Electric Fields and Forces Practice Quiz (11 questions)

1. Two small objects carry charges with the same sign. What electrical interaction do they experience?

A. They repel one another because their charges have opposite signs.

B. They exert no electrical force because their charges have the same sign.

C. They repel one another because their charges have the same sign.

D. They attract one another because their charges have the same sign.

Answer: (C)

Explanation

Charges with the same sign repel, while charges with opposite signs attract. The attraction choice reverses the interaction associated with like charges.

2. When two objects are rubbed together inside an isolated system, what happens to their total electric charge?

A. It decreases because rubbing removes charge from the isolated system.

B. It remains conserved as charge is transferred between the objects.

C. It increases because rubbing creates additional electric charge.

D. It remains unchanged because no charge moves between the objects.

Answer: (B)

Explanation

In an isolated system, total electric charge is conserved, and rubbing transfers charge rather than creating it. The claim that no charge moves is wrong because rubbing can transfer charge between objects.

3. Which expression describes the allowed values of electric charge, where $N$ is an integer and $e$ is the fundamental unit of charge?

A. $q=N+e$

B. $q=Ne$

C. $q=N^2e$

D. $q=\frac{e}{N}$

Answer: (B)

Explanation

Electric charge is quantized in integer multiples of the fundamental charge, so it is expressed as $q=Ne$. The quotient form would not represent the stated rule that charge occurs in integer multiples of $e$.

4. A material contains electrons that are not bound to individual atoms and can move relatively freely through it. How is the material classified?

A. As an electrical conductor, because its electrons cannot move through it.

B. As an electrical conductor, because mobile electrons can move through it.

C. As an electrical insulator, because its electrons remain bound to atoms.

D. As an electrical insulator, because mobile electrons cannot move through it.

Answer: (B)

Explanation

A conductor has some electrons that are not bound to atoms and can move relatively freely through the material. The insulator classification does not fit because it describes electrons that are bound to atoms and cannot move relatively freely.

5. What does charging by induction mean?

A. Charging an object without touching it with the object that causes the charge

B. Charging an object by rubbing it against a different material

C. Charging an object by transferring electrons through direct contact

D. Charging an object by connecting it to ground without a nearby charged body

Answer: (A)

Explanation

Induction charges an object without contact with the object that induces the charge. Direct contact can transfer charge, but it is a different charging process.

6. If the separation between two stationary point charges is tripled while their charge magnitudes remain unchanged, how does the magnitude of the Coulomb force change?

A. It becomes one-ninth as large.

B. It becomes nine times as large.

C. It becomes one-third as large.

D. It becomes three times as large.

Answer: (A)

Explanation

Coulomb force varies inversely with the square of separation, so tripling the distance reduces the force by a factor of nine. A one-third reduction would apply to a force varying inversely with distance rather than its square.

7. What is the role of a positive test charge in defining the electric field at a point?

A. It creates the electric field whose strength is measured at that point.

B. It provides a reference for measuring the force per unit charge at that point.

C. It determines the distance between the source charges at that point.

D. It cancels the influence of other source charges at that point.

Answer: (B)

Explanation

The electric field is defined as the force on a positive test charge divided by the test charge, so it serves as a reference for measuring force per unit charge. A test charge detects the field rather than creating the source field.

8. How do linear and surface charge densities differ for uniform distributions?

A. Linear density is charge per unit length, while surface density is charge per unit area.

B. Linear density is charge per unit area, while surface density is charge per unit volume.

C. Linear density is total charge divided by volume, while surface density is total charge divided by length.

D. Linear density is charge per unit volume, while surface density is charge per unit length.

Answer: (A)

Explanation

Linear density describes how much charge is distributed along each unit of length, whereas surface density describes charge per unit area. Volume density, a plausible alternative, instead measures charge per unit volume.

9. In a diagram of an electric field, what does the local spacing of field lines indicate about the field?

A. Closer spacing represents a smaller electric-field magnitude.

B. Closer spacing represents a greater electric-field magnitude.

C. The spacing represents the sign of the source charge.

D. The spacing represents the direction of the electric force on a negative charge.

Answer: (B)

Explanation

The density of field lines through a surface perpendicular to them is proportional to the electric-field magnitude, so denser lines indicate a stronger field. Line direction, rather than spacing, conveys the field’s direction; a negative charge experiences force opposite that direction.

10. An electron enters a uniform electric field between two plates with an initial velocity perpendicular to the field. What motion results from the field’s effect on the electron?

A. It accelerates along the field and follows a parabolic path between the plates.

B. It accelerates opposite the field and follows a circular path between the plates.

C. It accelerates opposite the field and follows a parabolic path between the plates.

D. It moves at constant speed opposite the field in a straight path between the plates.

Answer: (C)

Explanation

The electric force on a negative electron points opposite the field, producing constant acceleration in that direction while its perpendicular velocity persists, so its path is parabolic. A positive particle accelerates along the field, not opposite it.

11. A glass rod is rubbed with silk in an isolated system. If electrons move from the glass to the silk, what should happen to the total charge of the two objects?

A. It remains unchanged because charge is transferred between the objects.

B. It becomes zero because rubbing neutralizes both objects.

C. It becomes positive because the glass loses electrons.

D. It becomes negative because the silk gains electrons.

Answer: (A)

Explanation

Rubbing transfers electrons from the glass to the silk, so the objects acquire equal and opposite charges while the isolated system’s total charge is conserved. The silk becomes negative, but that change does not make the combined charge negative.