Q: A radon atom undergoes radioactive decay, emitting an alpha particle ($q = +2e$) and a polonium nucleus ($q = +84e$). If they start $1.0 \times 10^{-10}$ m apart, what is the initial electric potential energy of the system? ($k = 9 \times 10^9$ N m$^2$/C$^2$, $e = 1.6 \times 10^{-19}$ C)

$U = 9 \times 10^9 \frac{(2 \times 1.6 \times 10^{-19})(84 \times 1.6 \times 10^{-19})}{1.0 \times 10^{-10}}$

Question 1

Which of the following best describes electric potential energy in a system of two point charges?

Accepted Answer

It is the energy stored in the configuration of the charges due to their positions.

Question 2

What is the SI unit of electric potential energy?

Accepted Answer

Joule

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

Which equation gives the electric potential energy $U$ between two point charges $q_1$ and $q_2$ separated by a distance $r$?

Accepted Answer

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

Question 5

A positive charge and a negative charge are released from rest in vacuum and move toward each other. As they do, what happens to the electric potential energy of the system?

Accepted Answer

It becomes more negative.

Question 6

If work is done by an external force to increase the separation between two oppositely charged particles, what happens to the electric potential energy?

Accepted Answer

It increases.

Question 7

Which of the following statements is true about the electric potential energy of two like charges as the distance between them increases?

Accepted Answer

The potential energy decreases.

Question 8

A system contains two charges: $q_1 = +5 \mu C$ and $q_2 = -3 \mu C$ separated by $r = 0.2$ m. What is the sign of the electric potential energy?

Accepted Answer

Negative

Question 9

Which of the following best explains why work must be done to separate two oppositely charged particles?

Accepted Answer

Because separating them increases the system's potential energy.

Question 10

A radon atom undergoes radioactive decay, emitting an alpha particle ($q = +2e$) and a polonium nucleus ($q = +84e$). If they start $1.0 	imes 10^{-10}$ m apart, what is the initial electric potential energy of the system? ($k = 9 	imes 10^9$ N m$^2$/C$^2$, $e = 1.6 	imes 10^{-19}$ C)

Accepted Answer

$U = 9 	imes 10^9 \frac{(2 	imes 1.6 	imes 10^{-19})(84 	imes 1.6 	imes 10^{-19})}{1.0 	imes 10^{-10}}$

Question 11

Which graph best represents the electric potential energy $U_e$ versus distance $r$ for two like charges?

Accepted Answer

A curve that increases as $r$ decreases.

Question 12

Which graph best represents the electric potential energy $U_e$ versus distance $r$ for two unlike charges?

Accepted Answer

A curve that becomes more negative as $r$ decreases.

Question 13

A charge is moved from position $i$ to position $f$ in an electric field. If the electric potential energy increases, what can you infer about the work done?

Accepted Answer

Work was done by an external force against the field.

Question 14

Which of the following is true about the relationship between electric potential energy and configuration of charges?

Accepted Answer

Potential energy depends on both the magnitude and sign of the charges and their separation.

Question 15

A system of two charges $q_1$ and $q_2$ is assembled by bringing $q_2$ from infinity to a distance $r$ from $q_1$. What is the work done by the external force?

Accepted Answer

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

Question 16

If the electric potential energy of a system is negative, what does this indicate about the configuration of the charges?

Accepted Answer

The charges are unlike (opposite) charges.

Question 17

A positive charge moves left in a uniform electric field at constant velocity. What can you say about the net force acting on the charge?

Accepted Answer

The net force is zero.

Question 18

Which of the following is a correct qualitative justification for why increasing the distance between two oppositely charged objects increases their potential energy?

Accepted Answer

Because you are working against the attractive force between them.

Question 19

A system of two charges has electric potential energy $U$. If the charges are moved farther apart, what happens to $U$ for unlike charges?

Accepted Answer

$U$ becomes less negative.

Question 20

Which of the following is a real-world consequence of electric potential energy in atomic nuclei, as seen in radon decay?

Accepted Answer

It can result in the emission of energetic particles that damage biological tissue.

Electric Potential Energy: Concepts, Diagrams, and Applications

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