# Electrostatics Problems (all with worked out answers but wrong)

• clairez93
In summary: For question 2, I don't think that the answer should be negative. If it's negative, then the net force on the charge would be negative, which is not the case.For question 3, the answer should be that the angle theta is 2.0 degrees.
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

## 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

#### Attachments

• electrostatics1.jpg
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• electrostatics2.jpg
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• electrostatics3.jpg
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Last edited by a moderator:
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.

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

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.

## 1. How do I solve an electrostatics problem?

Solving an electrostatics problem involves following a few key steps. First, draw a diagram of the problem and label all known values. Then, use Coulomb's Law to calculate the magnitude and direction of the electric force between the charged objects. Next, use the principle of superposition to calculate the total electric field at a specific point. Finally, use the electric field to calculate the electric potential at that point.

## 2. What is the difference between electric potential and electric field?

Electric potential is a scalar quantity that represents the amount of potential energy per unit charge at a given point. It is measured in volts. Electric field, on the other hand, is a vector quantity that represents the force experienced by a charged particle at a given point. It is measured in newtons per coulomb.

## 3. How do I determine the direction of an electric field?

The direction of an electric field is always in the direction of the force that a positive test charge would experience if placed at that point. This means that the direction of the electric field points away from positive charges and towards negative charges.

## 4. What is the relationship between electric force and distance?

According to Coulomb's Law, the force between two charged objects is directly proportional to the product of their charges and inversely proportional to the square of the distance between them. This means that as the distance between two charged objects increases, the force between them decreases.

## 5. How do I account for multiple charged objects in an electrostatics problem?

To account for multiple charged objects, you must use the principle of superposition. This states that the total electric field at a point is equal to the vector sum of the individual electric fields produced by each charged object. This can also be applied to electric potential by using the principle of superposition for electric potential.

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