Electrostatics Problems (all with worked out answers but wrong)

[clairez93]

Homework Statement

1. A thin rod bent in the shape of a semicircle of radius 20 cm is uniformly charged along its length with a total charge of 8.0 microC. What is the electric field at the center of the semicircle?
a. 7.3 x 10^4 N/C
b. 8.4 x 10^4
c. 5.7 x 10^4
d. 3.9 x 10^4
e. None
Answer: C

2. Two charges, q1 = +2.0 microC and q2 = -3.0 microC are placed on the two corners A and B of a square as shown. What should be the value a charge q3 placed at the corner D such that the net electrostatic force on a test charge Q on corner C is zero?
a. 3.0 microC
b. -3.0
c. -2.0
d. +2.0
e. None
Answer: E

3. Two charges of +5.0 microC are placed on two corners of an equilateral triangle of side 0.50 m. The magnitude of the electric field at the third corner is:
a. 3.9 x 10^3 N/C
b. 2.8 x 10^4
c. 1.6 x 10^5
d. Zero
e. None
Answer: C

4. A charge of +3.0 microC is located at the origin and another charge of -4.0 microC is located on the y-axis at y = 0.30 m. The magnitude of the electric field at 0.40 m is:
a. 2.5 x 10^3 N/C
b. 3.2 x 10^4
c. 5.7 x 10^4
d. 9.5 x 10^4
e. None
Answer: D

5. The figure shows two 5.0 g spheres suspended from 1.0-m-long threads. The spheres repel each other after being charged to +31 nC. What is the angle theta?
a. 4.0 degrees
b. 6.0
c. 2.0
d. 7.9
Answer: C

6. Three charges of magnitude 9.0 x 10^-4 C each are located at x = 1.0m, y = 0.0 m, at x = 0.0 m, y = 0.0 m, and a t x = -1.0 m, y = 0.0 m. The one in the middle is negative, while the other two are positive. What is the net Coulombic force exerted by them on a negative 9.0 x 10^-5 C charge located at x = 0.0 m, y = -2.0 m?
a. 790 N repulsive
b. 1100 N attractive
c. 1100 N repulsive
d. 790 N attractive
Answer: D

Homework Equations

The Attempt at a Solution

*see pictures*

For second 3:

http://img695.imageshack.us/img695/8262/electrostatics4.jpg
http://img189.imageshack.us/img189/3112/electrostatics5.jpg
http://img706.imageshack.us/img706/4039/electrostatics6.jpg

 

[clairez93]

Please, I know there's a lot, but my exam is coming up and I would really appreciate if someone could show me what I'm doing wrong.

 

[Oerg]

I don't think i see anything wrong with your working for question 1. Are the questions from a reliable source?

 

[clairez93]

I think so; they are the questions that I got wrong on my last test for electrostatics. But it may be that the test bank is wrong, that has happened before. I'll have to speak with my teacher.