Q: A $$2\mu C$$ charge is placed at the top of a triangle, with a $$7\mu C$$ charge 6 m to the left and a $$-3.5\mu C$$ charge 6 m to the right. What is the net force on the $$2\mu C$$ charge?

$$F = 4.9 \times 10$$^{-3}$$ \text{ N}$$

Question 1

According to Coulomb's Law, what is the direction of the force between two point charges of opposite sign?

Accepted Answer

The force is attractive and points toward each other.

Question 2

Given three point charges $$q_1$$, $$q_2$$, and $$q_3$$ arranged in a triangle, what is the total electric potential energy of the system?

Accepted Answer

$$U = \frac{k q_1$ q_2$}{r_{12}} + \frac{k q_2$ q_3$}{r_{23}} + \frac{k q_1$ q_3$}{r_{13}}$$

Question 3

A $$2\mu C$$ charge is placed at the top of a triangle, with a $$7\mu C$$ charge 6 m to the left and a $$-3.5\mu C$$ charge 6 m to the right. What is the net force on the $$2\mu C$$ charge?

Accepted Answer

$$F = 4.9 	imes 10$$^{-3}$$ 	ext{ N}$$

Question 4

If two charges $$q_1$$ and $$q_2$$ are brought from infinite separation to a distance $$r$$, what is the work done by an external force?

Accepted Answer

$$W = \frac{k q_1$ q_2$}{r}$$

Question 5

A negative charge moves in a circle at constant speed around a positive charge. What happens to the electric potential energy of the system?

Accepted Answer

It is constant since potential energy depends only on separation.

Question 6

Which of the following best describes the relationship between electric potential energy and separation distance for two point charges?

Accepted Answer

Electric potential energy decreases as separation increases.

Question 7

What is the SI unit for electric potential energy?

Accepted Answer

Joule (J)

Question 8

A $$2\mu C$$ charge and a $$-12nC$$ charge are separated by $$0.10 m. If $$the $$-12nC$$ charge is moved to $$0.05 m$$, what happens to the electric potential energy?

Accepted Answer

It decreases and becomes more negative.

Question 9

Which strategy is NOT recommended for solving problems involving electric forces?

Accepted Answer

Ignore the vector nature of force.

Question 10

If the force between two charges is $$F = \frac{k |q_1$||q_2$|}{r^2$}$$, what happens to the force if the distance between the charges is doubled?

Accepted Answer

The force is reduced to one-fourth.

Question 11

A person moves two $$2\mu C$$ charges from infinite separation to $$0.03 m $$apart. What is the potential energy stored in the system?

Accepted Answer

$$U = 1.2 	ext{ J}$$

Question 12

Which equation correctly relates the work done by external forces to the change in kinetic and potential energy for a system?

Accepted Answer

$$K_i + U_i + W = K_f + U_f$$

Question 13

For a system of two charges, $$q_1$$ and $$q_2$$, what happens to the electric potential energy as $$r \rightarrow \infty$$?

Accepted Answer

It approaches zero.

Question 14

In a triangle arrangement of three charges, which step is necessary to find the net force on one charge?

Accepted Answer

Calculate the force from each charge and add them as vectors.

Question 15

If the initial and final positions of two opposite charges are $$r_i$$ and $$r_f$$, what is the change in electric potential energy?

Accepted Answer

$$\Delta U = k q_1$ q_2$ \left( \frac{1}{r_f} - \frac{1}{r_i} \right)$$

Question 16

Which of the following is true about the force between two charges according to Coulomb's Law?

Accepted Answer

The force can be attractive or repulsive depending on the sign of the charges.

Question 17

A bead is pushed along a wire by a constant force of $$10 N $$for a horizontal displacement of $$10 m. $$What is the work done?

Accepted Answer

$$100 	ext{ J}$$

Question 18

If the electric potential energy between two charges is $$U_q = \frac{k q_1$ q_2$}{r}$$, what happens to $$U_q if$$ $$q_1$$ and $$q_2$$ are both positive and $$r$$ decreases?

Accepted Answer

The potential energy increases.

Question 19

Which of the following is a correct problem-solving strategy for electric force problems?

Accepted Answer

Draw a clear force diagram.

Electric Potential Energy and Coulomb’s Law: Study Notes

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