 * (-1.3*10^-3)(1.5*10^-8)/(.073m)^2 I got 3.3*10^-4 but apparently that's not the correct answer.

You've found the force between the particles, but the question is asking for the electric field. E=kq/r² for each particle.

d. I honestly don't know have to solve this one either. :(

Can you find the horizontal component of the electric field? The vertical component? (Hint: Use E=kq/r².)

 

[Anthem26]

ok, i tried to work out a and b, but still no luck. here's my work:

(a) Given: r=.23m, q1=q2, f=.04N

Equations needed:F=k((q1q2)/r^2), where k is approx 8.9910^9 Nm^2/C^2

Solution: Since q1=q2, we can substitute q2 for q1 in the equation

F=k((q1q1)/r^2)

F=k*((q1^2)/r^2)

F/k=(q1^2)/r^2

r^2*(F/k)=(q1^2)

q1=q2=sqrt(r^2*(F/k))

plug in the given values...sqrt(.23^2(.04/(8.9910^9)))

i get approx -4.85*10^-7 C

*EDIT ok i managed to get this right, I just had to rewrite 4.85*10^7 to .000000485
(b) Using the E=kq/r2 provided. I plugged in:
E1 = 8.99*10^9 * (13*10^-6)/(.073m)^2
= 2.2*10^7

E2 = 8.99*10^9 * (15*10^-6)/(.073m)^2
= 2.5*10^7

E = 2.5*10^7 - 2.2*10^7 = 3*10^6

What did I do wrong?