Question 1

According to Coulomb's Law, what is the magnitude of the force between two point charges $$q_1$$ and $$q_2$$ separated by a distance $$r in $$vacuum?

Accepted Answer

$$F = k \frac{|q_1$||q_2$|}{r^2$}$$

Question 2

Three charges are arranged in a triangle as shown: $$q_1 = +7.0$$\,\mu C$$, q_2$ = +2.0\,\mu C$$, $$q_3 = -3.5$$\,\mu C$$. What is the first step in calculating the net force on q_2$?

Accepted Answer

Calculate the force on $$q_2$$ due to $$q_1$$ and $$q_3$$ separately using Coulomb's Law.

Question 3

If two like charges are brought closer together, what happens to the electric potential energy of the system?

Accepted Answer

It increases.

Question 4

A $$2\,\mu C$$ charge is placed at the vertex of a right triangle with sides $$3\,m$$ and $$4\,m. $$What is the distance from this charge to the $$-3.5\,\mu C$$ charge at the other vertex?

Accepted Answer

$$5\,m$$

Question 5

Which of the following best describes the work done by an external force to assemble two point charges $$q_1$$ and $$q_2$$ from infinity to a separation $$r$$?

Accepted Answer

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

Question 6

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

Accepted Answer

It remains constant.

Question 7

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

Accepted Answer

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

Question 8

Which of the following is NOT a recommended problem-solving strategy for electric force problems?

Accepted Answer

Ignore the vector nature of force.

Question 9

If $$q_1 = +2$$\,\mu C$$ and q_2$ = +2\,\mu C$$ are brought from infinity to a separation of $$0.03\,m$$, what is the potential energy stored in the system?

Accepted Answer

$$U = 1.2\,J$$

Question 10

A $$-12\,nC$$ charge is released from rest $$10\,cm$$ above a $$+2\,\mu C$$ fixed charge. If the mass of the moving charge is $$8\,g$$, what is its speed when it is $$5\,cm$$ above the fixed charge? (Neglect gravity.)

Accepted Answer

$$v = 1.2\,m/s$$

Question 11

Which equation correctly relates the change in kinetic energy to the change in potential energy for a system with only conservative forces?

Accepted Answer

$$\Delta K = -\Delta U$$

Question 12

If two oppositely charged objects are pulled farther apart, what happens to the electric potential energy of the system?

Accepted Answer

It increases.

Question 13

What is the value of the Coulomb constant $$k$$ used in Coulomb's Law?

Accepted Answer

$$9 	imes 10^9$\,N\,m^2$/C^2$

Question 14

Which of the following best describes the direction of the force between two charges of the same sign?

Accepted Answer

The force is repulsive and directed away from each other.

Question 15

A person moves a charge $$q_2$$ from infinity to a distance $$r$$ from a fixed charge $$q_1. $$What is the total work done by the person?

Accepted Answer

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

Question 16

When adding the forces from multiple charges on a single charge, what must you always remember to do?

Accepted Answer

Add the forces as vectors, not just magnitudes.

Electric Potential Energy and Coulomb’s Law: Study Notes

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